Re: [PATCH RFC v2 2/3] ext4: never fall back to buffered I/O for atomic direct writes

[sashiko-bot@kernel.org]

> ext4_dio_write_iter() finishes a short direct write through the page
> cache. ext4_buffered_write_iter() now rejects IOCB_ATOMIC, so a short
> atomic write would warn and then fail with -EOPNOTSUPP, as if atomic
> writes were unsupported on the inode.
> 
> This came out of a block device fix [1], where an IOCB_ATOMIC direct
> write could complete short when only part of the buffer could be
> pinned, with the rest going through the buffered fallback. Christoph
> Hellwig noted that ext4 has the same fallback and only warns in it [2],
> and John Garry agreed it should reject the write instead [3].
> 
> In contrast to the block device, nothing reaches the branch on ext4
> today: iomap requires the mapping to cover the whole atomic write, a
> buffer that can't be pinned in full fails in iomap before the bio is
> submitted, and a failed page cache invalidation returns -EAGAIN rather
> than reaching the buffered fallback. A short atomic write is therefore
> a bug, and neither a short count nor -EOPNOTSUPP is a valid result for
> one. Fail the write with -EIO before the fallback, keeping the WARN()
> so any future bug is visible.
> 
> Link: https://lore.kernel.org/linux-block/20260828-blkdev-fixes-v2-0-32f3f40cebed@columbia.edu/ [1]
> Link: https://lore.kernel.org/linux-block/20260907071217.GD934@lst.de/ [2]
> Link: https://lore.kernel.org/linux-block/a7752d1e-8ec1-44d1-a266-c287c315e8ab@oracle.com/ [3]
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261012-ext4-atomic-no-fallback-v2-0-c4b30c60e500@columbia.edu?part=2