[PATCH v3 2/8] md: ensure REQ_NOMERGE is set on P2PDMA bios

[Mykola Marzhan]

md_submit_bio() strips REQ_NOMERGE from every bio.  For P2PDMA bios
the flag is what keeps requests single-provider (see
__bio_add_page()): merging a P2PDMA bio with one over a different
pgmap, or over host memory, maps the merged request with the wrong
bus address.  Set the flag on P2PDMA bios -- bios built via
bio_iov_bvec_set() arrive without it -- and keep stripping it
otherwise.

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 <mykola@xxxxxxxxxxx>
Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
---
 drivers/md/md.c | 10 ++++++++--
 drivers/md/md.h | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index d1465bcd86c8..3770bdb4d4b2 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -451,8 +451,14 @@ static void md_submit_bio(struct bio *bio)
 		return;
 	}
 
-	/* bio could be mergeable after passing to underlayer */
-	bio->bi_opf &= ~REQ_NOMERGE;
+	/*
+	 * A bio md split may merge again below md -- except P2PDMA bios,
+	 * which must stay single-provider (see __bio_add_page()).
+	 */
+	if (md_bio_is_p2pdma(bio))
+		bio->bi_opf |= REQ_NOMERGE;
+	else
+		bio->bi_opf &= ~REQ_NOMERGE;
 
 	md_handle_request(mddev, bio);
 }
diff --git a/drivers/md/md.h b/drivers/md/md.h
index d8daf0f75cbb..73df1a6eccd8 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -11,8 +11,10 @@
 #include <linux/blkdev.h>
 #include <linux/backing-dev.h>
 #include <linux/badblocks.h>
+#include <linux/bio.h>
 #include <linux/kobject.h>
 #include <linux/list.h>
+#include <linux/memremap.h>
 #include <linux/mm.h>
 #include <linux/mutex.h>
 #include <linux/timer.h>
@@ -22,6 +24,19 @@
 #include <trace/events/block.h>
 
 #define MaxSector (~(sector_t)0)
+
+/*
+ * P2P and host pages never mix within a bio, so the first bvec is
+ * representative.  Read bi_io_vec directly: bio_first_bvec_all()
+ * WARNs on the split clones md handles, and data-less bios have no
+ * bi_io_vec.  Not valid after the bio's iterator is consumed.
+ */
+static inline bool md_bio_is_p2pdma(struct bio *bio)
+{
+	return bio_has_data(bio) && bio->bi_io_vec &&
+	       is_pci_p2pdma_page(bio->bi_io_vec->bv_page);
+}
+
 /*
  * Number of guaranteed raid bios in case of extreme VM load:
  */
-- 
2.52.0