Re: [PATCH v2 5/7] block: fail atomic writes instead of falling back to buffered I/O

[Hannes Reinecke]

On 8/28/26 3:49 PM, Tal Zussman wrote:
> An IOCB_ATOMIC direct write to a block device can silently lose its
> torn-write guarantee in two ways:
>
>    1. blkdev_direct_write() turns an -EBUSY from page cache invalidation
>       into a 0 return, so the whole write is retried through
>       blkdev_buffered_write(), with no atomicity guarantee.
>
>    2. On a partial page pin, __blkdev_direct_IO_simple() and
>       __blkdev_direct_IO_async() submit what was pinned with REQ_ATOMIC
>       set and leave the rest to the buffered fallback.
>
> The second case can be triggered deterministically. A 16K
> pwritev2(RWF_ATOMIC) whose last page is PROT_NONE, on a scsi_debug
> device with atomic_wr=1, completes short with only three of the four
> pages written, violating RWF_ATOMIC semantics.
>
> Fail the I/O instead. Return -EAGAIN when page cache invalidation fails
> for IOCB_ATOMIC rather than retrying through the page cache, matching
> __iomap_dio_rw(), which treats the failure as transient and lets the
> caller retry. Release a short atomic pin and return -EFAULT before
> submission, which is what a direct write already returns when none of
> the buffer can be pinned. A sync atomic write can then never return
> short with a remainder, so the buffered fallback is never reached.
>
> ext4 has the same fallback and only warns in it. For block devices both
> ways in can be detected before any I/O is submitted, so fail early instead.
>
> Fixes: caf336f81b3a ("block: Add fops atomic write support")
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Link: https://sashiko.dev/#/patchset/20260802-blkdev-fixes-v1-0-a82fc549fd74%40columbia.edu?part=2
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> ---
>   block/fops.c | 21 ++++++++++++++++++++-
>   1 file changed, 20 insertions(+), 1 deletion(-)
Reviewed-by: Hannes Reinecke <hare@xxxxxxxxxx>

Cheers,

Hannes
--
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@xxxxxxx                                +49 911 74053 688
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