linux-kernel - Re: [PATCH 04/21] fs: Add RWF_ATOMIC and IOCB

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Message-ID: <ZSR4jeSKlppLWjQy@dread.disaster.area>
Date:   Tue, 10 Oct 2023 09:02:53 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Jeremy Bongio <jbongio@...gle.com>
Cc:     John Garry <john.g.garry@...cle.com>, axboe@...nel.dk,
        kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu,
        linux-api@...r.kernel.org,
        Prasad Singamsetty <prasad.singamsetty@...cle.com>
Subject: Re: [PATCH 04/21] fs: Add RWF_ATOMIC and IOCB_ATOMIC flags for
 atomic write support

On Fri, Oct 06, 2023 at 11:15:11AM -0700, Jeremy Bongio wrote:
> What is the advantage of using write flags instead of using an atomic
> open flag (O_ATOMIC)? With an open flag, write, writev, pwritev would
> all be supported for atomic writes. And this would potentially require
> less application changes to take advantage of atomic writes.

Atomic writes are not a property of the file or even the inode
itself, they are an attribute of the specific IO being issued by
the application.

Most applications that want atomic writes are using it as a
performance optimisation. They are likely already using DIO with
either AIO, pwritev2 or io_uring and so are already using the
interfaces that support per-IO attributes. Not every IO to every
file needs to be atomic, so a per-IO attribute makes a lot of sense
for these applications.

Add to that that implementing atomic IO semantics in the generic IO
paths (e.g. for buffered writes) is much more difficult. It's
not an unsolvable problem (especially now with high-order folio
support in the page cache), it's just way outside the scope of this
patchset.

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