linux-kernel - Re: [PATCH RFC 2/2] io_uring: add support for getdents

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Message-ID: <20230423224045.GS447837@dread.disaster.area>
Date:   Mon, 24 Apr 2023 08:40:45 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        Stefan Roesch <shr@...com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [PATCH RFC 2/2] io_uring: add support for getdents

On Sat, Apr 22, 2023 at 05:40:19PM +0900, Dominique Martinet wrote:
> 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.
> 
> Additionally, since io_uring has no way of issuing a seek, a flag
> IORING_GETDENTS_REWIND has been added that will seek to the start of the
> directory like rewinddir(3) for users that might require such a thing.
> This will act exactly as if seek then getdents64 have been called
> sequentially with no atomicity guarantee:
> if this wasn't clear it is the responsibility of the caller to not use
> getdents multiple time on a single file in parallel if they want useful
> results, as is currently the case when using the syscall from multiple
> threads.
> 
> Signed-off-by: Dominique Martinet <asmadeus@...ewreck.org>
> ---
>  include/uapi/linux/io_uring.h |  7 ++++++
>  io_uring/fs.c                 | 51 +++++++++++++++++++++++++++++++++++++++++++
>  io_uring/fs.h                 |  3 +++
>  io_uring/opdef.c              |  8 +++++++
>  4 files changed, 69 insertions(+)

This doesn't actually introduce non-blocking getdents operations, so
what's the point? If it just shuffles the getdents call off to a
background thread, why bother with io_uring in the first place?

Filesystems like XFS can easily do non-blocking getdents calls - we
just need the NOWAIT plumbing (like we added to the IO path with
IOCB_NOWAIT) to tell the filesystem not to block on locks or IO.
Indeed, filesystems often have async readahead built into their
getdents paths (XFS does), so it seems to me that we really want
non-blocking getdents to allow filesystems to take full advantage of
doing work without blocking and then shuffling the remainder off to
a background thread when it actually needs to wait for IO....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com