On 9/10/25 3:26 AM, Bart Van Assche wrote:
> Splitting a bio into an empty flush bio and a non-flush data bio is
> only necessary if there are multiple underlying devices. Do not split
> REQ_PREFLUSH bios if there is only one underlying device. This patch
> preserves the order of REQ_PREFLUSH writes if there is only one
> underlying device and if multiple writes are queued. This patch fixes
> a write error observed with zoned storage devices and with write
> pipelining enabled.
Write pipelining is not upstream. So is this fixing an actual problem with
upstream kernels ?
If that is the case and you have a reproducer, please share it (and maybe make
it a blktest case).
>
> Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
> Cc: Damien Le Moal <dlemoal@xxxxxxxxxx>
> Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
> ---
> drivers/md/dm.c | 16 ++++++++++------
> 1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 66dd5f6ce778..d0791eef21f7 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -490,9 +490,13 @@ u64 dm_start_time_ns_from_clone(struct bio *bio)
> }
> EXPORT_SYMBOL_GPL(dm_start_time_ns_from_clone);
>
> -static inline bool bio_is_flush_with_data(struct bio *bio)
> +static inline bool bio_is_flush_with_data(struct mapped_device *md,
> + struct bio *bio)
> {
> - return ((bio->bi_opf & REQ_PREFLUSH) && bio->bi_iter.bi_size);
> + guard(rcu)();
> +
> + return bio->bi_opf & REQ_PREFLUSH && bio->bi_iter.bi_size &&
> + ((struct dm_table *)rcu_dereference(md->map))->num_targets > 1;
> }
>
> static inline unsigned int dm_io_sectors(struct dm_io *io, struct bio *bio)
> @@ -501,7 +505,7 @@ static inline unsigned int dm_io_sectors(struct dm_io *io, struct bio *bio)
> * If REQ_PREFLUSH set, don't account payload, it will be
> * submitted (and accounted) after this flush completes.
> */
> - if (bio_is_flush_with_data(bio))
> + if (bio_is_flush_with_data(io->md, bio))
> return 0;
> if (unlikely(dm_io_flagged(io, DM_IO_WAS_SPLIT)))
> return io->sectors;
> @@ -976,7 +980,7 @@ static void __dm_io_complete(struct dm_io *io, bool first_stage)
> if (requeued)
> return;
>
> - if (bio_is_flush_with_data(bio)) {
> + if (bio_is_flush_with_data(md, bio)) {
> /*
> * Preflush done for flush with data, reissue
> * without REQ_PREFLUSH.
> @@ -1715,7 +1719,7 @@ static void dm_queue_poll_io(struct bio *bio, struct dm_io *io)
> }
>
> /*
> - * Select the correct strategy for processing a non-flush bio.
> + * Select the correct strategy for processing a bio.
> */
> static blk_status_t __split_and_process_bio(struct clone_info *ci)
> {
> @@ -1996,7 +2000,7 @@ static void dm_split_and_process_bio(struct mapped_device *md,
> }
> init_clone_info(&ci, io, map, bio, is_abnormal);
>
> - if (bio->bi_opf & REQ_PREFLUSH) {
> + if (bio->bi_opf & REQ_PREFLUSH && ci.map->num_targets > 1) {
> __send_empty_flush(&ci);
> /* dm_io_complete submits any data associated with flush */
> goto out;
--
Damien Le Moal
Western Digital Research
