* Loophole in async page I/O
@ 2020-10-12 21:13 Matthew Wilcox
2020-10-12 22:08 ` Jens Axboe
2020-10-13 5:13 ` Hao_Xu
0 siblings, 2 replies; 14+ messages in thread
From: Matthew Wilcox @ 2020-10-12 21:13 UTC (permalink / raw)
To: io-uring; +Cc: Johannes Weiner, Jens Axboe
This one's pretty unlikely, but there's a case in buffered reads where
an IOCB_WAITQ read can end up sleeping.
generic_file_buffered_read():
page = find_get_page(mapping, index);
...
if (!PageUptodate(page)) {
...
if (iocb->ki_flags & IOCB_WAITQ) {
...
error = wait_on_page_locked_async(page,
iocb->ki_waitq);
wait_on_page_locked_async():
if (!PageLocked(page))
return 0;
(back to generic_file_buffered_read):
if (!mapping->a_ops->is_partially_uptodate(page,
offset, iter->count))
goto page_not_up_to_date_locked;
page_not_up_to_date_locked:
if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) {
unlock_page(page);
put_page(page);
goto would_block;
}
...
error = mapping->a_ops->readpage(filp, page);
(will unlock page on I/O completion)
if (!PageUptodate(page)) {
error = lock_page_killable(page);
So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage()
and wait for the I/O to complete. I can't quite figure out if this is
intentional -- I think not; if I understand the semantics right, we
should be returning -EIOCBQUEUED and punting to an I/O thread to
kick off the I/O and wait.
I think the right fix is to return -EIOCBQUEUED from
wait_on_page_locked_async() if the page isn't locked. ie this:
@@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page,
struct wait_page_queue *wait)
{
if (!PageLocked(page))
- return 0;
+ return -EIOCBQUEUED;
return __wait_on_page_locked_async(compound_head(page), wait, false);
}
But as I said, I'm not sure what the semantics are supposed to be.
^ permalink raw reply [flat|nested] 14+ messages in thread* Re: Loophole in async page I/O 2020-10-12 21:13 Loophole in async page I/O Matthew Wilcox @ 2020-10-12 22:08 ` Jens Axboe 2020-10-12 22:22 ` Jens Axboe 2020-10-13 5:31 ` Hao_Xu 2020-10-13 5:13 ` Hao_Xu 1 sibling, 2 replies; 14+ messages in thread From: Jens Axboe @ 2020-10-12 22:08 UTC (permalink / raw) To: Matthew Wilcox, io-uring; +Cc: Johannes Weiner On 10/12/20 3:13 PM, Matthew Wilcox wrote: > This one's pretty unlikely, but there's a case in buffered reads where > an IOCB_WAITQ read can end up sleeping. > > generic_file_buffered_read(): > page = find_get_page(mapping, index); > ... > if (!PageUptodate(page)) { > ... > if (iocb->ki_flags & IOCB_WAITQ) { > ... > error = wait_on_page_locked_async(page, > iocb->ki_waitq); > wait_on_page_locked_async(): > if (!PageLocked(page)) > return 0; > (back to generic_file_buffered_read): > if (!mapping->a_ops->is_partially_uptodate(page, > offset, iter->count)) > goto page_not_up_to_date_locked; > > page_not_up_to_date_locked: > if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { > unlock_page(page); > put_page(page); > goto would_block; > } > ... > error = mapping->a_ops->readpage(filp, page); > (will unlock page on I/O completion) > if (!PageUptodate(page)) { > error = lock_page_killable(page); > > So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() > and wait for the I/O to complete. I can't quite figure out if this is > intentional -- I think not; if I understand the semantics right, we > should be returning -EIOCBQUEUED and punting to an I/O thread to > kick off the I/O and wait. > > I think the right fix is to return -EIOCBQUEUED from > wait_on_page_locked_async() if the page isn't locked. ie this: > > @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, > struct wait_page_queue *wait) > { > if (!PageLocked(page)) > - return 0; > + return -EIOCBQUEUED; > return __wait_on_page_locked_async(compound_head(page), wait, false); > } > > But as I said, I'm not sure what the semantics are supposed to be. If NOWAIT isn't set, then the issue attempt is from the helper thread already, and IOCB_WAITQ shouldn't be set either (the latter doesn't matter for this discussion). So it's totally fine and expected to block at that point. Hmm actually, I believe that: commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 Author: Hao Xu <[email protected]> Date: Tue Sep 29 20:00:45 2020 +0800 io_uring: fix async buffered reads when readahead is disabled maybe messed up that case, so we could block off the retry-path. I'll take a closer look, looks like that can be the case if read-ahead is disabled. In general, we can only return -EIOCBQUEUED if the IO has been started or is in progress already. That means we can safely rely on being told when it's unlocked/done. If we need to block, we should be returning -EAGAIN, which would punt to a worker thread. -- Jens Axboe ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-12 22:08 ` Jens Axboe @ 2020-10-12 22:22 ` Jens Axboe 2020-10-12 22:42 ` Jens Axboe 2020-10-13 5:31 ` Hao_Xu 1 sibling, 1 reply; 14+ messages in thread From: Jens Axboe @ 2020-10-12 22:22 UTC (permalink / raw) To: Matthew Wilcox, io-uring; +Cc: Johannes Weiner, Hao_Xu On 10/12/20 4:08 PM, Jens Axboe wrote: > On 10/12/20 3:13 PM, Matthew Wilcox wrote: >> This one's pretty unlikely, but there's a case in buffered reads where >> an IOCB_WAITQ read can end up sleeping. >> >> generic_file_buffered_read(): >> page = find_get_page(mapping, index); >> ... >> if (!PageUptodate(page)) { >> ... >> if (iocb->ki_flags & IOCB_WAITQ) { >> ... >> error = wait_on_page_locked_async(page, >> iocb->ki_waitq); >> wait_on_page_locked_async(): >> if (!PageLocked(page)) >> return 0; >> (back to generic_file_buffered_read): >> if (!mapping->a_ops->is_partially_uptodate(page, >> offset, iter->count)) >> goto page_not_up_to_date_locked; >> >> page_not_up_to_date_locked: >> if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { >> unlock_page(page); >> put_page(page); >> goto would_block; >> } >> ... >> error = mapping->a_ops->readpage(filp, page); >> (will unlock page on I/O completion) >> if (!PageUptodate(page)) { >> error = lock_page_killable(page); >> >> So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() >> and wait for the I/O to complete. I can't quite figure out if this is >> intentional -- I think not; if I understand the semantics right, we >> should be returning -EIOCBQUEUED and punting to an I/O thread to >> kick off the I/O and wait. >> >> I think the right fix is to return -EIOCBQUEUED from >> wait_on_page_locked_async() if the page isn't locked. ie this: >> >> @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, >> struct wait_page_queue *wait) >> { >> if (!PageLocked(page)) >> - return 0; >> + return -EIOCBQUEUED; >> return __wait_on_page_locked_async(compound_head(page), wait, false); >> } >> >> But as I said, I'm not sure what the semantics are supposed to be. > > If NOWAIT isn't set, then the issue attempt is from the helper thread > already, and IOCB_WAITQ shouldn't be set either (the latter doesn't > matter for this discussion). So it's totally fine and expected to block > at that point. > > Hmm actually, I believe that: > > commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 > Author: Hao Xu <[email protected]> > Date: Tue Sep 29 20:00:45 2020 +0800 > > io_uring: fix async buffered reads when readahead is disabled > > maybe messed up that case, so we could block off the retry-path. I'll > take a closer look, looks like that can be the case if read-ahead is > disabled. > > In general, we can only return -EIOCBQUEUED if the IO has been started > or is in progress already. That means we can safely rely on being told > when it's unlocked/done. If we need to block, we should be returning > -EAGAIN, which would punt to a worker thread. Something like the below might be a better solution - just always use the read-ahead to generate the IO, for the requested range. That won't issue any IO beyond what we asked for. And ensure we don't clear NOWAIT on the io_uring side for retry. Totally untested... Just trying to get the idea across. We might need some low cap on req_count in case the range is large. Hao Xu, can you try with this? Thinking of your read-ahead disabled slowdown as well, this could very well be the reason why. diff --git a/fs/io_uring.c b/fs/io_uring.c index aae0ef2ec34d..9a2dfe132665 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -3107,7 +3107,6 @@ static bool io_rw_should_retry(struct io_kiocb *req) wait->wait.flags = 0; INIT_LIST_HEAD(&wait->wait.entry); kiocb->ki_flags |= IOCB_WAITQ; - kiocb->ki_flags &= ~IOCB_NOWAIT; kiocb->ki_waitq = wait; io_get_req_task(req); diff --git a/mm/readahead.c b/mm/readahead.c index 3c9a8dd7c56c..693af86d171d 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -568,15 +568,16 @@ void page_cache_sync_readahead(struct address_space *mapping, struct file_ra_state *ra, struct file *filp, pgoff_t index, unsigned long req_count) { - /* no read-ahead */ - if (!ra->ra_pages) - return; - if (blk_cgroup_congested()) return; - /* be dumb */ - if (filp && (filp->f_mode & FMODE_RANDOM)) { + /* + * Even if read-ahead is disabled, issue this request as read-ahead + * as we'll need it to satisfy the requested range. The forced + * read-ahead will do the right thing and limit the read to just the + * requested range. + */ + if (!ra->ra_pages || (filp && (filp->f_mode & FMODE_RANDOM))) { force_page_cache_readahead(mapping, filp, index, req_count); return; } -- Jens Axboe ^ permalink raw reply related [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-12 22:22 ` Jens Axboe @ 2020-10-12 22:42 ` Jens Axboe 2020-10-14 20:31 ` Hao_Xu 0 siblings, 1 reply; 14+ messages in thread From: Jens Axboe @ 2020-10-12 22:42 UTC (permalink / raw) To: Matthew Wilcox, io-uring; +Cc: Johannes Weiner, Hao_Xu, Andrew Morton On 10/12/20 4:22 PM, Jens Axboe wrote: > On 10/12/20 4:08 PM, Jens Axboe wrote: >> On 10/12/20 3:13 PM, Matthew Wilcox wrote: >>> This one's pretty unlikely, but there's a case in buffered reads where >>> an IOCB_WAITQ read can end up sleeping. >>> >>> generic_file_buffered_read(): >>> page = find_get_page(mapping, index); >>> ... >>> if (!PageUptodate(page)) { >>> ... >>> if (iocb->ki_flags & IOCB_WAITQ) { >>> ... >>> error = wait_on_page_locked_async(page, >>> iocb->ki_waitq); >>> wait_on_page_locked_async(): >>> if (!PageLocked(page)) >>> return 0; >>> (back to generic_file_buffered_read): >>> if (!mapping->a_ops->is_partially_uptodate(page, >>> offset, iter->count)) >>> goto page_not_up_to_date_locked; >>> >>> page_not_up_to_date_locked: >>> if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { >>> unlock_page(page); >>> put_page(page); >>> goto would_block; >>> } >>> ... >>> error = mapping->a_ops->readpage(filp, page); >>> (will unlock page on I/O completion) >>> if (!PageUptodate(page)) { >>> error = lock_page_killable(page); >>> >>> So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() >>> and wait for the I/O to complete. I can't quite figure out if this is >>> intentional -- I think not; if I understand the semantics right, we >>> should be returning -EIOCBQUEUED and punting to an I/O thread to >>> kick off the I/O and wait. >>> >>> I think the right fix is to return -EIOCBQUEUED from >>> wait_on_page_locked_async() if the page isn't locked. ie this: >>> >>> @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, >>> struct wait_page_queue *wait) >>> { >>> if (!PageLocked(page)) >>> - return 0; >>> + return -EIOCBQUEUED; >>> return __wait_on_page_locked_async(compound_head(page), wait, false); >>> } >>> >>> But as I said, I'm not sure what the semantics are supposed to be. >> >> If NOWAIT isn't set, then the issue attempt is from the helper thread >> already, and IOCB_WAITQ shouldn't be set either (the latter doesn't >> matter for this discussion). So it's totally fine and expected to block >> at that point. >> >> Hmm actually, I believe that: >> >> commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 >> Author: Hao Xu <[email protected]> >> Date: Tue Sep 29 20:00:45 2020 +0800 >> >> io_uring: fix async buffered reads when readahead is disabled >> >> maybe messed up that case, so we could block off the retry-path. I'll >> take a closer look, looks like that can be the case if read-ahead is >> disabled. >> >> In general, we can only return -EIOCBQUEUED if the IO has been started >> or is in progress already. That means we can safely rely on being told >> when it's unlocked/done. If we need to block, we should be returning >> -EAGAIN, which would punt to a worker thread. > > Something like the below might be a better solution - just always use > the read-ahead to generate the IO, for the requested range. That won't > issue any IO beyond what we asked for. And ensure we don't clear NOWAIT > on the io_uring side for retry. > > Totally untested... Just trying to get the idea across. We might need > some low cap on req_count in case the range is large. Hao Xu, can you > try with this? Thinking of your read-ahead disabled slowdown as well, > this could very well be the reason why. Here's one that caps us at 1 page, if read-ahead is disabled or we're congested. Should still be fine in terms of being async, and it allows us to use the same path for this instead of special casing it. I ran some quick testing on this, and it seems to Work For Me. I'll do some more targeted testing. diff --git a/fs/io_uring.c b/fs/io_uring.c index aae0ef2ec34d..9a2dfe132665 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -3107,7 +3107,6 @@ static bool io_rw_should_retry(struct io_kiocb *req) wait->wait.flags = 0; INIT_LIST_HEAD(&wait->wait.entry); kiocb->ki_flags |= IOCB_WAITQ; - kiocb->ki_flags &= ~IOCB_NOWAIT; kiocb->ki_waitq = wait; io_get_req_task(req); diff --git a/mm/readahead.c b/mm/readahead.c index 3c9a8dd7c56c..d0f556612fd6 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -568,15 +568,20 @@ void page_cache_sync_readahead(struct address_space *mapping, struct file_ra_state *ra, struct file *filp, pgoff_t index, unsigned long req_count) { - /* no read-ahead */ - if (!ra->ra_pages) - return; + bool do_forced_ra = filp && (filp->f_mode & FMODE_RANDOM); - if (blk_cgroup_congested()) - return; + /* + * Even if read-ahead is disabled, issue this request as read-ahead + * as we'll need it to satisfy the requested range. The forced + * read-ahead will do the right thing and limit the read to just the + * requested range, which we'll set to 1 page for this case. + */ + if (!ra->ra_pages || blk_cgroup_congested()) { + req_count = 1; + do_forced_ra = true; + } - /* be dumb */ - if (filp && (filp->f_mode & FMODE_RANDOM)) { + if (do_forced_ra) { force_page_cache_readahead(mapping, filp, index, req_count); return; } -- Jens Axboe ^ permalink raw reply related [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-12 22:42 ` Jens Axboe @ 2020-10-14 20:31 ` Hao_Xu 2020-10-14 20:57 ` Jens Axboe 0 siblings, 1 reply; 14+ messages in thread From: Hao_Xu @ 2020-10-14 20:31 UTC (permalink / raw) To: Jens Axboe, Matthew Wilcox, io-uring; +Cc: Johannes Weiner, Andrew Morton 在 2020/10/13 上午6:42, Jens Axboe 写道: > On 10/12/20 4:22 PM, Jens Axboe wrote: >> On 10/12/20 4:08 PM, Jens Axboe wrote: >>> On 10/12/20 3:13 PM, Matthew Wilcox wrote: >>>> This one's pretty unlikely, but there's a case in buffered reads where >>>> an IOCB_WAITQ read can end up sleeping. >>>> >>>> generic_file_buffered_read(): >>>> page = find_get_page(mapping, index); >>>> ... >>>> if (!PageUptodate(page)) { >>>> ... >>>> if (iocb->ki_flags & IOCB_WAITQ) { >>>> ... >>>> error = wait_on_page_locked_async(page, >>>> iocb->ki_waitq); >>>> wait_on_page_locked_async(): >>>> if (!PageLocked(page)) >>>> return 0; >>>> (back to generic_file_buffered_read): >>>> if (!mapping->a_ops->is_partially_uptodate(page, >>>> offset, iter->count)) >>>> goto page_not_up_to_date_locked; >>>> >>>> page_not_up_to_date_locked: >>>> if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { >>>> unlock_page(page); >>>> put_page(page); >>>> goto would_block; >>>> } >>>> ... >>>> error = mapping->a_ops->readpage(filp, page); >>>> (will unlock page on I/O completion) >>>> if (!PageUptodate(page)) { >>>> error = lock_page_killable(page); >>>> >>>> So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() >>>> and wait for the I/O to complete. I can't quite figure out if this is >>>> intentional -- I think not; if I understand the semantics right, we >>>> should be returning -EIOCBQUEUED and punting to an I/O thread to >>>> kick off the I/O and wait. >>>> >>>> I think the right fix is to return -EIOCBQUEUED from >>>> wait_on_page_locked_async() if the page isn't locked. ie this: >>>> >>>> @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, >>>> struct wait_page_queue *wait) >>>> { >>>> if (!PageLocked(page)) >>>> - return 0; >>>> + return -EIOCBQUEUED; >>>> return __wait_on_page_locked_async(compound_head(page), wait, false); >>>> } >>>> >>>> But as I said, I'm not sure what the semantics are supposed to be. >>> >>> If NOWAIT isn't set, then the issue attempt is from the helper thread >>> already, and IOCB_WAITQ shouldn't be set either (the latter doesn't >>> matter for this discussion). So it's totally fine and expected to block >>> at that point. >>> >>> Hmm actually, I believe that: >>> >>> commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 >>> Author: Hao Xu <[email protected]> >>> Date: Tue Sep 29 20:00:45 2020 +0800 >>> >>> io_uring: fix async buffered reads when readahead is disabled >>> >>> maybe messed up that case, so we could block off the retry-path. I'll >>> take a closer look, looks like that can be the case if read-ahead is >>> disabled. >>> >>> In general, we can only return -EIOCBQUEUED if the IO has been started >>> or is in progress already. That means we can safely rely on being told >>> when it's unlocked/done. If we need to block, we should be returning >>> -EAGAIN, which would punt to a worker thread. >> >> Something like the below might be a better solution - just always use >> the read-ahead to generate the IO, for the requested range. That won't >> issue any IO beyond what we asked for. And ensure we don't clear NOWAIT >> on the io_uring side for retry. >> >> Totally untested... Just trying to get the idea across. We might need >> some low cap on req_count in case the range is large. Hao Xu, can you >> try with this? Thinking of your read-ahead disabled slowdown as well, >> this could very well be the reason why. > > Here's one that caps us at 1 page, if read-ahead is disabled or we're > congested. Should still be fine in terms of being async, and it allows > us to use the same path for this instead of special casing it. > > I ran some quick testing on this, and it seems to Work For Me. I'll do > some more targeted testing. > > diff --git a/fs/io_uring.c b/fs/io_uring.c > index aae0ef2ec34d..9a2dfe132665 100644 > --- a/fs/io_uring.c > +++ b/fs/io_uring.c > @@ -3107,7 +3107,6 @@ static bool io_rw_should_retry(struct io_kiocb *req) > wait->wait.flags = 0; > INIT_LIST_HEAD(&wait->wait.entry); > kiocb->ki_flags |= IOCB_WAITQ; > - kiocb->ki_flags &= ~IOCB_NOWAIT; > kiocb->ki_waitq = wait; > > io_get_req_task(req); > diff --git a/mm/readahead.c b/mm/readahead.c > index 3c9a8dd7c56c..d0f556612fd6 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -568,15 +568,20 @@ void page_cache_sync_readahead(struct address_space *mapping, > struct file_ra_state *ra, struct file *filp, > pgoff_t index, unsigned long req_count) > { > - /* no read-ahead */ > - if (!ra->ra_pages) > - return; > + bool do_forced_ra = filp && (filp->f_mode & FMODE_RANDOM); > > - if (blk_cgroup_congested()) > - return; > + /* > + * Even if read-ahead is disabled, issue this request as read-ahead > + * as we'll need it to satisfy the requested range. The forced > + * read-ahead will do the right thing and limit the read to just the > + * requested range, which we'll set to 1 page for this case. > + */ > + if (!ra->ra_pages || blk_cgroup_congested()) { > + req_count = 1; > + do_forced_ra = true; > + } > > - /* be dumb */ > - if (filp && (filp->f_mode & FMODE_RANDOM)) { > + if (do_forced_ra) { > force_page_cache_readahead(mapping, filp, index, req_count); > return; > } > Hi Jens, I've done some tests for the new fix code with readahead disabled from userspace. Here comes some results. For the perf reports, since I'm new to kernel stuff, still investigating on it. I'll keep addressing the issue which causes the difference among the four perf reports(in which the copy_user_enhanced_fast_string() catches my eyes) my environment is: server: physical server kernel: mainline 5.9.0-rc8+ latest commit 6f2f486d57c4d562cdf4 fs: ext4 device: nvme ssd fio: 3.20 I did the tests by setting and commenting the code: filp->f_mode |= FMODE_BUF_RASYNC; in fs/ext4/file.c ext4_file_open() the IOPS with readahead disabled from userspace is below: with new fix code(force readahead) QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set 1 10.8k 10.3k 2 21.2k 20.1k 4 41.1k 39.1k 8 76.1k 72.2k 16 133k 126k 32 169k 147k 64 176k 160k 128 (1)187k (2)156k now async buffered reads feature looks better in terms of IOPS, but it still looks similar with the async buffered reads feature in the mainline code. with mainline code(the fix code in commit c8d317aa1887 ("io_uring: fix async buffered reads when readahead is disabled")) QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set 1 10.9k 10.2k 2 21.6k 20.2k 4 41.0k 39.9k 8 79.7k 75.9k 16 141k 138k 32 169k 237k 64 190k 316k 128 (3)195k (4)315k Considering the number in place (1)(2)(3)(4), the new fix doesn't seem to fix the slow down but make the number (4) become number (2) the perf reports of (1)(2)(3)(4) situations are: (1) 9 # Overhead Command Shared Object Symbol 10 # ........ ....... .................. .............................................. 11 # 12 10.19% fio [kernel.vmlinux] [k] copy_user_enhanced_fast_string 13 8.53% fio fio [.] clock_thread_fn 14 4.67% fio [kernel.vmlinux] [k] xas_load 15 2.18% fio [kernel.vmlinux] [k] clear_page_erms 16 2.02% fio libc-2.24.so [.] __memset_avx2_erms 17 1.55% fio [kernel.vmlinux] [k] mutex_unlock 18 1.51% fio [kernel.vmlinux] [k] shmem_getpage_gfp 19 1.48% fio [kernel.vmlinux] [k] native_irq_return_iret 20 1.48% fio [kernel.vmlinux] [k] get_page_from_freelist 21 1.46% fio [kernel.vmlinux] [k] generic_file_buffered_read 22 1.45% fio [nvme] [k] nvme_irq 23 1.25% fio [kernel.vmlinux] [k] __list_del_entry_valid 24 1.22% fio [kernel.vmlinux] [k] free_pcppages_bulk 25 1.15% fio [kernel.vmlinux] [k] _raw_spin_lock 26 1.12% fio fio [.] get_io_u 27 0.81% fio [ext4] [k] ext4_mpage_readpages 28 0.78% fio fio [.] fio_gettime 29 0.76% fio [kernel.vmlinux] [k] find_get_entries 30 0.75% fio [vdso] [.] __vdso_clock_gettime 31 0.73% fio [kernel.vmlinux] [k] release_pages 32 0.68% fio [kernel.vmlinux] [k] find_get_entry 33 0.68% fio fio [.] io_u_queued_complete 34 0.67% fio [kernel.vmlinux] [k] io_async_buf_func 35 0.65% fio [kernel.vmlinux] [k] io_submit_sqes ... (2) 9 # Overhead Command Shared Object Symbol 10 # ........ ....... .................. .............................................. 11 # 12 7.94% fio fio [.] clock_thread_fn 13 3.83% fio [kernel.vmlinux] [k] xas_load 14 2.57% fio [kernel.vmlinux] [k] io_prep_async_work 15 2.24% fio [kernel.vmlinux] [k] clear_page_erms 16 1.99% fio [kernel.vmlinux] [k] _raw_spin_lock_irqsave 17 1.94% fio libc-2.24.so [.] __memset_avx2_erms 18 1.83% fio [kernel.vmlinux] [k] get_page_from_freelist 19 1.78% fio [kernel.vmlinux] [k] __fget_files 20 1.67% fio [kernel.vmlinux] [k] __list_del_entry_valid 21 1.50% fio fio [.] get_io_u 22 1.41% fio [kernel.vmlinux] [k] shmem_getpage_gfp 23 1.40% fio [kernel.vmlinux] [k] io_prep_rw 24 1.39% fio [kernel.vmlinux] [k] mutex_unlock 25 1.28% fio [kernel.vmlinux] [k] _raw_spin_lock_irq 26 1.21% fio [kernel.vmlinux] [k] free_pcppages_bulk 27 1.17% fio [kernel.vmlinux] [k] generic_file_buffered_read 28 1.11% fio [ext4] [k] ext4_mpage_readpages 29 1.11% fio fio [.] fio_gettime 30 1.09% fio [kernel.vmlinux] [k] __pagevec_lru_add_fn 31 1.04% fio [kernel.vmlinux] [k] kmem_cache_alloc_bulk 32 0.99% fio [kernel.vmlinux] [k] io_submit_sqes 33 0.95% fio fio [.] io_u_queued_complete 34 0.90% fio [kernel.vmlinux] [k] io_wqe_wake_worker 35 0.78% fio [vdso] [.] __vdso_clock_gettime ... (3) 9 # Overhead Command Shared Object Symbol 10 # ........ ....... .................. .............................................. 11 # 12 9.06% fio fio [.] clock_thread_fn 13 6.05% fio [kernel.vmlinux] [k] copy_user_enhanced_fast_string 14 4.27% fio [kernel.vmlinux] [k] xas_load 15 2.31% fio [kernel.vmlinux] [k] clear_page_erms 16 2.09% fio libc-2.24.so [.] __memset_avx2_erms 17 1.70% fio fio [.] get_io_u 18 1.67% fio [kernel.vmlinux] [k] get_page_from_freelist 19 1.67% fio [kernel.vmlinux] [k] shmem_getpage_gfp 20 1.61% fio [kernel.vmlinux] [k] native_irq_return_iret 21 1.56% fio [kernel.vmlinux] [k] generic_file_buffered_read 22 1.34% fio [kernel.vmlinux] [k] __list_del_entry_valid 23 1.29% fio [kernel.vmlinux] [k] mutex_unlock 24 1.24% fio [kernel.vmlinux] [k] free_pcppages_bulk 25 1.11% fio fio [.] fio_gettime 26 1.01% fio [kernel.vmlinux] [k] _raw_spin_lock 27 0.90% fio [ext4] [k] ext4_mpage_readpages 28 0.89% fio [vdso] [.] __vdso_clock_gettime 29 0.82% fio [kernel.vmlinux] [k] audit_filter_syscall.constprop.20 30 0.74% fio [kernel.vmlinux] [k] xas_store 31 0.73% fio [kernel.vmlinux] [k] find_get_entries 32 0.72% fio [kernel.vmlinux] [k] find_get_entry 33 0.70% fio fio [.] io_u_queued_complete 34 0.66% fio [kernel.vmlinux] [k] release_pages 35 0.66% fio [kernel.vmlinux] [k] io_submit_sqes ... (4) 9 # Overhead Command Shared Object Symbol 10 # ........ ....... .................. .............................................. 11 # 12 12.30% fio fio [.] clock_thread_fn 13 4.69% fio [kernel.vmlinux] [k] xas_load 14 3.12% fio [kernel.vmlinux] [k] clear_page_erms 15 2.87% fio libc-2.24.so [.] __memset_avx2_erms 16 2.80% fio [kernel.vmlinux] [k] io_prep_async_work 17 2.43% fio [kernel.vmlinux] [k] io_prep_rw 18 2.32% fio [kernel.vmlinux] [k] shmem_getpage_gfp 19 2.24% fio [kernel.vmlinux] [k] __fget_files 20 2.18% fio [kernel.vmlinux] [k] get_page_from_freelist 21 2.15% fio [kernel.vmlinux] [k] __list_del_entry_valid 22 2.10% fio [kernel.vmlinux] [k] _raw_spin_lock_irqsave 23 1.81% fio [kernel.vmlinux] [k] native_queued_spin_lock_slowpath 24 1.77% fio [kernel.vmlinux] [k] lru_cache_add 25 1.69% fio [kernel.vmlinux] [k] _raw_spin_lock_irq 26 1.65% fio [kernel.vmlinux] [k] free_pcppages_bulk 27 1.36% fio [kernel.vmlinux] [k] __pagevec_lru_add_fn 28 1.27% fio fio [.] get_io_u 29 1.26% fio [kernel.vmlinux] [k] generic_file_buffered_read 30 1.21% fio [kernel.vmlinux] [k] io_submit_sqes 31 1.19% fio fio [.] account_io_completion 32 1.16% fio [vdso] [.] __vdso_clock_gettime 33 1.14% fio fio [.] fio_gettime 34 1.12% fio [kernel.vmlinux] [k] allocate_slab 35 0.97% fio [kernel.vmlinux] [k] __x64_sys_io_uring_enter ... the arguments of fio I use are: fio_test.sh: fio -filename=/mnt/nvme0n1/haul.xh/fio_read_test.txt \ -buffered=1 \ -iodepth $1 \ -rw=randread \ -ioengine=io_uring \ -direct=0 \ -bs=4k \ -size=4G \ -name=rand_read_4k \ -numjobs=1 Thanks && Regards, Hao ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-14 20:31 ` Hao_Xu @ 2020-10-14 20:57 ` Jens Axboe 2020-10-15 11:27 ` Hao_Xu 0 siblings, 1 reply; 14+ messages in thread From: Jens Axboe @ 2020-10-14 20:57 UTC (permalink / raw) To: Hao_Xu, Matthew Wilcox, io-uring; +Cc: Johannes Weiner, Andrew Morton On 10/14/20 2:31 PM, Hao_Xu wrote: > Hi Jens, > I've done some tests for the new fix code with readahead disabled from > userspace. Here comes some results. > For the perf reports, since I'm new to kernel stuff, still investigating > on it. > I'll keep addressing the issue which causes the difference among the > four perf reports(in which the copy_user_enhanced_fast_string() catches > my eyes) > > my environment is: > server: physical server > kernel: mainline 5.9.0-rc8+ latest commit 6f2f486d57c4d562cdf4 > fs: ext4 > device: nvme ssd > fio: 3.20 > > I did the tests by setting and commenting the code: > filp->f_mode |= FMODE_BUF_RASYNC; > in fs/ext4/file.c ext4_file_open() You don't have to modify the kernel, if you use a newer fio then you can essentially just add: --force_async=1 after setting the engine to io_uring to get the same effect. Just a heads up, as that might make it easier for you. > the IOPS with readahead disabled from userspace is below: > > with new fix code(force readahead) > QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set > 1 10.8k 10.3k > 2 21.2k 20.1k > 4 41.1k 39.1k > 8 76.1k 72.2k > 16 133k 126k > 32 169k 147k > 64 176k 160k > 128 (1)187k (2)156k > > now async buffered reads feature looks better in terms of IOPS, > but it still looks similar with the async buffered reads feature in the > mainline code. I'd say it looks better all around. And what you're completely forgetting here is that when FMODE_BUF_RASYNC isn't set, then you're using QD number of async workers to achieve that result. Hence you have 1..128 threads potentially running on that one, vs having a _single_ process running with FMODE_BUF_RASYNC. > with mainline code(the fix code in commit c8d317aa1887 ("io_uring: fix > async buffered reads when readahead is disabled")) > QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set > 1 10.9k 10.2k > 2 21.6k 20.2k > 4 41.0k 39.9k > 8 79.7k 75.9k > 16 141k 138k > 32 169k 237k > 64 190k 316k > 128 (3)195k (4)315k > > Considering the number in place (1)(2)(3)(4), the new fix doesn't seem > to fix the slow down > but make the number (4) become number (2) Not sure why there would be a difference between 2 and 4, that does seem odd. I'll see if I can reproduce that. More questions below. > the perf reports of (1)(2)(3)(4) situations are: > (1) > 9 # Overhead Command Shared Object Symbol > 10 # ........ ....... .................. > .............................................. > 11 # > 12 10.19% fio [kernel.vmlinux] [k] > copy_user_enhanced_fast_string > 13 8.53% fio fio [.] clock_thread_fn > 14 4.67% fio [kernel.vmlinux] [k] xas_load > 15 2.18% fio [kernel.vmlinux] [k] clear_page_erms > 16 2.02% fio libc-2.24.so [.] __memset_avx2_erms > 17 1.55% fio [kernel.vmlinux] [k] mutex_unlock > 18 1.51% fio [kernel.vmlinux] [k] shmem_getpage_gfp > 19 1.48% fio [kernel.vmlinux] [k] native_irq_return_iret > 20 1.48% fio [kernel.vmlinux] [k] get_page_from_freelist > 21 1.46% fio [kernel.vmlinux] [k] generic_file_buffered_read > 22 1.45% fio [nvme] [k] nvme_irq > 23 1.25% fio [kernel.vmlinux] [k] __list_del_entry_valid > 24 1.22% fio [kernel.vmlinux] [k] free_pcppages_bulk > 25 1.15% fio [kernel.vmlinux] [k] _raw_spin_lock > 26 1.12% fio fio [.] get_io_u > 27 0.81% fio [ext4] [k] ext4_mpage_readpages > 28 0.78% fio fio [.] fio_gettime > 29 0.76% fio [kernel.vmlinux] [k] find_get_entries > 30 0.75% fio [vdso] [.] __vdso_clock_gettime > 31 0.73% fio [kernel.vmlinux] [k] release_pages > 32 0.68% fio [kernel.vmlinux] [k] find_get_entry > 33 0.68% fio fio [.] io_u_queued_complete > 34 0.67% fio [kernel.vmlinux] [k] io_async_buf_func > 35 0.65% fio [kernel.vmlinux] [k] io_submit_sqes These profiles are of marginal use, as you're only profiling fio itself, not all of the async workers that are running for !FMODE_BUF_RASYNC. How long does the test run? It looks suspect that clock_thread_fn shows up in the profiles at all. And is it actually doing IO, or are you using shm/tmpfs for this test? Isn't ext4 hosting the file? I see a lot of shmem_getpage_gfp(), makes me a little confused. -- Jens Axboe ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-14 20:57 ` Jens Axboe @ 2020-10-15 11:27 ` Hao_Xu 2020-10-15 12:17 ` Hao_Xu 0 siblings, 1 reply; 14+ messages in thread From: Hao_Xu @ 2020-10-15 11:27 UTC (permalink / raw) To: Jens Axboe, Matthew Wilcox, io-uring; +Cc: Johannes Weiner, Andrew Morton 在 2020/10/15 上午4:57, Jens Axboe 写道: > On 10/14/20 2:31 PM, Hao_Xu wrote: >> Hi Jens, >> I've done some tests for the new fix code with readahead disabled from >> userspace. Here comes some results. >> For the perf reports, since I'm new to kernel stuff, still investigating >> on it. >> I'll keep addressing the issue which causes the difference among the >> four perf reports(in which the copy_user_enhanced_fast_string() catches >> my eyes) >> >> my environment is: >> server: physical server >> kernel: mainline 5.9.0-rc8+ latest commit 6f2f486d57c4d562cdf4 >> fs: ext4 >> device: nvme ssd >> fio: 3.20 >> >> I did the tests by setting and commenting the code: >> filp->f_mode |= FMODE_BUF_RASYNC; >> in fs/ext4/file.c ext4_file_open() > > You don't have to modify the kernel, if you use a newer fio then you can > essentially just add: > > --force_async=1 > > after setting the engine to io_uring to get the same effect. Just a > heads up, as that might make it easier for you. > >> the IOPS with readahead disabled from userspace is below: >> >> with new fix code(force readahead) >> QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set >> 1 10.8k 10.3k >> 2 21.2k 20.1k >> 4 41.1k 39.1k >> 8 76.1k 72.2k >> 16 133k 126k >> 32 169k 147k >> 64 176k 160k >> 128 (1)187k (2)156k >> >> now async buffered reads feature looks better in terms of IOPS, >> but it still looks similar with the async buffered reads feature in the >> mainline code. > > I'd say it looks better all around. And what you're completely > forgetting here is that when FMODE_BUF_RASYNC isn't set, then you're > using QD number of async workers to achieve that result. Hence you have > 1..128 threads potentially running on that one, vs having a _single_ > process running with FMODE_BUF_RASYNC. I totally agree with this, the server I use has many cpus which makes the multiple async workers works exactly parallelly. > >> with mainline code(the fix code in commit c8d317aa1887 ("io_uring: fix >> async buffered reads when readahead is disabled")) >> QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set >> 1 10.9k 10.2k >> 2 21.6k 20.2k >> 4 41.0k 39.9k >> 8 79.7k 75.9k >> 16 141k 138k >> 32 169k 237k >> 64 190k 316k >> 128 (3)195k (4)315k >> >> Considering the number in place (1)(2)(3)(4), the new fix doesn't seem >> to fix the slow down >> but make the number (4) become number (2) > > Not sure why there would be a difference between 2 and 4, that does seem > odd. I'll see if I can reproduce that. More questions below. > >> the perf reports of (1)(2)(3)(4) situations are: >> (1) >> 9 # Overhead Command Shared Object Symbol >> 10 # ........ ....... .................. >> .............................................. >> 11 # >> 12 10.19% fio [kernel.vmlinux] [k] >> copy_user_enhanced_fast_string >> 13 8.53% fio fio [.] clock_thread_fn >> 14 4.67% fio [kernel.vmlinux] [k] xas_load >> 15 2.18% fio [kernel.vmlinux] [k] clear_page_erms >> 16 2.02% fio libc-2.24.so [.] __memset_avx2_erms >> 17 1.55% fio [kernel.vmlinux] [k] mutex_unlock >> 18 1.51% fio [kernel.vmlinux] [k] shmem_getpage_gfp >> 19 1.48% fio [kernel.vmlinux] [k] native_irq_return_iret >> 20 1.48% fio [kernel.vmlinux] [k] get_page_from_freelist >> 21 1.46% fio [kernel.vmlinux] [k] generic_file_buffered_read >> 22 1.45% fio [nvme] [k] nvme_irq >> 23 1.25% fio [kernel.vmlinux] [k] __list_del_entry_valid >> 24 1.22% fio [kernel.vmlinux] [k] free_pcppages_bulk >> 25 1.15% fio [kernel.vmlinux] [k] _raw_spin_lock >> 26 1.12% fio fio [.] get_io_u >> 27 0.81% fio [ext4] [k] ext4_mpage_readpages >> 28 0.78% fio fio [.] fio_gettime >> 29 0.76% fio [kernel.vmlinux] [k] find_get_entries >> 30 0.75% fio [vdso] [.] __vdso_clock_gettime >> 31 0.73% fio [kernel.vmlinux] [k] release_pages >> 32 0.68% fio [kernel.vmlinux] [k] find_get_entry >> 33 0.68% fio fio [.] io_u_queued_complete >> 34 0.67% fio [kernel.vmlinux] [k] io_async_buf_func >> 35 0.65% fio [kernel.vmlinux] [k] io_submit_sqes > > These profiles are of marginal use, as you're only profiling fio itself, > not all of the async workers that are running for !FMODE_BUF_RASYNC. > Ah, I got it. Thanks. > How long does the test run? It looks suspect that clock_thread_fn shows > up in the profiles at all. > it runs about 5 msec, randread 4G with bs=4k > And is it actually doing IO, or are you using shm/tmpfs for this test? > Isn't ext4 hosting the file? I see a lot of shmem_getpage_gfp(), makes > me a little confused. > I'm using ext4 on real nvme ssd device. from the call stack, the shm_getpage_gfp is from __memset_avx2_erms in libc. there are ext4 related functions in all the four reports. I'm doing more to check if it is my test process causing high IOPS in case (4). ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-15 11:27 ` Hao_Xu @ 2020-10-15 12:17 ` Hao_Xu 0 siblings, 0 replies; 14+ messages in thread From: Hao_Xu @ 2020-10-15 12:17 UTC (permalink / raw) To: Jens Axboe, Matthew Wilcox, io-uring; +Cc: Johannes Weiner, Andrew Morton 在 2020/10/15 下午7:27, Hao_Xu 写道: > 在 2020/10/15 上午4:57, Jens Axboe 写道: >> On 10/14/20 2:31 PM, Hao_Xu wrote: >>> Hi Jens, >>> I've done some tests for the new fix code with readahead disabled from >>> userspace. Here comes some results. >>> For the perf reports, since I'm new to kernel stuff, still investigating >>> on it. >>> I'll keep addressing the issue which causes the difference among the >>> four perf reports(in which the copy_user_enhanced_fast_string() catches >>> my eyes) >>> >>> my environment is: >>> server: physical server >>> kernel: mainline 5.9.0-rc8+ latest commit 6f2f486d57c4d562cdf4 >>> fs: ext4 >>> device: nvme ssd >>> fio: 3.20 >>> >>> I did the tests by setting and commenting the code: >>> filp->f_mode |= FMODE_BUF_RASYNC; >>> in fs/ext4/file.c ext4_file_open() >> >> You don't have to modify the kernel, if you use a newer fio then you can >> essentially just add: >> >> --force_async=1 >> >> after setting the engine to io_uring to get the same effect. Just a >> heads up, as that might make it easier for you. >> >>> the IOPS with readahead disabled from userspace is below: >>> >>> with new fix code(force readahead) >>> QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set >>> 1 10.8k 10.3k >>> 2 21.2k 20.1k >>> 4 41.1k 39.1k >>> 8 76.1k 72.2k >>> 16 133k 126k >>> 32 169k 147k >>> 64 176k 160k >>> 128 (1)187k (2)156k >>> >>> now async buffered reads feature looks better in terms of IOPS, >>> but it still looks similar with the async buffered reads feature in the >>> mainline code. >> >> I'd say it looks better all around. And what you're completely >> forgetting here is that when FMODE_BUF_RASYNC isn't set, then you're >> using QD number of async workers to achieve that result. Hence you have >> 1..128 threads potentially running on that one, vs having a _single_ >> process running with FMODE_BUF_RASYNC. > I totally agree with this, the server I use has many cpus which makes > the multiple async workers works exactly parallelly. > >> >>> with mainline code(the fix code in commit c8d317aa1887 ("io_uring: fix >>> async buffered reads when readahead is disabled")) >>> QD/Test FMODE_BUF_RASYNC set FMODE_BUF_RASYNC not set >>> 1 10.9k 10.2k >>> 2 21.6k 20.2k >>> 4 41.0k 39.9k >>> 8 79.7k 75.9k >>> 16 141k 138k >>> 32 169k 237k >>> 64 190k 316k >>> 128 (3)195k (4)315k >>> >>> Considering the number in place (1)(2)(3)(4), the new fix doesn't seem >>> to fix the slow down >>> but make the number (4) become number (2) >> >> Not sure why there would be a difference between 2 and 4, that does seem >> odd. I'll see if I can reproduce that. More questions below. >> >>> the perf reports of (1)(2)(3)(4) situations are: >>> (1) >>> 9 # Overhead Command Shared Object Symbol >>> 10 # ........ ....... .................. >>> .............................................. >>> 11 # >>> 12 10.19% fio [kernel.vmlinux] [k] >>> copy_user_enhanced_fast_string >>> 13 8.53% fio fio [.] clock_thread_fn >>> 14 4.67% fio [kernel.vmlinux] [k] xas_load >>> 15 2.18% fio [kernel.vmlinux] [k] clear_page_erms >>> 16 2.02% fio libc-2.24.so [.] __memset_avx2_erms >>> 17 1.55% fio [kernel.vmlinux] [k] mutex_unlock >>> 18 1.51% fio [kernel.vmlinux] [k] shmem_getpage_gfp >>> 19 1.48% fio [kernel.vmlinux] [k] >>> native_irq_return_iret >>> 20 1.48% fio [kernel.vmlinux] [k] >>> get_page_from_freelist >>> 21 1.46% fio [kernel.vmlinux] [k] >>> generic_file_buffered_read >>> 22 1.45% fio [nvme] [k] nvme_irq >>> 23 1.25% fio [kernel.vmlinux] [k] >>> __list_del_entry_valid >>> 24 1.22% fio [kernel.vmlinux] [k] free_pcppages_bulk >>> 25 1.15% fio [kernel.vmlinux] [k] _raw_spin_lock >>> 26 1.12% fio fio [.] get_io_u >>> 27 0.81% fio [ext4] [k] ext4_mpage_readpages >>> 28 0.78% fio fio [.] fio_gettime >>> 29 0.76% fio [kernel.vmlinux] [k] find_get_entries >>> 30 0.75% fio [vdso] [.] __vdso_clock_gettime >>> 31 0.73% fio [kernel.vmlinux] [k] release_pages >>> 32 0.68% fio [kernel.vmlinux] [k] find_get_entry >>> 33 0.68% fio fio [.] io_u_queued_complete >>> 34 0.67% fio [kernel.vmlinux] [k] io_async_buf_func >>> 35 0.65% fio [kernel.vmlinux] [k] io_submit_sqes >> >> These profiles are of marginal use, as you're only profiling fio itself, >> not all of the async workers that are running for !FMODE_BUF_RASYNC. >> > Ah, I got it. Thanks. >> How long does the test run? It looks suspect that clock_thread_fn shows >> up in the profiles at all. >> > it runs about 5 msec, randread 4G with bs=4k Sorry, 5 seconds not 5 msec. >> And is it actually doing IO, or are you using shm/tmpfs for this test? >> Isn't ext4 hosting the file? I see a lot of shmem_getpage_gfp(), makes >> me a little confused. >> > I'm using ext4 on real nvme ssd device. from the call stack, the > shm_getpage_gfp is from __memset_avx2_erms in libc. > there are ext4 related functions in all the four reports. > I'm doing more to check if it is my test process causing high IOPS in > case (4). ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-12 22:08 ` Jens Axboe 2020-10-12 22:22 ` Jens Axboe @ 2020-10-13 5:31 ` Hao_Xu 2020-10-13 17:50 ` Jens Axboe 1 sibling, 1 reply; 14+ messages in thread From: Hao_Xu @ 2020-10-13 5:31 UTC (permalink / raw) To: Jens Axboe, Matthew Wilcox, io-uring; +Cc: Johannes Weiner 在 2020/10/13 上午6:08, Jens Axboe 写道: > On 10/12/20 3:13 PM, Matthew Wilcox wrote: >> This one's pretty unlikely, but there's a case in buffered reads where >> an IOCB_WAITQ read can end up sleeping. >> >> generic_file_buffered_read(): >> page = find_get_page(mapping, index); >> ... >> if (!PageUptodate(page)) { >> ... >> if (iocb->ki_flags & IOCB_WAITQ) { >> ... >> error = wait_on_page_locked_async(page, >> iocb->ki_waitq); >> wait_on_page_locked_async(): >> if (!PageLocked(page)) >> return 0; >> (back to generic_file_buffered_read): >> if (!mapping->a_ops->is_partially_uptodate(page, >> offset, iter->count)) >> goto page_not_up_to_date_locked; >> >> page_not_up_to_date_locked: >> if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { >> unlock_page(page); >> put_page(page); >> goto would_block; >> } >> ... >> error = mapping->a_ops->readpage(filp, page); >> (will unlock page on I/O completion) >> if (!PageUptodate(page)) { >> error = lock_page_killable(page); >> >> So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() >> and wait for the I/O to complete. I can't quite figure out if this is >> intentional -- I think not; if I understand the semantics right, we >> should be returning -EIOCBQUEUED and punting to an I/O thread to >> kick off the I/O and wait. >> >> I think the right fix is to return -EIOCBQUEUED from >> wait_on_page_locked_async() if the page isn't locked. ie this: >> >> @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, >> struct wait_page_queue *wait) >> { >> if (!PageLocked(page)) >> - return 0; >> + return -EIOCBQUEUED; >> return __wait_on_page_locked_async(compound_head(page), wait, false); >> } >> >> But as I said, I'm not sure what the semantics are supposed to be. > > If NOWAIT isn't set, then the issue attempt is from the helper thread > already, and IOCB_WAITQ shouldn't be set either (the latter doesn't > matter for this discussion). So it's totally fine and expected to block > at that point. > > Hmm actually, I believe that: > > commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 > Author: Hao Xu <[email protected]> > Date: Tue Sep 29 20:00:45 2020 +0800 > > io_uring: fix async buffered reads when readahead is disabled > > maybe messed up that case, so we could block off the retry-path. I'll > take a closer look, looks like that can be the case if read-ahead is > disabled. > > In general, we can only return -EIOCBQUEUED if the IO has been started > or is in progress already. That means we can safely rely on being told > when it's unlocked/done. If we need to block, we should be returning > -EAGAIN, which would punt to a worker thread. > Hi Jens, My undertanding of io_uring buffered reads process after the commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 has been merged is: the first io_uring IO try is with IOCB_NOWAIT, the second retry in the same context is with IOCB_WAITQ but without IOCB_NOWAIT. so in Matthew's case, lock_page_async() will be called after calling mapping->a_ops->readpage(), So it won't end up sleeping. Actually this case is what happens when readahead is disabled or somehow skipped for reasons like blk_cgroup_congested() returns true. And this case is my commit c8d317aa1887b40b188e for. Regards, Hao ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-13 5:31 ` Hao_Xu @ 2020-10-13 17:50 ` Jens Axboe 2020-10-13 19:50 ` Hao_Xu 0 siblings, 1 reply; 14+ messages in thread From: Jens Axboe @ 2020-10-13 17:50 UTC (permalink / raw) To: Hao_Xu, Matthew Wilcox, io-uring; +Cc: Johannes Weiner [-- Attachment #1: Type: text/plain, Size: 4211 bytes --] On 10/12/20 11:31 PM, Hao_Xu wrote: > 在 2020/10/13 上午6:08, Jens Axboe 写道: >> On 10/12/20 3:13 PM, Matthew Wilcox wrote: >>> This one's pretty unlikely, but there's a case in buffered reads where >>> an IOCB_WAITQ read can end up sleeping. >>> >>> generic_file_buffered_read(): >>> page = find_get_page(mapping, index); >>> ... >>> if (!PageUptodate(page)) { >>> ... >>> if (iocb->ki_flags & IOCB_WAITQ) { >>> ... >>> error = wait_on_page_locked_async(page, >>> iocb->ki_waitq); >>> wait_on_page_locked_async(): >>> if (!PageLocked(page)) >>> return 0; >>> (back to generic_file_buffered_read): >>> if (!mapping->a_ops->is_partially_uptodate(page, >>> offset, iter->count)) >>> goto page_not_up_to_date_locked; >>> >>> page_not_up_to_date_locked: >>> if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { >>> unlock_page(page); >>> put_page(page); >>> goto would_block; >>> } >>> ... >>> error = mapping->a_ops->readpage(filp, page); >>> (will unlock page on I/O completion) >>> if (!PageUptodate(page)) { >>> error = lock_page_killable(page); >>> >>> So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() >>> and wait for the I/O to complete. I can't quite figure out if this is >>> intentional -- I think not; if I understand the semantics right, we >>> should be returning -EIOCBQUEUED and punting to an I/O thread to >>> kick off the I/O and wait. >>> >>> I think the right fix is to return -EIOCBQUEUED from >>> wait_on_page_locked_async() if the page isn't locked. ie this: >>> >>> @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, >>> struct wait_page_queue *wait) >>> { >>> if (!PageLocked(page)) >>> - return 0; >>> + return -EIOCBQUEUED; >>> return __wait_on_page_locked_async(compound_head(page), wait, false); >>> } >>> >>> But as I said, I'm not sure what the semantics are supposed to be. >> >> If NOWAIT isn't set, then the issue attempt is from the helper thread >> already, and IOCB_WAITQ shouldn't be set either (the latter doesn't >> matter for this discussion). So it's totally fine and expected to block >> at that point. >> >> Hmm actually, I believe that: >> >> commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 >> Author: Hao Xu <[email protected]> >> Date: Tue Sep 29 20:00:45 2020 +0800 >> >> io_uring: fix async buffered reads when readahead is disabled >> >> maybe messed up that case, so we could block off the retry-path. I'll >> take a closer look, looks like that can be the case if read-ahead is >> disabled. >> >> In general, we can only return -EIOCBQUEUED if the IO has been started >> or is in progress already. That means we can safely rely on being told >> when it's unlocked/done. If we need to block, we should be returning >> -EAGAIN, which would punt to a worker thread. >> > Hi Jens, > My undertanding of io_uring buffered reads process after the commit > c8d317aa1887b40b188ec3aaa6e9e524333caed1 has been merged is: > the first io_uring IO try is with IOCB_NOWAIT, the second retry in the > same context is with IOCB_WAITQ but without IOCB_NOWAIT. > so in Matthew's case, lock_page_async() will be called after calling > mapping->a_ops->readpage(), So it won't end up sleeping. > Actually this case is what happens when readahead is disabled or somehow > skipped for reasons like blk_cgroup_congested() returns true. And this > case is my commit c8d317aa1887b40b188e for. Well, try the patches. I agree it's not going to sleep with the previous fix, but we're definitely driving a lower utilization by not utilizing read-ahead even if disabled. Re-run your previous tests with these two applied and see what you get. -- Jens Axboe [-- Attachment #2: 0002-io_uring-don-t-clear-IOCB_NOWAIT-for-async-buffered-.patch --] [-- Type: text/x-patch, Size: 1173 bytes --] From 19185e0ea3a91a1d8b9c7e013a32f96bf006052a Mon Sep 17 00:00:00 2001 From: Jens Axboe <[email protected]> Date: Mon, 12 Oct 2020 16:48:57 -0600 Subject: [PATCH 2/2] io_uring: don't clear IOCB_NOWAIT for async buffered retry If we do, and read-ahead is disabled, we can be blocking on the page to finish before making progress. This defeats the purpose of async IO. Now that we know that read-ahead will most likely trigger the IO, we can make progress even for ra_pages == 0 without punting to io-wq to satisfy the IO in a blocking fashion. Fixes: c8d317aa1887 ("io_uring: fix async buffered reads when readahead is disabled") Reported-by: Matthew Wilcox (Oracle) <[email protected]> Signed-off-by: Jens Axboe <[email protected]> --- fs/io_uring.c | 1 - 1 file changed, 1 deletion(-) diff --git a/fs/io_uring.c b/fs/io_uring.c index c043d889a2eb..be70f3e38fb2 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -3248,7 +3248,6 @@ static bool io_rw_should_retry(struct io_kiocb *req) wait->wait.flags = 0; INIT_LIST_HEAD(&wait->wait.entry); kiocb->ki_flags |= IOCB_WAITQ; - kiocb->ki_flags &= ~IOCB_NOWAIT; kiocb->ki_waitq = wait; return true; } -- 2.28.0 [-- Attachment #3: 0001-readahead-use-limited-read-ahead-to-satisfy-read.patch --] [-- Type: text/x-patch, Size: 2221 bytes --] From 10b8c31e8085a85d5a71c7e271387c2edbcf7b96 Mon Sep 17 00:00:00 2001 From: Jens Axboe <[email protected]> Date: Mon, 12 Oct 2020 16:44:23 -0600 Subject: [PATCH 1/2] readahead: use limited read-ahead to satisfy read Willy reports that there's a case where async buffered reads will be blocking, and that's due to not using read-ahead to generate the reads when read-ahead is disabled. io_uring relies on read-ahead triggering the reads, if not, it needs to fallback to threaded helpers. For the case where read-ahead is disabled on the file, or if the cgroup is congested, ensure that we can at least do 1 page of read-ahead to make progress on the read in an async fashion. This could potentially be larger, but it's not needed in terms of functionality, so let's error on the side of caution as larger counts of pages may run into reclaim issues (particularly if we're congested). Reported-by: Matthew Wilcox (Oracle) <[email protected]> Acked-by: Johannes Weiner <[email protected]> Signed-off-by: Jens Axboe <[email protected]> --- mm/readahead.c | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/mm/readahead.c b/mm/readahead.c index 3c9a8dd7c56c..e5975f4e0ee5 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -568,15 +568,21 @@ void page_cache_sync_readahead(struct address_space *mapping, struct file_ra_state *ra, struct file *filp, pgoff_t index, unsigned long req_count) { - /* no read-ahead */ - if (!ra->ra_pages) - return; + bool do_forced_ra = filp && (filp->f_mode & FMODE_RANDOM); - if (blk_cgroup_congested()) - return; + /* + * Even if read-ahead is disabled, start this request as read-ahead. + * This makes regular read-ahead disabled use the same path as normal + * reads, instead of having to punt to ->readpage() manually. We limit + * ourselves to 1 page for this case, to avoid causing problems if + * we're congested or tight on memory. + */ + if (!ra->ra_pages || blk_cgroup_congested()) { + req_count = 1; + do_forced_ra = true; + } - /* be dumb */ - if (filp && (filp->f_mode & FMODE_RANDOM)) { + if (do_forced_ra) { force_page_cache_readahead(mapping, filp, index, req_count); return; } -- 2.28.0 ^ permalink raw reply related [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-13 17:50 ` Jens Axboe @ 2020-10-13 19:50 ` Hao_Xu 0 siblings, 0 replies; 14+ messages in thread From: Hao_Xu @ 2020-10-13 19:50 UTC (permalink / raw) To: Jens Axboe, Matthew Wilcox, io-uring; +Cc: Johannes Weiner 在 2020/10/14 上午1:50, Jens Axboe 写道: > On 10/12/20 11:31 PM, Hao_Xu wrote: >> 在 2020/10/13 上午6:08, Jens Axboe 写道: >>> On 10/12/20 3:13 PM, Matthew Wilcox wrote: >>>> This one's pretty unlikely, but there's a case in buffered reads where >>>> an IOCB_WAITQ read can end up sleeping. >>>> >>>> generic_file_buffered_read(): >>>> page = find_get_page(mapping, index); >>>> ... >>>> if (!PageUptodate(page)) { >>>> ... >>>> if (iocb->ki_flags & IOCB_WAITQ) { >>>> ... >>>> error = wait_on_page_locked_async(page, >>>> iocb->ki_waitq); >>>> wait_on_page_locked_async(): >>>> if (!PageLocked(page)) >>>> return 0; >>>> (back to generic_file_buffered_read): >>>> if (!mapping->a_ops->is_partially_uptodate(page, >>>> offset, iter->count)) >>>> goto page_not_up_to_date_locked; >>>> >>>> page_not_up_to_date_locked: >>>> if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { >>>> unlock_page(page); >>>> put_page(page); >>>> goto would_block; >>>> } >>>> ... >>>> error = mapping->a_ops->readpage(filp, page); >>>> (will unlock page on I/O completion) >>>> if (!PageUptodate(page)) { >>>> error = lock_page_killable(page); >>>> >>>> So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() >>>> and wait for the I/O to complete. I can't quite figure out if this is >>>> intentional -- I think not; if I understand the semantics right, we >>>> should be returning -EIOCBQUEUED and punting to an I/O thread to >>>> kick off the I/O and wait. >>>> >>>> I think the right fix is to return -EIOCBQUEUED from >>>> wait_on_page_locked_async() if the page isn't locked. ie this: >>>> >>>> @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, >>>> struct wait_page_queue *wait) >>>> { >>>> if (!PageLocked(page)) >>>> - return 0; >>>> + return -EIOCBQUEUED; >>>> return __wait_on_page_locked_async(compound_head(page), wait, false); >>>> } >>>> >>>> But as I said, I'm not sure what the semantics are supposed to be. >>> >>> If NOWAIT isn't set, then the issue attempt is from the helper thread >>> already, and IOCB_WAITQ shouldn't be set either (the latter doesn't >>> matter for this discussion). So it's totally fine and expected to block >>> at that point. >>> >>> Hmm actually, I believe that: >>> >>> commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 >>> Author: Hao Xu <[email protected]> >>> Date: Tue Sep 29 20:00:45 2020 +0800 >>> >>> io_uring: fix async buffered reads when readahead is disabled >>> >>> maybe messed up that case, so we could block off the retry-path. I'll >>> take a closer look, looks like that can be the case if read-ahead is >>> disabled. >>> >>> In general, we can only return -EIOCBQUEUED if the IO has been started >>> or is in progress already. That means we can safely rely on being told >>> when it's unlocked/done. If we need to block, we should be returning >>> -EAGAIN, which would punt to a worker thread. >>> >> Hi Jens, >> My undertanding of io_uring buffered reads process after the commit >> c8d317aa1887b40b188ec3aaa6e9e524333caed1 has been merged is: >> the first io_uring IO try is with IOCB_NOWAIT, the second retry in the >> same context is with IOCB_WAITQ but without IOCB_NOWAIT. >> so in Matthew's case, lock_page_async() will be called after calling >> mapping->a_ops->readpage(), So it won't end up sleeping. >> Actually this case is what happens when readahead is disabled or somehow >> skipped for reasons like blk_cgroup_congested() returns true. And this >> case is my commit c8d317aa1887b40b188e for. > > Well, try the patches. I agree it's not going to sleep with the previous > fix, but we're definitely driving a lower utilization by not utilizing > read-ahead even if disabled. > > Re-run your previous tests with these two applied and see what you get. > Sure I agree, looks good to me. I'll try the tests with the new code. Thanks ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-12 21:13 Loophole in async page I/O Matthew Wilcox 2020-10-12 22:08 ` Jens Axboe @ 2020-10-13 5:13 ` Hao_Xu 2020-10-13 12:01 ` Matthew Wilcox 1 sibling, 1 reply; 14+ messages in thread From: Hao_Xu @ 2020-10-13 5:13 UTC (permalink / raw) To: Matthew Wilcox, io-uring; +Cc: Johannes Weiner, Jens Axboe 在 2020/10/13 上午5:13, Matthew Wilcox 写道: > This one's pretty unlikely, but there's a case in buffered reads where > an IOCB_WAITQ read can end up sleeping. > > generic_file_buffered_read(): > page = find_get_page(mapping, index); > ... > if (!PageUptodate(page)) { > ... > if (iocb->ki_flags & IOCB_WAITQ) { > ... > error = wait_on_page_locked_async(page, > iocb->ki_waitq); > wait_on_page_locked_async(): > if (!PageLocked(page)) > return 0; > (back to generic_file_buffered_read): > if (!mapping->a_ops->is_partially_uptodate(page, > offset, iter->count)) > goto page_not_up_to_date_locked; > > page_not_up_to_date_locked: > if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { > unlock_page(page); > put_page(page); > goto would_block; > } > ... > error = mapping->a_ops->readpage(filp, page); > (will unlock page on I/O completion) > if (!PageUptodate(page)) { > error = lock_page_killable(page); > > So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() > and wait for the I/O to complete. I can't quite figure out if this is > intentional -- I think not; if I understand the semantics right, we > should be returning -EIOCBQUEUED and punting to an I/O thread to > kick off the I/O and wait. > > I think the right fix is to return -EIOCBQUEUED from > wait_on_page_locked_async() if the page isn't locked. ie this: > > @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, > struct wait_page_queue *wait) > { > if (!PageLocked(page)) > - return 0; > + return -EIOCBQUEUED; > return __wait_on_page_locked_async(compound_head(page), wait, false); > } > > But as I said, I'm not sure what the semantics are supposed to be. > Hi Matthew, which kernel version are you use, I believe I've fixed this case in the commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 in this commit, I did the modification: diff --git a/mm/filemap.c b/mm/filemap.c index 1aaea26556cc..ea383478fc22 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2267,7 +2267,11 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, } if (!PageUptodate(page)) { - error = lock_page_killable(page); + if (iocb->ki_flags & IOCB_WAITQ) + error = lock_page_async(page, iocb->ki_waitq); + else + error = lock_page_killable(page); + if (unlikely(error)) goto readpage_error; if (!PageUptodate(page)) { lock_page_killable() won't be called in this case. ^ permalink raw reply related [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-13 5:13 ` Hao_Xu @ 2020-10-13 12:01 ` Matthew Wilcox 2020-10-13 19:57 ` Hao_Xu 0 siblings, 1 reply; 14+ messages in thread From: Matthew Wilcox @ 2020-10-13 12:01 UTC (permalink / raw) To: Hao_Xu; +Cc: io-uring, Johannes Weiner, Jens Axboe On Tue, Oct 13, 2020 at 01:13:48PM +0800, Hao_Xu wrote: > 在 2020/10/13 上午5:13, Matthew Wilcox 写道: > > This one's pretty unlikely, but there's a case in buffered reads where > > an IOCB_WAITQ read can end up sleeping. > > > > generic_file_buffered_read(): > > page = find_get_page(mapping, index); > > ... > > if (!PageUptodate(page)) { > > ... > > if (iocb->ki_flags & IOCB_WAITQ) { > > ... > > error = wait_on_page_locked_async(page, > > iocb->ki_waitq); > > wait_on_page_locked_async(): > > if (!PageLocked(page)) > > return 0; > > (back to generic_file_buffered_read): > > if (!mapping->a_ops->is_partially_uptodate(page, > > offset, iter->count)) > > goto page_not_up_to_date_locked; > > > > page_not_up_to_date_locked: > > if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { > > unlock_page(page); > > put_page(page); > > goto would_block; > > } > > ... > > error = mapping->a_ops->readpage(filp, page); > > (will unlock page on I/O completion) > > if (!PageUptodate(page)) { > > error = lock_page_killable(page); > > > > So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() > > and wait for the I/O to complete. I can't quite figure out if this is > > intentional -- I think not; if I understand the semantics right, we > > should be returning -EIOCBQUEUED and punting to an I/O thread to > > kick off the I/O and wait. > > > > I think the right fix is to return -EIOCBQUEUED from > > wait_on_page_locked_async() if the page isn't locked. ie this: > > > > @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, > > struct wait_page_queue *wait) > > { > > if (!PageLocked(page)) > > - return 0; > > + return -EIOCBQUEUED; > > return __wait_on_page_locked_async(compound_head(page), wait, false); > > } > > But as I said, I'm not sure what the semantics are supposed to be. > > > Hi Matthew, > which kernel version are you use, I believe I've fixed this case in the > commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 Ah, I don't have that commit in my tree. Nevertheless, there is still a problem. The ->readpage implementation is not required to execute asynchronously. For example, it may enter page reclaim by using GFP_KERNEL. Indeed, I feel it is better if it works synchronously as it can then report the actual error from an I/O instead of the almost-meaningless -EIO. This patch series documents 12 filesystems which implement ->readpage in a synchronous way today (for at least some cases) and converts iomap to be synchronous (making two more filesystems synchronous). https://lore.kernel.org/linux-fsdevel/[email protected]/ ^ permalink raw reply [flat|nested] 14+ messages in thread
* Re: Loophole in async page I/O 2020-10-13 12:01 ` Matthew Wilcox @ 2020-10-13 19:57 ` Hao_Xu 0 siblings, 0 replies; 14+ messages in thread From: Hao_Xu @ 2020-10-13 19:57 UTC (permalink / raw) To: Matthew Wilcox; +Cc: io-uring, Johannes Weiner, Jens Axboe 在 2020/10/13 下午8:01, Matthew Wilcox 写道: > On Tue, Oct 13, 2020 at 01:13:48PM +0800, Hao_Xu wrote: >> 在 2020/10/13 上午5:13, Matthew Wilcox 写道: >>> This one's pretty unlikely, but there's a case in buffered reads where >>> an IOCB_WAITQ read can end up sleeping. >>> >>> generic_file_buffered_read(): >>> page = find_get_page(mapping, index); >>> ... >>> if (!PageUptodate(page)) { >>> ... >>> if (iocb->ki_flags & IOCB_WAITQ) { >>> ... >>> error = wait_on_page_locked_async(page, >>> iocb->ki_waitq); >>> wait_on_page_locked_async(): >>> if (!PageLocked(page)) >>> return 0; >>> (back to generic_file_buffered_read): >>> if (!mapping->a_ops->is_partially_uptodate(page, >>> offset, iter->count)) >>> goto page_not_up_to_date_locked; >>> >>> page_not_up_to_date_locked: >>> if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { >>> unlock_page(page); >>> put_page(page); >>> goto would_block; >>> } >>> ... >>> error = mapping->a_ops->readpage(filp, page); >>> (will unlock page on I/O completion) >>> if (!PageUptodate(page)) { >>> error = lock_page_killable(page); >>> >>> So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() >>> and wait for the I/O to complete. I can't quite figure out if this is >>> intentional -- I think not; if I understand the semantics right, we >>> should be returning -EIOCBQUEUED and punting to an I/O thread to >>> kick off the I/O and wait. >>> >>> I think the right fix is to return -EIOCBQUEUED from >>> wait_on_page_locked_async() if the page isn't locked. ie this: >>> >>> @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, >>> struct wait_page_queue *wait) >>> { >>> if (!PageLocked(page)) >>> - return 0; >>> + return -EIOCBQUEUED; >>> return __wait_on_page_locked_async(compound_head(page), wait, false); >>> } >>> But as I said, I'm not sure what the semantics are supposed to be. >>> >> Hi Matthew, >> which kernel version are you use, I believe I've fixed this case in the >> commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 > > Ah, I don't have that commit in my tree. > > Nevertheless, there is still a problem. The ->readpage implementation > is not required to execute asynchronously. For example, it may enter > page reclaim by using GFP_KERNEL. Indeed, I feel it is better if it > works synchronously as it can then report the actual error from an I/O > instead of the almost-meaningless -EIO. > > This patch series documents 12 filesystems which implement ->readpage > in a synchronous way today (for at least some cases) and converts iomap > to be synchronous (making two more filesystems synchronous). > > https://lore.kernel.org/linux-fsdevel/[email protected]/ > Thanks, Matthew. I didn't have this knowledge before, thank you for your share and information. It's really kind of you. I'll look into it soon. ^ permalink raw reply [flat|nested] 14+ messages in thread
end of thread, other threads:[~2020-10-15 12:17 UTC | newest] Thread overview: 14+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2020-10-12 21:13 Loophole in async page I/O Matthew Wilcox 2020-10-12 22:08 ` Jens Axboe 2020-10-12 22:22 ` Jens Axboe 2020-10-12 22:42 ` Jens Axboe 2020-10-14 20:31 ` Hao_Xu 2020-10-14 20:57 ` Jens Axboe 2020-10-15 11:27 ` Hao_Xu 2020-10-15 12:17 ` Hao_Xu 2020-10-13 5:31 ` Hao_Xu 2020-10-13 17:50 ` Jens Axboe 2020-10-13 19:50 ` Hao_Xu 2020-10-13 5:13 ` Hao_Xu 2020-10-13 12:01 ` Matthew Wilcox 2020-10-13 19:57 ` Hao_Xu
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox