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

[Christian Brauner <brauner@kernel.org>]

On Thu, Jul 27, 2023 at 04:12:12PM +0100, Pavel Begunkov wrote:
> 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

It would also solve it for writes which is what my kiocb_modified()
comment was about. So right now you have:

kiocb_modified(IOCB_NOWAI)
-> file_modified_flags(IOCB_NOWAI)
   -> file_remove_privs(IOCB_NOWAIT) // already fully non-blocking
   -> file_accessed(IOCB_NOWAIT)
      -> i_op->update_time(S_ATIME | S_NOWAIT)

and since xfs_file_write_iter() calls xfs_file_write_checks() before
doing any actual work you'd now be fine.

For reads xfs_file_read_iter() would need to be changed to a similar
logic but that's for xfs to decide ultimately.

> 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?

I would prefer 2) which seems cleaner to me. But I might miss why this
won't work. So input needed/wanted.