Re: [PATCH 3/5] io_uring: add support for getdents

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/27/23 15:27, Christian Brauner wrote:
> On Thu, Jul 27, 2023 at 07:51:19PM +0800, Hao Xu wrote:
>> On 7/26/23 23:00, Christian Brauner wrote:
>>> On Tue, Jul 18, 2023 at 09:21:10PM +0800, Hao Xu wrote:
>>>> From: Hao Xu <howeyxu@tencent.com>
>>>>
>>>> This add support for getdents64 to io_uring, acting exactly like the
>>>> syscall: the directory is iterated from it's current's position as
>>>> stored in the file struct, and the file's position is updated exactly as
>>>> if getdents64 had been called.
>>>>
>>>> For filesystems that support NOWAIT in iterate_shared(), try to use it
>>>> first; if a user already knows the filesystem they use do not support
>>>> nowait they can force async through IOSQE_ASYNC in the sqe flags,
>>>> avoiding the need to bounce back through a useless EAGAIN return.
>>>>
>>>> Co-developed-by: Dominique Martinet <asmadeus@codewreck.org>
>>>> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
>>>> Signed-off-by: Hao Xu <howeyxu@tencent.com>
>>>> ---
[...]
>> I actually saw this semaphore, and there is another xfs lock in
>> file_accessed
>>    --> touch_atime
>>      --> inode_update_time
>>        --> inode->i_op->update_time == xfs_vn_update_time
>>
>> Forgot to point them out in the cover-letter..., I didn't modify them
>> since I'm not very sure about if we should do so, and I saw Stefan's
>> patchset didn't modify them too.
>>
>> My personnal thinking is we should apply trylock logic for this
>> inode->i_rwsem. For xfs lock in touch_atime, we should do that since it
>> doesn't make sense to rollback all the stuff while we are almost at the
>> end of getdents because of a lock.
> 
> That manoeuvres around the problem. Which I'm slightly more sensitive
> too as this review is a rather expensive one.
> 
> Plus, it seems fixable in at least two ways:
> 
> For both we need to be able to tell the filesystem that a nowait atime
> update is requested. Simple thing seems to me to add a S_NOWAIT flag to
> file_time_flags and passing that via i_op->update_time() which already
> has a flag argument. That would likely also help kiocb_modified().

fwiw, we've just recently had similar problems with io_uring read/write
and atime/mtime in prod environment, so we're interested in solving that
regardless of this patchset. I.e. io_uring issues rw with NOWAIT, {a,m}time
touch ignores that, that stalls other submissions and completely screws
latency.

> file_accessed()
> -> touch_atime()
>     -> inode_update_time()
>        -> i_op->update_time == xfs_vn_update_time()
> 
> Then we have two options afaict:
> 
> (1) best-effort atime update
> 
> file_accessed() already has the builtin assumption that updating atime
> might fail for other reasons - see the comment in there. So it is
> somewhat best-effort already.
> 
> (2) move atime update before calling into filesystem
> 
> If we want to be sure that access time is updated when a readdir request
> is issued through io_uring then we need to have file_accessed() give a
> return value and expose a new helper for io_uring or modify
> vfs_getdents() to do something like:
> 
> vfs_getdents()
> {
> 	if (nowait)
> 		down_read_trylock()
> 
> 	if (!IS_DEADDIR(inode)) {
> 		ret = file_accessed(file);
> 		if (ret == -EAGAIN)
> 			goto out_unlock;
> 
> 		f_op->iterate_shared()
> 	}
> }
> 
> It's not unprecedented to do update atime before the actual operation
> has been done afaict. That's already the case in xfs_file_write_checks()
> which is called before anything is written. So that seems ok.
> 
> Does any of these two options work for the xfs maintainers and Jens?

It doesn't look (2) will solve it for reads/writes, at least without
the pain of changing the {write,read}_iter callbacks. 1) sounds good
to me from the io_uring perspective, but I guess it won't work
for mtime?

-- 
Pavel Begunkov