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

[Hao Xu <hao.xu@linux.dev>]

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
>>>> 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().
>>
>> Wait - didn't we already fix this for mtime updates on IOCB_NOWAIT
>> modification operations? Yeah, we did:
>>
>> kiocb_modified(iocb)
>>    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>
>>
>>> file_accessed()
>>> -> 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:
>>>
>>> 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()
>>> 	}
>>> }
>>
>> 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.