Re: Loophole in async page I/O

[Hao_Xu <haoxu@linux.alibaba.com>]

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 <haoxu@linux.alibaba.com>
>>> 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