The dm core splits REQ_PREFLUSH bios that have data into two bios. First, a REQ_PREFLUSH bio with no data is submitted to all underlying dm devices. Next, the REQ_PREFLUSH flag is cleared and the same bio is resubmitted. This approach is essential if there are multiple underlying devices to provide correct REQ_PREFLUSH semantics.
Splitting a bio into an empty flush bio and a non-flush data bio is not necessary if there is only a single underlying device. Hence this patch that does 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 one or more write bios have been queued past the REQ_PREFLUSH bio before the REQ_PREFLUSH bio is processed. Cc: Mike Snitzer <[email protected]> Cc: Damien Le Moal <[email protected]> Signed-off-by: Bart Van Assche <[email protected]> --- Changes compared to v1: - Made the patch description more detailed. - Removed the reference to write pipelining from the patch description. 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;
