[PATCH v6 04/12] md/raid1: don't use write-behind for P2PDMA bios
From: Mykola Marzhan <hidden>
Date: 2026-07-26 11:32:45
Also in:
linux-block, linux-nvme, linux-pci, linux-rdma, lkml, stable
Subsystem:
software raid (multiple disks) support, the rest · Maintainers:
Song Liu, Yu Kuai, Linus Torvalds
alloc_behind_master_bio() copies the bio's data with bio_copy_data(),
a CPU copy. P2PDMA pages are peer device (BAR) memory; generic code
must not assume CPU load/store access to them is safe or fast on
every architecture, and bouncing peer memory through the CPU defeats
the point of a peer-to-peer transfer.
Skip write-behind for P2PDMA bios: they are written directly to all
members, including write-mostly ones. Ordering against write-behind
I/O in flight to overlapping sectors is preserved: the non-behind
clone path serializes on CollisionCheck rdevs (see the preceding
fix), which covers these bios like any other write that bypasses
write-behind.
Fixes: 02666132403a ("md: propagate BLK_FEAT_PCI_P2PDMA from member devices to RAID device")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Mykola Marzhan <redacted>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
---
drivers/md/raid1.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 997f79c05db7..7d769b6460ca 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c@@ -1523,6 +1523,7 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio, bool write_behind = false; bool nowait = bio->bi_opf & REQ_NOWAIT; bool is_discard = op_is_discard(bio->bi_opf); + bool is_p2pdma = md_bio_is_p2pdma(bio); sector_t sector = bio->bi_iter.bi_sector; if (mddev_is_clustered(mddev) &&
@@ -1575,9 +1576,12 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio, /* * The write-behind io is only attempted on drives marked as * write-mostly, which means we could allocate write behind - * bio later. + * bio later. P2PDMA bios are excluded: write-behind copies + * the data with bio_copy_data(), a CPU copy that cannot be + * assumed safe or fast on P2PDMA (device BAR) pages. */ - if (!is_discard && rdev && test_bit(WriteMostly, &rdev->flags)) + if (!is_discard && !is_p2pdma && rdev && + test_bit(WriteMostly, &rdev->flags)) write_behind = true; r1_bio->bios[i] = NULL;
--
2.52.0