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

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On 7/31/23 16:13, Christian Brauner wrote:
On Mon, Jul 31, 2023 at 11:33:05AM +1000, Dave Chinner wrote:
On Thu, Jul 27, 2023 at 04:27:30PM +0200, Christian Brauner wrote:
On Thu, Jul 27, 2023 at 07:51:19PM +0800, Hao Xu wrote:
I actually saw this semaphore, and there is another xfs lock in
   --> 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().

Wait - didn't we already fix this for mtime updates on IOCB_NOWAIT
modification operations? Yeah, we did:

   file_modified_flags(iocb->ki_file, iocb->ki_flags)
     ret = inode_needs_update_time()
     if (ret <= 0)
	return ret;
     if (flags & IOCB_NOWAIT)
	return -EAGAIN;
     <does timestamp update>

-> touch_atime()
    -> inode_update_time()
       -> i_op->update_time == xfs_vn_update_time()

Yeah, so this needs the same treatment as file_modified_flags() -
touch_atime() needs a flag variant that passes IOCB_NOWAIT, and
after atime_needs_update() returns trues we should check IOCB_NOWAIT
and return EAGAIN if it is set. That will punt the operation that
needs to the update to a worker thread that can block....

As I tried to explain, I would prefer if we could inform the filesystem
through i_op->update_time() itself that this is async and give the
filesystem the ability to try and acquire the locks it needs and return
EAGAIN from i_op->update_time() itself if it can't acquire them.

I browse code in i_op->update_time = xfs_vn_update_time, it's mainly
about xfs journal code. It creates a transaction and adds a item to
it, not familiar with this part, from a quick look I found some
locks and sleep point in it to modify. I think I need some time to sort
out this part. Or maybe we can do it like what Dave said as a short term
solution and change the block points in journal code later as a separate
patchset, those journal code I believe are common code for xfs IO
operations. I'm ok with both though.

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:

	if (nowait)

	if (!IS_DEADDIR(inode)) {
		ret = file_accessed(file);
		if (ret == -EAGAIN)
			goto out_unlock;


Yup, that's the sort of thing that needs to be done.

But as I said in the "llseek for io-uring" thread, we need to stop
the game of whack-a-mole passing random nowait boolean flags to VFS
operations before it starts in earnest.  We really need a common
context structure (like we have a kiocb for IO operations) that
holds per operation control state so we have consistency across all
the operations that we need different behaviours for.

Yes, I tend to agree and thought about the same. But right now we don't
have a lot of context. So I would lean towards a flag argument at most.

But I also wouldn't consider it necessarily wrong to start with booleans
or a flag first and in a couple of months if the need for more context
arises we know what kind of struct we want or need.

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