[PATCH v3 6/8] md/raid1: skip futile retries on P2PDMA mapping failures

[Mykola Marzhan]

A member that cannot DMA-map peer memory fails the leg bio with
BLK_STS_TARGET.  The failure is a property of the peer/member
pairing -- retrying the same pages against the same member cannot
succeed, and there is nothing on the medium to repair -- but the
error machinery treats it as a medium error: narrow_write_error()
grinds through hundreds of doomed chunk retries per write,
WantReplacement burns a spare and evicts the healthy member,
fix_read_error()'s host-page probe "succeeds" so mixed host/P2P
reads charge the read-error budget until a healthy member is
kicked (~1s in testing), and FailFast evicts on the first
unroutable I/O.

Flag P2PDMA master bios with a new R1BIO_P2PDMA state bit at
submission.  On a BLK_STS_TARGET leg failure for a flagged bio:
retry writes once as a single whole range, recording one bad range
if that also fails (TARGET can be a transient device condition,
e.g. NVME_SC_NS_NOT_READY, so the retry is not skipped outright);
don't set WantReplacement; mark failed read legs IO_BLOCKED without
charging the read-error budget; and don't treat the failure as
FailFast evidence -- the request never reached the wire -- as
commit f7b24c7b41f2 ("md/raid1,raid10: don't fail devices for
invalid IO errors") did for BLK_STS_INVAL.

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>
---
 drivers/md/raid1.c | 54 ++++++++++++++++++++++++++++++++++------------
 drivers/md/raid1.h |  2 ++
 2 files changed, 42 insertions(+), 14 deletions(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 16b7465c233a..6334032c4bb8 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -483,16 +483,22 @@ static void raid1_end_write_request(struct bio *bio)
 	 * 'one mirror IO has finished' event handler:
 	 */
 	if (bio->bi_status && !ignore_error) {
-		set_bit(WriteErrorSeen,	&rdev->flags);
-		if (!test_and_set_bit(WantReplacement, &rdev->flags))
-			set_bit(MD_RECOVERY_NEEDED, &
-				conf->mddev->recovery);
+		/* Peer/member pairing failure, not member health. */
+		bool p2pdma_unmappable = bio->bi_status == BLK_STS_TARGET &&
+			test_bit(R1BIO_P2PDMA, &r1_bio->state);
 
-		if (test_bit(FailFast, &rdev->flags) &&
-		    (bio->bi_opf & MD_FAILFAST) &&
-		    /* We never try FailFast to WriteMostly devices */
-		    !test_bit(WriteMostly, &rdev->flags)) {
-			md_error(r1_bio->mddev, rdev);
+		set_bit(WriteErrorSeen,	&rdev->flags);
+		if (!p2pdma_unmappable) {
+			if (!test_and_set_bit(WantReplacement, &rdev->flags))
+				set_bit(MD_RECOVERY_NEEDED,
+					&conf->mddev->recovery);
+
+			if (test_bit(FailFast, &rdev->flags) &&
+			    (bio->bi_opf & MD_FAILFAST) &&
+			    /* We never try FailFast to WriteMostly devices */
+			    !test_bit(WriteMostly, &rdev->flags)) {
+				md_error(r1_bio->mddev, rdev);
+			}
 		}
 
 		/*
@@ -1378,6 +1384,8 @@ static void raid1_read_request(struct mddev *mddev, struct bio *bio,
 	else
 		init_r1bio(r1_bio, mddev, bio);
 	r1_bio->sectors = max_read_sectors;
+	if (md_bio_is_p2pdma(bio))
+		set_bit(R1BIO_P2PDMA, &r1_bio->state);
 
 	/*
 	 * make_request() can abort the operation when read-ahead is being
@@ -1557,6 +1565,8 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio,
 
 	r1_bio = alloc_r1bio(mddev, bio);
 	r1_bio->sectors = max_sectors;
+	if (md_bio_is_p2pdma(bio))
+		set_bit(R1BIO_P2PDMA, &r1_bio->state);
 
 	/* first select target devices under rcu_lock and
 	 * inc refcount on their rdev.  Record them by setting
@@ -2522,7 +2532,7 @@ static void fix_read_error(struct r1conf *conf, struct r1bio *r1_bio)
 	}
 }
 
-static void narrow_write_error(struct r1bio *r1_bio, int i)
+static void narrow_write_error(struct r1bio *r1_bio, int i, bool coarse)
 {
 	struct mddev *mddev = r1_bio->mddev;
 	struct r1conf *conf = mddev->private;
@@ -2536,6 +2546,9 @@ static void narrow_write_error(struct r1bio *r1_bio, int i)
 	 * It is conceivable that the bio doesn't exactly align with
 	 * blocks.  We must handle this somehow.
 	 *
+	 * With 'coarse', retry the whole range as one bio: P2PDMA
+	 * mapping failures fail every block identically.
+	 *
 	 * We currently own a reference on the rdev.
 	 */
 
@@ -2550,9 +2563,12 @@ static void narrow_write_error(struct r1bio *r1_bio, int i)
 		block_sectors = roundup(1 << rdev->badblocks.shift, lbs);
 
 	sector = r1_bio->sector;
-	sectors = ((sector + block_sectors)
-		   & ~(sector_t)(block_sectors - 1))
-		- sector;
+	if (coarse)
+		sectors = sect_to_write;
+	else
+		sectors = ((sector + block_sectors)
+			   & ~(sector_t)(block_sectors - 1))
+			- sector;
 
 	while (sect_to_write) {
 		struct bio *wbio;
@@ -2633,8 +2649,12 @@ static void handle_write_finished(struct r1conf *conf, struct r1bio *r1_bio)
 			 * narrow down and record precise write
 			 * errors.
 			 */
+			bool coarse = r1_bio->bios[m]->bi_status ==
+					BLK_STS_TARGET &&
+				test_bit(R1BIO_P2PDMA, &r1_bio->state);
+
 			fail = true;
-			narrow_write_error(r1_bio, m);
+			narrow_write_error(r1_bio, m, coarse);
 			rdev_dec_pending(conf->mirrors[m].rdev,
 					 conf->mddev);
 		}
@@ -2661,6 +2681,9 @@ static void handle_read_error(struct r1conf *conf, struct r1bio *r1_bio)
 {
 	struct md_rdev *rdev = conf->mirrors[r1_bio->read_disk].rdev;
 	struct bio *bio = r1_bio->bios[r1_bio->read_disk];
+	/* evaluate before the bio_put() below */
+	bool p2pdma_error = bio->bi_status == BLK_STS_TARGET &&
+		test_bit(R1BIO_P2PDMA, &r1_bio->state);
 	struct mddev *mddev = conf->mddev;
 	sector_t sector;
 
@@ -2680,6 +2703,9 @@ static void handle_read_error(struct r1conf *conf, struct r1bio *r1_bio)
 	 */
 	if (mddev->ro) {
 		r1_bio->bios[r1_bio->read_disk] = IO_BLOCKED;
+	} else if (p2pdma_error) {
+		/* Peer can't reach this member: just redirect the read. */
+		r1_bio->bios[r1_bio->read_disk] = IO_BLOCKED;
 	} else if (test_bit(FailFast, &rdev->flags)) {
 		md_error(mddev, rdev);
 	} else {
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index c98d43a7ae99..61b788a99d14 100644
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -184,6 +184,8 @@ enum r1bio_state {
 	R1BIO_MadeGood,
 	R1BIO_WriteError,
 	R1BIO_FailFast,
+/* the master bio carries PCI P2PDMA (peer device memory) pages */
+	R1BIO_P2PDMA,
 };
 
 static inline int sector_to_idx(sector_t sector)
-- 
2.52.0