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From: Dave Chinner <[email protected]>
To: "Darrick J. Wong" <[email protected]>
Cc: Christian Brauner <[email protected]>, Hao Xu <[email protected]>,
	Jens Axboe <[email protected]>,
	[email protected],
	Dominique Martinet <[email protected]>,
	Pavel Begunkov <[email protected]>,
	Alexander Viro <[email protected]>,
	Stefan Roesch <[email protected]>, Clay Harris <[email protected]>,
	[email protected], Wanpeng Li <[email protected]>
Subject: Re: [PATCH 3/5] io_uring: add support for getdents
Date: Tue, 1 Aug 2023 08:18:27 +1000	[thread overview]
Message-ID: <ZMgzM9YF1WyJBOd/@dread.disaster.area> (raw)
In-Reply-To: <20230731152623.GC11336@frogsfrogsfrogs>

On Mon, Jul 31, 2023 at 08:26:23AM -0700, Darrick J. Wong wrote:
> On Mon, Jul 31, 2023 at 10:13:21AM +0200, 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:
> > > 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.
> 
> I'm probably missing a ton of context (because at the end of the day I
> don't care all that much about NOWAIT and still have never installed
> liburing) but AFAICT the goal seems to be that for a given io request,
> uring tries to execute it with trylocks in the originating process
> context.  If that attempt fails, it'll punt the io to a workqueue and
> rerun the request with blocking locks.  Right?

Yes, that might be the case for the VFS level code we are talking
about right now....

... but, for example, I have no clue what task context
nvmet_file_execute_rw() runs in but it definitely issues file IO
with IOCB_NOWAIT...

> I've watched quite a bit of NOWAIT whackamole going on over the past few
> years (i_rwsem, the ILOCK, the IO layer, memory allocations...).  IIRC
> these filesystem ios all have to run in process context, right?  If so,
> why don't we capture the NOWAIT state in a PF flag?  We already do that
> for NOFS/NOIO memory allocations to make sure that /all/ reclaim
> attempts cannot recurse into the fs/io stacks.

Interesting idea.

That would mean the high level code would have to toggle the task
flags before calling into the VFS, which means we'd have to capture
RWF_NOWAIT flags at the syscall level rather than propagating them
into the iocb. That may impact speed racers, because the RWF flag
propagation has been a target of significant micro-optimisation
since io_uring came along....

> "I prefer EAGAIN errors to this process blocking" doesn't seem all that
> much different.  But, what do I know...

Yeah, I can see how that might be advantageous from an API
persepective, though my gut says "that's a can of worms" but I
haven't spent enough time thinking about it to work out why I feel
that way.

-Dave.
-- 
Dave Chinner
[email protected]

  reply	other threads:[~2023-07-31 22:18 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-18 13:21 [PATCH v4 0/5] io_uring getdents Hao Xu
2023-07-18 13:21 ` [PATCH 1/5] fs: split off vfs_getdents function of getdents64 syscall Hao Xu
2023-07-18 13:21 ` [PATCH 2/5] vfs_getdents/struct dir_context: add flags field Hao Xu
2023-07-18 13:21 ` [PATCH 3/5] io_uring: add support for getdents Hao Xu
2023-07-19  8:56   ` Hao Xu
2023-07-26 15:00   ` Christian Brauner
2023-07-27 11:51     ` Hao Xu
2023-07-27 14:27       ` Christian Brauner
2023-07-27 15:12         ` Pavel Begunkov
2023-07-27 15:52           ` Christian Brauner
2023-07-27 16:17             ` Pavel Begunkov
2023-07-27 16:28               ` Christian Brauner
2023-07-31  1:58                 ` Dave Chinner
2023-07-31  7:34                   ` Hao Xu
2023-07-31  7:50                     ` Christian Brauner
2023-07-31  7:40                   ` Christian Brauner
2023-07-30 18:02         ` Hao Xu
2023-07-31  8:18           ` Christian Brauner
2023-07-31  9:31             ` Hao Xu
2023-07-31  1:33         ` Dave Chinner
2023-07-31  8:13           ` Christian Brauner
2023-07-31 15:26             ` Darrick J. Wong
2023-07-31 22:18               ` Dave Chinner [this message]
2023-08-01  0:28               ` Jens Axboe
2023-08-01  0:47                 ` Matthew Wilcox
2023-08-01  0:49                   ` Jens Axboe
2023-08-01  1:01                     ` Matthew Wilcox
2023-08-01  7:00                       ` Christian Brauner
2023-08-01  6:59                     ` Christian Brauner
2023-08-01  7:17                 ` Christian Brauner
2023-08-08  4:34                 ` Hao Xu
2023-08-08  5:18                   ` Hao Xu
2023-08-08  9:33                 ` Hao Xu
2023-08-08 22:55                   ` Jens Axboe
2023-08-01 18:39             ` Hao Xu
2023-07-18 13:21 ` [PATCH 4/5] xfs: add NOWAIT semantics for readdir Hao Xu
2023-07-19  2:35   ` kernel test robot
2023-07-18 13:21 ` [PATCH RFC 5/5] disable fixed file for io_uring getdents for now Hao Xu
2023-07-26 14:23   ` Christian Brauner
2023-07-27 12:09     ` Hao Xu
2023-07-19  6:04 ` [PATCH v4 0/5] io_uring getdents Christian Brauner

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