Re: [PATCH v4 6/8] block: don't fall back to buffered I/O for atomic writes

[John Garry <john.garry@linux.dev>]

On 9/22/26 03:54, Tal Zussman wrote:
> blkdev_direct_write() turns an -EBUSY from page cache invalidation into
> a 0 return, so an IOCB_ATOMIC write is retried in full through
> blkdev_buffered_write(), with no atomicity guarantee.
> 
> Skip the buffered fallback in blkdev_write_iter() for IOCB_ATOMIC, as
> it already does for IOCB_NOWAIT, so the -EBUSY case returns -EAGAIN and
> the caller retries, matching __iomap_dio_rw().
> 
> ext4 has the same fallback and only warns in it. For block devices the
> fallback can be skipped before any I/O is submitted, so fail early
> instead.
> 
> Fixes: caf336f81b3a ("block: Add fops atomic write support")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> 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@columbia.edu>


Note that I still think that you can add an IOCB_ATOMIC check in 
iomap_file_buffered_write() (until it does handle IOCB_ATOMIC).

Reviewed-by: John Garry <john.garry@linux.dev>

> ---
>   block/fops.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/block/fops.c b/block/fops.c
> index 90777e8a9a6c..7f23778f6b81 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -766,10 +766,11 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
>   	if (iocb->ki_flags & IOCB_DIRECT) {
>   		ret = blkdev_direct_write(iocb, from);
>   		if (ret >= 0 && iov_iter_count(from)) {
> -			if (iocb->ki_flags & IOCB_NOWAIT) {
> +			if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_ATOMIC)) {
>   				/*
>   				 * The buffered fallback blocks on i_rwsem and
> -				 * on writeback of the data it copied: return
> +				 * on writeback of the data it copied, and
> +				 * can't provide torn-write protection: return
>   				 * the short direct write instead and let the
>   				 * caller retry.
>   				 */
>