[PATCH v2 0/7] block,md,nvme: correct handling of unsupported P2PDMA transfers

[PATCH v2 0/7] block,md,nvme: correct handling of unsupported P2PDMA transfers

[Mykola Marzhan]

Driving peer-to-peer I/O (NVMe CMB source) through md arrays and
nvme-rdma legs on asymmetric PCIe topologies -- where the peer device
reaches some members/paths but not others -- turns up two failures on
v7.2-rc, both rooted in how an unsupported-P2PDMA mapping failure is
reported. On md: silent data loss -- an unreachable-leg P2PDMA write
counts as written, mirrors silently diverge, and where no member is
reachable the write reports success with zero copies on stable
storage; reads picked to an unreachable leg fail with EIO and are
never retried on the mirror that holds the data. On nvme-rdma: the
transport swallows the DMA layer's mapping errno into a path error,
which under the default multipath configuration livelocks the I/O (a
stacked md mirror hangs) and with nvme_core.multipath=N surfaces as
retryable BLK_STS_TRANSPORT.

The root cause is a block-layer status-code regression, not an md or
nvme bug. Commit 91fb2b6052f7 ("nvme-pci: convert to using
dma_map_sgtable()") deliberately mapped unsupported P2PDMA transfers
to BLK_STS_TARGET ("... return BLK_STS_TARGET so the request isn't
retried"). Commit 858299dc6160 ("block: add scatterlist-less DMA
mapping helpers") silently changed that to BLK_STS_INVAL, and since
commit 7ce3c1dd78fc ("nvme-pci: convert the data mapping to
blk_rq_dma_map") in v6.17 the failure surfaces to direct NVMe
consumers as EINVAL instead of the documented -EREMOTEIO, with
blk_path_error() now classifying it as retryable. md/raid1,raid10
ignore BLK_STS_INVAL leg failures per commit f7b24c7b41f2
("md/raid1,raid10: don't fail devices for invalid IO errors"), where
it means a request-shaped error that fails identically on every
member -- hence the silent divergence. The md leg of the regression
became reachable when md started advertising BLK_FEAT_PCI_P2PDMA in
v7.2-rc1 (commit 02666132403a ("md: propagate BLK_FEAT_PCI_P2PDMA
from member devices to RAID device")).

Patch 1 restores BLK_STS_TARGET at the source. md then handles an
unreachable leg like any other per-device error: badblocks on the
affected member, the master bio succeeds while an In_sync leg holds
the data, and reads are redirected to the other mirror.

Patches 2, 4 and 5 fix independent md-side P2PDMA bugs of the same
vintage: md_submit_bio() strips REQ_NOMERGE (the only request-level
protection against merging P2PDMA segments across pgmaps); raid1
write-behind CPU-copies device BAR memory via bio_copy_data(); and
narrow_write_error()'s retry clones reset bi_opf, dropping the
REQ_NOMERGE protection exactly on the error-retry path.  Patch 3 is
a standalone raid1 fix for a pre-existing ordering hole the
write-behind bypass in patch 4 would otherwise widen: writes that
skip write-behind (queue saturation, a waiting reader, allocation
failure, discards -- and now P2PDMA bios) never serialized against
in-flight write-behind I/O to overlapping sectors.

Patch 6 stops the restored device-error machinery from misfiring
where its medium-error assumptions don't hold. A mapping failure is a
property of the peer/member pairing: retrying the same peer pages
against the same member cannot succeed, and there is nothing on the
medium to repair. Without it, narrow_write_error() serializes hundreds
of guaranteed-to-fail chunk retries per failed write; on reads each
unreachable-leg pick costs a full freeze_array() quiesce plus a tick
of the read-error budget -- measured below, a mixed host/P2P read
workload on an asymmetric topology kicks the perfectly healthy far
leg after ~20 failed picks, in under a second; and on FailFast
members a single unroutable I/O evicts a healthy mirror outright.
Because BLK_STS_TARGET is also produced for transient device
conditions md cannot distinguish from bi_status alone, writes probe
the whole range once instead of predicting futility: a real mapping
failure is recorded in one call, a cleared transient recovers with
nothing recorded. Patches 1 and 6 belong in the same release: with
patch 1 alone, restoring the error surfaces the retry storms and
read-path evictions above on 7.2's newly reachable md path.

Patch 7 closes the same hole in the rdma transport. nvme-rdma maps
the data scatterlist with ib_dma_map_sg(), which returns 0 on a
peer-unreachable failure, discarding the -EREMOTEIO that
dma_map_sgtable() documents for exactly this case; the driver reports
it as a path error. Because the multipath head node advertises
BLK_FEAT_PCI_P2PDMA (commit fb0eeeed91f3 ("nvme-multipath: enable PCI
P2PDMA for multipath devices"), v7.2-rc1) and the mapping failure is
deterministic, nvme_failover_req() requeues the bios with a fresh
retry budget every cycle and the I/O never completes -- a hot requeue
livelock that hangs a stacked md mirror instead of failing over; with
nvme_core.multipath=N it burns nvme_max_retries requeues and
completes as retryable BLK_STS_TRANSPORT. Patch 7 maps with
ib_dma_map_sgtable_attrs() to preserve the errno and returns
BLK_STS_TARGET, matching nvme-pci so patch 6's handling covers rdma
legs too, and starts the request only after the map succeeds so
nvme_mpath_start_request() accounting cannot leak on the direct
blk-mq completion. nvme-rdma stays on the scatterlist DMA API: its
fast-reg MR path (ib_map_mr_sg()) consumes scatterlists, so a
blk_rq_dma_map conversion is separate modernization, out of scope for
a fix.

Behavior on v7.2-rc2, QEMU rig (one CMB provider, two NVMe members,
q35 PCIe topologies), 90 8KiB peer-memory reads under concurrent
host reads for the read rows:

  topology     op          v7.2-rc2                patched (md set)
  symmetric    write+read  ok                      ok
  asymmetric   write       "ok", silent diverge    ok + badblocks on
                                                   far leg (1 probe)
  asymmetric   read        41/90 EIO, no mirror    90/90 ok, no
               under load  retry, no eviction      eviction
  unreachable  write       "ok", NO data copied    EIO + badblocks
  unreachable  read        EIO                     EIO, no members
                                                   kicked

Also verified on the same rig: a transient injected TARGET recovers
through the single probe with nothing recorded; P2P MEDIUM keeps the
chunked path; host-page TARGET/INVAL handling is unchanged; FailFast
survives mapping failures but still evicts on genuine errors;
write-mostly legs are written directly (no write-behind CPU copy); a
degraded array whose only leg is unreachable gets plain EIO -- no
budget charge, no eviction; an exhausted badblocks table fails the
member as described below. The write/read/injection rows repeat
identically on raid10.

Validation of patch 7: the mechanism -- ib_dma_map_sg() returning 0
where ib_dma_map_sgtable_attrs() preserves -EREMOTEIO -- was
confirmed on a real mlx5 HCA with a map-only probe. The
start-after-map reorder ran over an rxe nvmet-rdma loopback under fio
with crc32c verification; multipath inflight and nr_active accounting
drain to zero, matching the pre-reorder kernel. rxe cannot produce
-EREMOTEIO, so the failure paths were driven by injecting that exact
mechanism at the map call site: unpatched, one 4KiB write livelocked
(132 failover requeues in 8s, a stacked raid1 hung; multipath=N:
retryable BLK_STS_TRANSPORT); patched, it failed immediately as
BLK_STS_TARGET with zero requeues, and the stacked raid1 badblocked
the leg without evicting it.

Known trade-offs of routing through the stock md write machinery:
an unroutable P2P write records badblocks and sets WriteErrorSeen
(which gates the write-path badblocks consult), so later writes
overlapping those ranges skip the member -- host writes do not clear
the entries and repair skips known-bad ranges -- leaving them
single-copy until a re-add rebuild (host memory, so it succeeds)
rewrites and clears them.  WantReplacement is deliberately not set
for mapping failures: replacing a member cannot fix a topology
property, and would burn a spare per unreachable member.  A member
whose badblocks table is exhausted or disabled is failed on the
first unroutable P2P write, as with any write error md cannot
record. FailFast members are exempted from md_error() for
mapping failures only (nothing reached the wire, so the error says
nothing about device health); genuine I/O errors keep their
immediate-eviction semantics. raid10 additionally fails a
replacement device outright on a P2P write mapping failure (its
stock replacement-write policy), and an IO_BLOCKED slot excludes
that slot's replacement from the read retry -- both pre-existing
raid10 behaviors with a new trigger. On reads, read_balance() keeps
no memory of unreachability, so each far-leg pick still costs one
failed submission before the redirect.

Routing: patch 1 is block-tree material and fixes a v6.17-rc1 status
regression (hence its Cc: stable); patches 2-6 are md (patch 3 fixes
a pre-existing raid1 ordering bug and carries its own Cc: stable;
the rest address exposure that only became reachable with v7.2-rc1);
patch 7 is nvme-rdma and also carries Cc: stable -- the transport
misreport predates the head-node change (Fixes: 23528aa3320a, v7.1)
and is reachable on v7.1 with multipath=N. Patch 7 sources the errno
from dma_map_sgtable() in rdma.c and does not depend on patch 1,
which fixes the separate blk_rq_dma_map path. Patches 1 (block) and
7 (nvme) are independently applicable bug fixes; a maintainer can
take them even if the md discussion runs longer -- if the md side
slips a release, patch 1 turns the silent divergence into loud
per-leg errors, which we consider the right interim behavior.
Single-cycle routing through the relevant trees with acks works for
us: patch 1 block, 2-6 md, 7 nvme.

Per Documentation/process/coding-assistants.rst: the patches were
developed with AI assistance (see the Assisted-by trailers); all code
was human-reviewed and tested as described above.

Series is against v7.2-rc2.

Changes in v2 (after Sashiko AI review of v1 and further internal
review):
 - new patch 3: writes that bypass write-behind (including discards)
   never serialized against in-flight write-behind I/O to overlapping
   sectors; gate the non-behind path on CollisionCheck so host
   fallback writes, discards and P2PDMA bios (patch 4) all order
   correctly.  Verified with an overlap scenario that hangs if the
   completion-side removal is missing.
 - patch 6: track P2PDMA masters with an r1bio/r10bio state bit set
   at submission instead of sniffing bvec pages at completion time;
   stop setting WantReplacement for mapping failures (a spare cannot
   fix a topology property; verified that a genuine write error
   still sets it); document why raid10 replacement legs keep their
   stock fail-on-error policy and why the read path charges no
   read-error budget.
 - patch 7: also convert the metadata scatterlist to
   ib_dma_map_sgtable_attrs() -- bio_integrity_map_user() accepts
   P2P pages when the queue advertises P2PDMA, so integrity buffers
   are not always host memory; state the asymmetric-multipath
   failover trade-off explicitly.
 - patch 1: version-annotate the stable Cc.
 - patches 2, 5: unchanged.

Link: https://lore.kernel.org/linux-raid/20260718162547.448892-1-mykola@meshstor.io/

Mykola Marzhan (7):
  blk-mq-dma: restore BLK_STS_TARGET for unsupported P2P transfers
  md: ensure REQ_NOMERGE is set on P2PDMA bios
  md/raid1: serialize non-write-behind writes on CollisionCheck rdevs
  md/raid1: don't use write-behind for P2PDMA bios
  md/raid1,raid10: keep REQ_NOMERGE on narrow_write_error() retry clones
  md/raid1,raid10: skip futile retries on P2PDMA mapping failures
  nvme-rdma: return BLK_STS_TARGET for unsupported P2P transfers

 block/blk-mq-dma.c       | 10 +++++-
 drivers/md/md.c          | 14 ++++++--
 drivers/md/md.h          | 18 ++++++++++
 drivers/md/raid1.c       | 78 ++++++++++++++++++++++++++++++++++------
 drivers/md/raid1.h       |  2 ++
 drivers/md/raid10.c      | 72 +++++++++++++++++++++++++++++++------
 drivers/md/raid10.h      |  2 ++
 drivers/nvme/host/rdma.c | 42 +++++++++++++---------
 8 files changed, 198 insertions(+), 40 deletions(-)


base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
-- 
2.43.0

[PATCH v2 1/7] blk-mq-dma: restore BLK_STS_TARGET for unsupported P2P transfers

[Mykola Marzhan]

Commit 91fb2b6052f7 ("nvme-pci: convert to using dma_map_sgtable()")
deliberately mapped unsupported P2PDMA transfers to BLK_STS_TARGET,
matching dma_map_sgtable()'s -EREMOTEIO: "When this happens, return
BLK_STS_TARGET so the request isn't retried." The conversion to
blk_rq_dma_map silently changed the status to BLK_STS_INVAL, which
regresses two consumers:

 - md/raid1 and raid10 ignore BLK_STS_INVAL leg failures
   (commit f7b24c7b41f2 ("md/raid1,raid10: don't fail devices for
   invalid IO errors"), where it means a request-shaped error that
   fails identically on every member).  Since commit 02666132403a
   ("md: propagate BLK_FEAT_PCI_P2PDMA from member devices to RAID device")
   P2PDMA bios reach md arrays: a peer-memory write to a member the
   peer cannot reach is counted as written, the master bio reports
   success, mirrors silently diverge, and on a topology where no
   member is reachable the write reports success with zero copies on
   stable storage.

 - the failure's classification flips: for direct NVMe consumers
   (nvme advertises BLK_FEAT_PCI_P2PDMA) the mapping failure has
   surfaced as EINVAL instead of the documented -EREMOTEIO since
   v6.17, and blk_path_error(BLK_STS_INVAL) is true, so a stacking
   consumer would treat it as a retryable path error (dm-mpath is
   the only in-tree blk_path_error() caller; P2P bios cannot
   currently reach it, but the classification is wrong on its face).

Restore BLK_STS_TARGET.  md then routes an unreachable leg through
its per-device error handling (badblocks, mirror
retry for reads); a later patch in this series teaches raid1/raid10
to handle mapping failures without the retry storms that machinery
was built around.

Fixes: 858299dc6160 ("block: add scatterlist-less DMA mapping helpers")
Fixes: 7ce3c1dd78fc ("nvme-pci: convert the data mapping to blk_rq_dma_map")
Cc: stable@vger.kernel.org # v6.17
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Mykola Marzhan <mykola@meshstor.io>
---
 block/blk-mq-dma.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-dma.c b/block/blk-mq-dma.c
index bfdb9ed70741..2eed06bfe791 100644
--- a/block/blk-mq-dma.c
+++ b/block/blk-mq-dma.c
@@ -190,7 +190,15 @@ static bool blk_dma_map_iter_start(struct request *req, struct device *dma_dev,
 	case PCI_P2PDMA_MAP_NONE:
 		break;
 	default:
-		iter->status = BLK_STS_INVAL;
+		/*
+		 * P2P transfers that the mapping layer cannot support
+		 * report BLK_STS_TARGET, matching dma_map_sgtable()'s
+		 * -EREMOTEIO and the pre-blk_rq_dma_map nvme behavior:
+		 * the failure is a property of this device pairing, so
+		 * it must not be retried on another path (blk_path_error)
+		 * nor be mistaken for an invalid request.
+		 */
+		iter->status = BLK_STS_TARGET;
 		return false;
 	}
 
-- 
2.43.0

Re: [PATCH v2 1/7] blk-mq-dma: restore BLK_STS_TARGET for unsupported P2P transfers

[Christoph Hellwig]

On Sun, Jul 19, 2026 at 10:53:21AM +0000, Mykola Marzhan wrote:
> matching dma_map_sgtable()'s -EREMOTEIO: "When this happens, return
> BLK_STS_TARGET so the request isn't retried." The conversion to
> blk_rq_dma_map silently changed the status to BLK_STS_INVAL, which
> regresses two consumers:

This is a really weird writing style, and suggested to me you neither
understand the code nor the problem.  Please actually think yourself,
write the patches and commit logs yourself instdad of this garbage.

And please also explain how you even generate the I/O that fails this
way.

Re: [PATCH v2 1/7] blk-mq-dma: restore BLK_STS_TARGET for unsupported P2P transfers

[Logan Gunthorpe]

On 2026-07-20 11:30, Mykola Marzhan wrote:
> On Mon, Jul 20, 2026 at 4:49 PM Christoph Hellwig <hch@lst.de
>> This is a really weird writing style, and suggested to me you neither
>> understand the code nor the problem.  Please actually think yourself,
>> write the patches and commit logs yourself instdad of this garbage.
>> 
>> And please also explain how you even generate the I/O that fails this
>> way.
> - md mirrors the write: the copy to member 1 succeeds, the copy to far
> member fails as BLK_STS_INVAL.
> - md deliberately ignores BLK_STS_INVAL, so the write is counted as
> written and the mirror silently diverges.

Seeing I just reviewed this and answered it for myself I'll expand on
what's going on here:

The switch statement will take the default branch when
pci_p2pdma_state() returns PCI_P2PDMA_MAP_NOT_SUPPORTED. This indicates
the memory that's trying to be mapped in this way will not succeed
because it's going through an unsupported host bridge (or if the device
isn't a PCI device, etc, but this is rarer or perhaps not possible).
Returning BLK_STS_INVAL doesn't seem correct here, to me. (Arguably it
might have been clearer if the switch case explicitly stated
PCI_P2PDMA_MAP_NOT_SUPPORTED instead of relying on the default).

Returning -EREMOTEIO/BLK_STS_TARGET was the convention for this I had
originally set when I wrote some of this and differentiating the error
does seem important in Patch 6 in this series.

Logan

Re: [PATCH v2 1/7] blk-mq-dma: restore BLK_STS_TARGET for unsupported P2P transfers

[Logan Gunthorpe]

On 2026-07-20 12:42, Logan Gunthorpe wrote:
> 
> 
> On 2026-07-20 11:30, Mykola Marzhan wrote:
>> On Mon, Jul 20, 2026 at 4:49 PM Christoph Hellwig <hch@lst.de
>>> This is a really weird writing style, and suggested to me you neither
>>> understand the code nor the problem.  Please actually think yourself,
>>> write the patches and commit logs yourself instdad of this garbage.
>>>
>>> And please also explain how you even generate the I/O that fails this
>>> way.
>> - md mirrors the write: the copy to member 1 succeeds, the copy to far
>> member fails as BLK_STS_INVAL.
>> - md deliberately ignores BLK_STS_INVAL, so the write is counted as
>> written and the mirror silently diverges.
> 
> Seeing I just reviewed this and answered it for myself I'll expand on
> what's going on here:
> 
> The switch statement will take the default branch when
> pci_p2pdma_state() returns PCI_P2PDMA_MAP_NOT_SUPPORTED. This indicates
> the memory that's trying to be mapped in this way will not succeed
> because it's going through an unsupported host bridge (or if the device
> isn't a PCI device, etc, but this is rarer or perhaps not possible).
> Returning BLK_STS_INVAL doesn't seem correct here, to me. (Arguably it
> might have been clearer if the switch case explicitly stated
> PCI_P2PDMA_MAP_NOT_SUPPORTED instead of relying on the default).
> 
> Returning -EREMOTEIO/BLK_STS_TARGET was the convention for this I had
> originally set when I wrote some of this and differentiating the error
> does seem important in Patch 6 in this series.

Sorry, one other note I almost forgot about which is probably important:
BLK_STS_INVAL will attempt a retry, where BLK_STS_TARGET will not (I
think that's part of why I chose that specific error in the first
place). A retry here doesn't make any sense because it will always hit
the same case and fail. So it's probably worth applying this patch even
without the specific error being used in Patch 6.

Logan

[PATCH v2 2/7] md: ensure REQ_NOMERGE is set on P2PDMA bios

[Mykola Marzhan]

md_submit_bio() unconditionally strips REQ_NOMERGE before passing the
bio to the personality, an optimization from commit 9c573de3283a ("MD:
make bio mergeable"): a bio that md has split may become mergeable
again below md.

For PCI P2PDMA bios the flag is load-bearing, not a hint. The block
layer sets REQ_NOMERGE on P2PDMA bios (__bio_add_page(), and the
extraction path of bio_iov_iter_get_pages()) because the DMA mapping
type of a request is resolved once, from its first segment
(blk_dma_map_iter_start()), and request-level merging is prevented
only by REQ_NOMERGE. Stripping it allows the member queue to merge a
P2PDMA bio with a bio carrying pages of a different pgmap, or host
memory, mapping the merged segments with the wrong bus address:
silent data corruption on the member.

Set the flag for P2PDMA bios instead of merely preserving it. No
in-tree path currently submits P2PDMA pages through the bvec-iter
path (bio_iov_bvec_set()), which skips the flagging -- but nothing
structural prevents one, so setting rather than preserving hardens
md against that gap at the cost of one branch. Everything else keeps
the original optimization of clearing the flag. This covers every
personality that advertises BLK_FEAT_PCI_P2PDMA (raid0, raid1,
raid10), which is why the fix lives in the shared md_submit_bio()
path.

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@meshstor.io>
---
 drivers/md/md.c | 14 ++++++++++++--
 drivers/md/md.h | 18 ++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index d1465bcd86c8..3ae4fd4ef381 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -451,8 +451,18 @@ 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 could be mergeable again below md, but for P2PDMA
+	 * bios REQ_NOMERGE is load-bearing: the DMA mapping type of a
+	 * request is resolved once, from its first segment, so requests
+	 * must stay single-provider (see __bio_add_page()). Set the flag
+	 * rather than merely preserve it -- bios built through the
+	 * bvec-iter path arrive without it.
+	 */
+	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..140e2b3670d8 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 +24,22 @@
 #include <trace/events/block.h>
 
 #define MaxSector (~(sector_t)0)
+
+/*
+ * Check if the bio carries PCI P2PDMA (peer device memory) pages. Read
+ * bi_io_vec directly rather than using bio_first_bvec_all(), which WARNs
+ * on cloned bios: md routinely handles split clones, which have
+ * bi_vcnt == 0 but a valid bi_io_vec shared with the parent. P2PDMA and
+ * host pages must not be mixed within one bio, so the first bvec is
+ * representative. Only valid before the bio's iterator is consumed:
+ * bio_has_data() is false at completion time.
+ */
+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:
  */
-- 
2.43.0

[PATCH v2 3/7] md/raid1: serialize non-write-behind writes on CollisionCheck rdevs

[Mykola Marzhan]

A write-behind write is acknowledged to the caller while its copy to
the write-mostly member is still in flight.  wait_for_serialization()
exists to order later overlapping writes behind it, but the non-behind
clone path only takes it under serialize_policy.  A write to a
write-behind array skips the behind path whenever the behind queue is
full, a reader is waiting on behind completion, or the behind bio
allocation fails (raid1_start_write_behind()) -- and then races the
in-flight behind write on the write-mostly member without any
ordering: if the older behind data lands last, the member keeps stale
data for sectors whose newer write the caller has already seen
acknowledged.

Gate the non-behind serialization on CollisionCheck alone.  The flag
marks exactly the rdevs that own a serial tree: every rdev present
when serialize_policy is enabled, and write-mostly members when
write-behind arms serialization, which is the case the current test
misses.  Take the matching remove_serial() at completion under the
same condition.  Discards take the same non-behind path and are now
ordered as well.  For an rdev hot-added while serialize_policy is
already enabled, mddev_create_serial_pool() never builds a serial
tree, so the old MD_SERIALIZE_POLICY gate sent it into
wait_for_serialization() with rdev->serial == NULL -- a latent oops
that the per-rdev gate avoids by construction.  REQ_NOWAIT writes
can wait here when they overlap an in-flight behind write, as they
already do on the behind path and under serialize_policy.

Fixes: d0d2d8ba0494 ("md/raid1: introduce wait_for_serialization")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-fable-5
Signed-off-by: Mykola Marzhan <mykola@meshstor.io>
---
 drivers/md/raid1.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index afe2ca96ad8c..8172df882f26 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -564,7 +564,7 @@ static void raid1_end_write_request(struct bio *bio)
 				call_bio_endio(r1_bio);
 			}
 		}
-	} else if (test_bit(MD_SERIALIZE_POLICY, &rdev->mddev->flags))
+	} else if (test_bit(CollisionCheck, &rdev->flags))
 		remove_serial(rdev, lo, hi);
 	if (r1_bio->bios[mirror] == NULL)
 		rdev_dec_pending(rdev, conf->mddev);
@@ -1677,7 +1677,14 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio,
 			mbio = bio_alloc_clone(rdev->bdev, bio, GFP_NOIO,
 					       &mddev->bio_set);
 
-			if (test_bit(MD_SERIALIZE_POLICY, &mddev->flags))
+			/*
+			 * Order against in-flight write-behind I/O: a
+			 * behind write is acked early, and an unordered
+			 * overwrite could land first, leaving its stale
+			 * data on the member last. CollisionCheck marks
+			 * every rdev that owns a serial tree.
+			 */
+			if (test_bit(CollisionCheck, &rdev->flags))
 				wait_for_serialization(rdev, r1_bio);
 		}
 
-- 
2.43.0

[PATCH v2 4/7] md/raid1: don't use write-behind for P2PDMA bios

[Mykola Marzhan]

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 <mykola@meshstor.io>
---
 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 8172df882f26..914fb86452c0 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;
-- 
2.43.0

Re: [PATCH v2 4/7] md/raid1: don't use write-behind for P2PDMA bios

[Logan Gunthorpe]

On 2026-07-19 04:53, Mykola Marzhan wrote:
> 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 <mykola@meshstor.io>

Looks correct to me.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

[PATCH v2 5/7] md/raid1,raid10: keep REQ_NOMERGE on narrow_write_error() retry clones

[Mykola Marzhan]

narrow_write_error() re-issues a failed write in badblock-granularity
chunks, cloning from the master bio and resetting bi_opf to a bare
REQ_OP_WRITE. For a P2PDMA bio that reset drops REQ_NOMERGE, which is
the only request-level protection against the member queue merging
P2PDMA segments across pgmaps or with host memory (see the preceding
md_submit_bio() fix): the retry path would quietly reopen the hole
the submission path closes. Restore the flag on P2PDMA retry clones.

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@meshstor.io>
---
 drivers/md/raid1.c  | 3 +++
 drivers/md/raid10.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 914fb86452c0..f562b6bd438b 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -2573,6 +2573,9 @@ static void narrow_write_error(struct r1bio *r1_bio, int i)
 		}
 
 		wbio->bi_opf = REQ_OP_WRITE;
+		/* Keep P2PDMA retry bios unmergeable, like the original */
+		if (md_bio_is_p2pdma(wbio))
+			wbio->bi_opf |= REQ_NOMERGE;
 		wbio->bi_iter.bi_sector = r1_bio->sector;
 		wbio->bi_iter.bi_size = r1_bio->sectors << 9;
 
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 0a3cfdd3f5df..f7ef903a3d4e 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -2831,6 +2831,9 @@ static void narrow_write_error(struct r10bio *r10_bio, int i)
 		wbio->bi_iter.bi_sector = wsector +
 				   choose_data_offset(r10_bio, rdev);
 		wbio->bi_opf = REQ_OP_WRITE;
+		/* Keep P2PDMA retry bios unmergeable, like the original */
+		if (md_bio_is_p2pdma(wbio))
+			wbio->bi_opf |= REQ_NOMERGE;
 
 		if (submit_bio_wait(wbio) &&
 		    !rdev_set_badblocks(rdev, wsector, sectors, 0)) {
-- 
2.43.0

Re: [PATCH v2 5/7] md/raid1,raid10: keep REQ_NOMERGE on narrow_write_error() retry clones

[Logan Gunthorpe]

On 2026-07-19 04:53, Mykola Marzhan wrote:
> narrow_write_error() re-issues a failed write in badblock-granularity
> chunks, cloning from the master bio and resetting bi_opf to a bare
> REQ_OP_WRITE. For a P2PDMA bio that reset drops REQ_NOMERGE, which is
> the only request-level protection against the member queue merging
> P2PDMA segments across pgmaps or with host memory (see the preceding
> md_submit_bio() fix): the retry path would quietly reopen the hole
> the submission path closes. Restore the flag on P2PDMA retry clones.
> 
> 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@meshstor.io>
Looks correct, thanks.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

[PATCH v2 6/7] md/raid1,raid10: skip futile retries on P2PDMA mapping failures

[Mykola Marzhan]

Since commit 02666132403a ("md: propagate BLK_FEAT_PCI_P2PDMA from
member devices to RAID device") raid1 and raid10 arrays accept P2PDMA
(peer device memory) bios, and a member that cannot DMA-map the
peer's pages fails its leg bio with BLK_STS_TARGET (from
blk_dma_map_iter_start()).  That mapping failure is a property of the
peer-device/member pairing, not of the medium: re-submitting the same
peer pages to the same member cannot succeed, and there is nothing on
the disk to repair.  Routing it through the stock error machinery
misfires on every path:

 - narrow_write_error() re-issues the failed write in badblock-sized
   chunks, serializing hundreds of guaranteed-to-fail synchronous bios
   through raid1d/raid10d per failed write (256 for a 1 MiB write on
   4K logical blocks, 2048 on 512e).

 - the write-error handler sets WantReplacement, so a hot spare is
   pulled in, rebuilt onto, and the healthy member is then evicted by
   spare_active() -- and if the spare is also unreachable from the
   peer, the cycle consumes the next spare.

 - fix_read_error() probes members with kernel pages, starting with
   the failing member itself.  That probe succeeds (the member is
   healthy for host memory), so the routine rewrites nothing, records
   nothing, and logs nothing -- but each invocation costs a full
   freeze_array() quiesce and a tick of the read-error budget.  The
   budget exists to evict members whose medium keeps producing
   corrected errors; its hourly decay is sized for sporadic medium
   errors, while mapping failures are deterministic and arrive at I/O
   rate.  On an asymmetric PCIe topology a mixed host/P2P read
   workload charges 20 errors to the unreachable leg within a second
   and kicks a perfectly healthy member.

 - On FailFast members both paths short-circuit into md_error(), so a
   single unroutable peer-memory I/O evicts a healthy mirror outright.

Track P2PDMA masters with a new r1bio/r10bio state bit, set at
submission where bio_has_data() is still meaningful; a leg's mapping
failure is then "BLK_STS_TARGET and the master carries P2PDMA pages",
cheap to test on every completion path.  Handle it explicitly:

 - Writes: probe the whole range with one retry and record one bad
   range if it also fails.  BLK_STS_TARGET is also produced for
   device conditions that narrow_write_error()'s retry loop recovers
   from (e.g. NVME_SC_NS_NOT_READY and NVME_SC_CMD_INTERRUPTED via
   nvme_error_status()), and md cannot tell the two apart from
   bi_status alone, so it must not skip the retry outright.  A single
   whole-range probe observes the outcome instead of predicting it.
   WriteErrorSeen is still set (it gates the write-path badblocks
   consult), but WantReplacement is not: replacing a member cannot
   fix a topology property, and spending a spare on it destroys
   redundancy management for real failures.

 - Reads: mark the leg IO_BLOCKED so read_balance() picks another
   mirror, and skip the freeze/fix cycle and the budget charge.  The
   same TARGET ambiguity exists here, but a redirected read leaves
   nothing to fence, and fix_read_error()'s host-page probe fails
   the same way under e.g. NVME_SC_NS_NOT_READY -- charging the
   budget would evict a healthy member for a firmware activation
   window.  A genuinely failing member is still evicted via host
   reads, writes and BLK_STS_MEDIUM errors.  If no mirror can serve
   the read the master bio fails with EIO as before (md completes
   masters by Uptodate state, as for any failed mirror I/O), now
   without kicking healthy members on the way.

 - FailFast: a mapping failure is rejected at map time and never
   reaches the wire, so it is no evidence of device unreliability;
   don't let it trigger the FailFast md_error() -- the same reasoning
   commit f7b24c7b41f2 ("md/raid1,raid10: don't fail devices for
   invalid IO errors") applied to BLK_STS_INVAL.

raid10 replacement legs keep their stock policy: badblocks are never
recorded on a replacement, so failing it is the only outcome that
cannot leave a silent hole in a rebuilding replacement.

This belongs in the same release as the BLK_STS_TARGET restoration:
with that fix alone, an unroutable leg costs the retry storms and
the healthy-member evictions above.

Measured on QEMU q35 rigs (one CMB provider, two NVMe members),
8KiB peer-memory I/O, reads x90 under concurrent host reads:

  scenario       stock error machinery     with this patch
  asym reads     90/90 ok, healthy far     90/90 ok, no eviction,
                 leg kicked after ~20      no budget charge
  unreach reads  all EIO, one healthy      all EIO, no members
                 member kicked             kicked
  p2p TARGET     16 chunk retries from     1 whole-range probe,
  write          raid1d, then badblocks    same badblocks, master ok

A transient TARGET recovers through the single probe with no
badblocks recorded, and a mapping failure no longer trips FailFast
eviction while a genuine I/O error still does -- both verified on
the same rig.

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@meshstor.io>
---
 drivers/md/raid1.c  | 56 +++++++++++++++++++++++++++++++-----
 drivers/md/raid1.h  |  2 ++
 drivers/md/raid10.c | 69 ++++++++++++++++++++++++++++++++++++++-------
 drivers/md/raid10.h |  2 ++
 4 files changed, 112 insertions(+), 17 deletions(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index f562b6bd438b..61f635463475 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -483,12 +483,22 @@ static void raid1_end_write_request(struct bio *bio)
 	 * 'one mirror IO has finished' event handler:
 	 */
 	if (bio->bi_status && !ignore_error) {
+		/*
+		 * A P2PDMA mapping failure reflects the peer/member
+		 * pairing, not member health: don't pull in a spare
+		 * or trip FailFast for it.
+		 */
+		bool p2pdma_unmappable = bio->bi_status == BLK_STS_TARGET &&
+			test_bit(R1BIO_P2PDMA, &r1_bio->state);
+
 		set_bit(WriteErrorSeen,	&rdev->flags);
-		if (!test_and_set_bit(WantReplacement, &rdev->flags))
+		if (!p2pdma_unmappable &&
+		    !test_and_set_bit(WantReplacement, &rdev->flags))
 			set_bit(MD_RECOVERY_NEEDED, &
 				conf->mddev->recovery);
 
-		if (test_bit(FailFast, &rdev->flags) &&
+		if (!p2pdma_unmappable &&
+		    test_bit(FailFast, &rdev->flags) &&
 		    (bio->bi_opf & MD_FAILFAST) &&
 		    /* We never try FailFast to WriteMostly devices */
 		    !test_bit(WriteMostly, &rdev->flags)) {
@@ -1378,6 +1388,8 @@ 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 +1569,8 @@ 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
@@ -2525,7 +2539,7 @@ 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;
@@ -2539,6 +2553,11 @@ 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 and record
+	 * one bad range if it fails: for P2PDMA mapping failures,
+	 * which fail every block identically, while the single retry
+	 * still lets a cleared transient error recover.
+	 *
 	 * We currently own a reference on the rdev.
 	 */
 
@@ -2553,9 +2572,12 @@ 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;
@@ -2636,8 +2658,18 @@ 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);
+			/*
+			 * A P2PDMA mapping failure fails the whole range
+			 * identically: probe it once (coarse) instead of
+			 * narrowing block by block. A transient TARGET
+			 * recovers via the probe with nothing recorded.
+			 */
+			narrow_write_error(r1_bio, m, coarse);
 			rdev_dec_pending(conf->mirrors[m].rdev,
 					 conf->mddev);
 		}
@@ -2664,6 +2696,9 @@ 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;
 
@@ -2683,6 +2718,13 @@ 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) {
+		/*
+		 * The peer cannot reach this member; nothing on the
+		 * medium to fix. Skip the read-error budget and
+		 * FailFast, just keep this leg out of the retry.
+		 */
+		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)
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index f7ef903a3d4e..144d07b6029c 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -482,13 +482,24 @@ static void raid10_end_write_request(struct bio *bio)
 			 */
 			md_error(rdev->mddev, rdev);
 		else {
+			/*
+			 * A P2PDMA mapping failure reflects the
+			 * peer/member pairing, not member health: don't
+			 * pull in a spare or trip FailFast for it.
+			 */
+			bool p2pdma_unmappable =
+				bio->bi_status == BLK_STS_TARGET &&
+				test_bit(R10BIO_P2PDMA, &r10_bio->state);
+
 			set_bit(WriteErrorSeen,	&rdev->flags);
-			if (!test_and_set_bit(WantReplacement, &rdev->flags))
+			if (!p2pdma_unmappable &&
+			    !test_and_set_bit(WantReplacement, &rdev->flags))
 				set_bit(MD_RECOVERY_NEEDED,
 					&rdev->mddev->recovery);
 
 			dec_rdev = 0;
-			if (test_bit(FailFast, &rdev->flags) &&
+			if (!p2pdma_unmappable &&
+			    test_bit(FailFast, &rdev->flags) &&
 			    (bio->bi_opf & MD_FAILFAST)) {
 				md_error(rdev->mddev, rdev);
 			}
@@ -1170,6 +1181,9 @@ static void raid10_read_request(struct mddev *mddev, struct bio *bio,
 	 */
 	gfp_t gfp = err_path ? (GFP_NOIO | __GFP_HIGH) : GFP_NOIO;
 
+	if (md_bio_is_p2pdma(bio))
+		set_bit(R10BIO_P2PDMA, &r10_bio->state);
+
 	if (slot >= 0 && r10_bio->devs[slot].rdev) {
 		/*
 		 * This is an error retry, but we cannot
@@ -1357,6 +1371,9 @@ static bool raid10_write_request(struct mddev *mddev, struct bio *bio,
 	sector_t sectors;
 	int max_sectors;
 
+	if (md_bio_is_p2pdma(bio))
+		set_bit(R10BIO_P2PDMA, &r10_bio->state);
+
 	if ((mddev_is_clustered(mddev) &&
 	     mddev->cluster_ops->area_resyncing(mddev, WRITE,
 						bio->bi_iter.bi_sector,
@@ -2786,7 +2803,7 @@ static void fix_read_error(struct r10conf *conf, struct mddev *mddev, struct r10
 	}
 }
 
-static void narrow_write_error(struct r10bio *r10_bio, int i)
+static void narrow_write_error(struct r10bio *r10_bio, int i, bool coarse)
 {
 	struct bio *bio = r10_bio->master_bio;
 	struct mddev *mddev = r10_bio->mddev;
@@ -2800,6 +2817,11 @@ static void narrow_write_error(struct r10bio *r10_bio, int i)
 	 * It is conceivable that the bio doesn't exactly align with
 	 * blocks.  We must handle this.
 	 *
+	 * With 'coarse', retry the whole range as one bio and record
+	 * one bad range if it fails: for P2PDMA mapping failures,
+	 * which fail every block identically, while the single retry
+	 * still lets a cleared transient error recover.
+	 *
 	 * We currently own a reference to the rdev.
 	 */
 
@@ -2814,9 +2836,12 @@ static void narrow_write_error(struct r10bio *r10_bio, int i)
 		block_sectors = roundup(1 << rdev->badblocks.shift, lbs);
 
 	sector = r10_bio->sector;
-	sectors = ((r10_bio->sector + block_sectors)
-		   & ~(sector_t)(block_sectors - 1))
-		- sector;
+	if (coarse)
+		sectors = sect_to_write;
+	else
+		sectors = ((r10_bio->sector + block_sectors)
+			   & ~(sector_t)(block_sectors - 1))
+			- sector;
 
 	while (sect_to_write) {
 		struct bio *wbio;
@@ -2856,6 +2881,7 @@ static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
 {
 	int slot = r10_bio->read_slot;
 	struct bio *bio;
+	bool p2pdma_error;
 	struct r10conf *conf = mddev->private;
 	struct md_rdev *rdev = r10_bio->devs[slot].rdev;
 
@@ -2868,17 +2894,28 @@ static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
 	 * frozen.
 	 */
 	bio = r10_bio->devs[slot].bio;
+	/* evaluate before the bio_put() below */
+	p2pdma_error = bio->bi_status == BLK_STS_TARGET &&
+		test_bit(R10BIO_P2PDMA, &r10_bio->state);
 	bio_put(bio);
 	r10_bio->devs[slot].bio = NULL;
 
 	if (mddev->ro)
 		r10_bio->devs[slot].bio = IO_BLOCKED;
-	else if (!test_bit(FailFast, &rdev->flags)) {
+	else if (p2pdma_error) {
+		/*
+		 * The peer cannot reach this member; nothing on the
+		 * medium to fix. Skip the read-error budget and
+		 * FailFast, just keep this leg out of the retry.
+		 */
+		r10_bio->devs[slot].bio = IO_BLOCKED;
+	} else if (test_bit(FailFast, &rdev->flags)) {
+		md_error(mddev, rdev);
+	} else {
 		freeze_array(conf, 1);
 		fix_read_error(conf, mddev, r10_bio);
 		unfreeze_array(conf);
-	} else
-		md_error(mddev, rdev);
+	}
 
 	rdev_dec_pending(rdev, mddev);
 	r10_bio->state = 0;
@@ -2947,8 +2984,20 @@ static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)
 					r10_bio->sectors, 0);
 				rdev_dec_pending(rdev, conf->mddev);
 			} else if (bio != NULL && bio->bi_status) {
+				bool coarse = bio->bi_status ==
+						BLK_STS_TARGET &&
+					test_bit(R10BIO_P2PDMA,
+						 &r10_bio->state);
+
 				fail = true;
-				narrow_write_error(r10_bio, m);
+				/*
+				 * A P2PDMA mapping failure fails the whole
+				 * range identically: probe it once (coarse)
+				 * instead of narrowing block by block.  A
+				 * transient TARGET recovers via the probe
+				 * with nothing recorded.
+				 */
+				narrow_write_error(r10_bio, m, coarse);
 				rdev_dec_pending(rdev, conf->mddev);
 			}
 			bio = r10_bio->devs[m].repl_bio;
diff --git a/drivers/md/raid10.h b/drivers/md/raid10.h
index ec79d87fb92f..a2e1554f77db 100644
--- a/drivers/md/raid10.h
+++ b/drivers/md/raid10.h
@@ -174,6 +174,8 @@ enum r10bio_state {
 	R10BIO_Previous,
 /* failfast devices did receive failfast requests. */
 	R10BIO_FailFast,
+/* the master bio carries PCI P2PDMA (peer device memory) pages */
+	R10BIO_P2PDMA,
 	R10BIO_Discard,
 };
 #endif
-- 
2.43.0

Re: [PATCH v2 6/7] md/raid1,raid10: skip futile retries on P2PDMA mapping failures

[Logan Gunthorpe]

On 2026-07-19 04:53, Mykola Marzhan wrote:
>  drivers/md/raid1.c  | 56 +++++++++++++++++++++++++++++++-----
>  drivers/md/raid1.h  |  2 ++
>  drivers/md/raid10.c | 69 ++++++++++++++++++++++++++++++++++++++-------
>  drivers/md/raid10.h |  2 ++

This is an intimidating and massive patch to review. I gotta second the
verbosity of the AI commit messages comment from Keith.

My eyes glazed over a bit. But one point is that maybe this could be
split into multiple patches: one patch introduces R1BIO_P2PDMA, one
patch for raid1 and another patch for raid10? That might make it a bit
more manageable.

Also, I'm not sure if it's possible without digging deeper but, instead
of adding a !p2pdma_unmappable check to every if() statement in the
raid*_end_write_request() functions, might it be possible to factor out
a helper function and return early from it? Maybe the helper could be
shared between raid10 and raid1 as they look very similar. The
differences between the two are minimal -- there might be a bug fix that
was applied to one that should be applied to both?

Thanks!

Logan

[PATCH v2 7/7] nvme-rdma: return BLK_STS_TARGET for unsupported P2P transfers

[Mykola Marzhan]

Since commit 23528aa3320a ("nvme: enable PCI P2PDMA support for RDMA
transport") nvme-rdma accepts P2PDMA bios, but a mapping failure for
peer memory the HCA cannot reach is misreported as a path error:
ib_dma_map_sg() returns 0, discarding the -EREMOTEIO that
dma_map_sgtable() documents for exactly this case, and the driver
converts it to -EIO -> nvme_host_path_error().

Under the default multipath configuration (the multipath head node
advertises BLK_FEAT_PCI_P2PDMA since commit fb0eeeed91f3
("nvme-multipath: enable PCI P2PDMA for multipath devices")) the
path-error status makes nvme_failover_req() requeue the bios with a
fresh retry budget each cycle, and since the mapping failure is a
deterministic property of the peer/device pairing the I/O simply
never completes: a hot requeue livelock, and a stacked md mirror
hangs instead of failing over to its other leg.  With
nvme_core.multipath=N the request burns nvme_max_retries requeues --
nothing ever reaching the wire -- and completes as
BLK_STS_TRANSPORT, which blk_path_error() classifies as retryable
and md/raid1,raid10 treat as a genuine device error: retry storms on
writes, read-error-budget eviction of a healthy member on reads (see
the preceding md patches, whose mapping-failure handling keys on
BLK_STS_TARGET).

Map both the data and the metadata scatterlists with
ib_dma_map_sgtable_attrs() so the DMA layer's error code is
preserved, and translate -EREMOTEIO to BLK_STS_TARGET: the
classification nvme-pci established in commit 91fb2b6052f7
("nvme-pci: convert to using dma_map_sgtable()") and the preceding
blk-mq-dma patch restores.  The metadata path needs the same
treatment because integrity buffers are not always host memory:
bio_integrity_map_user() allows P2P pages whenever the queue
advertises P2PDMA, and nvme-pci already maps P2P metadata.

Returning BLK_STS_TARGET from queue_rq is a deliberate trade-off: it
forecloses failover for a configuration where a sibling path through
a different HCA could reach the peer.  This matches nvme-pci, which
has returned BLK_STS_TARGET for unsupported P2P mappings since
that same commit and equally forecloses cross-controller failover;
distinguishing this-path-unreachable from all-paths-unreachable at
completion time would cost a spurious requeue cycle per I/O, and a
stacking consumer (md) routes around the leg one layer up.

Start the request only after mapping succeeds, as nvme-pci does: a
mapping failure now errors out of queue_rq on a not-yet-started
request, so nvme_mpath_start_request() accounting is never taken and
cannot leak on the direct blk-mq completion (this also closes the same
latent leak for mapping-failure BLK_STS_IOERR returns; the post-send
error tail is untouched). A mapping -ENOMEM now takes the existing
BLK_STS_RESOURCE branch instead of masquerading as a path error, and a
DMA-layer -EINVAL completes as BLK_STS_IOERR instead of a path error;
generic -EIO keeps today's host-path-error behavior, and virt-DMA
devices are unaffected. Ratelimit the map-failure message -- with an md
mirror steering peer-memory I/O around an unreachable leg it fires per
redirected I/O, not per rare event.

Fixes: 23528aa3320a ("nvme: enable PCI P2PDMA support for RDMA transport")
Cc: stable@vger.kernel.org # v7.1
Assisted-by: Claude:claude-fable-5
Signed-off-by: Mykola Marzhan <mykola@meshstor.io>
---
 drivers/nvme/host/rdma.c | 42 +++++++++++++++++++++++++---------------
 1 file changed, 26 insertions(+), 16 deletions(-)

diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
index 6909e3542794..9017d927edc4 100644
--- a/drivers/nvme/host/rdma.c
+++ b/drivers/nvme/host/rdma.c
@@ -1469,6 +1469,7 @@ static int nvme_rdma_dma_map_req(struct ib_device *ibdev, struct request *rq,
 		int *count, int *pi_count)
 {
 	struct nvme_rdma_request *req = blk_mq_rq_to_pdu(rq);
+	struct sg_table sgt;
 	int ret;
 
 	req->data_sgl.sg_table.sgl = (struct scatterlist *)(req + 1);
@@ -1480,12 +1481,14 @@ static int nvme_rdma_dma_map_req(struct ib_device *ibdev, struct request *rq,
 
 	req->data_sgl.nents = blk_rq_map_sg(rq, req->data_sgl.sg_table.sgl);
 
-	*count = ib_dma_map_sg(ibdev, req->data_sgl.sg_table.sgl,
-			       req->data_sgl.nents, rq_dma_dir(rq));
-	if (unlikely(*count <= 0)) {
-		ret = -EIO;
+	sgt = (struct sg_table) {
+		.sgl		= req->data_sgl.sg_table.sgl,
+		.orig_nents	= req->data_sgl.nents,
+	};
+	ret = ib_dma_map_sgtable_attrs(ibdev, &sgt, rq_dma_dir(rq), 0);
+	if (unlikely(ret))
 		goto out_free_table;
-	}
+	*count = sgt.nents;
 
 	if (blk_integrity_rq(rq)) {
 		req->metadata_sgl->sg_table.sgl =
@@ -1501,14 +1504,14 @@ static int nvme_rdma_dma_map_req(struct ib_device *ibdev, struct request *rq,
 
 		req->metadata_sgl->nents = blk_rq_map_integrity_sg(rq,
 				req->metadata_sgl->sg_table.sgl);
-		*pi_count = ib_dma_map_sg(ibdev,
-					  req->metadata_sgl->sg_table.sgl,
-					  req->metadata_sgl->nents,
-					  rq_dma_dir(rq));
-		if (unlikely(*pi_count <= 0)) {
-			ret = -EIO;
+		sgt = (struct sg_table) {
+			.sgl		= req->metadata_sgl->sg_table.sgl,
+			.orig_nents	= req->metadata_sgl->nents,
+		};
+		ret = ib_dma_map_sgtable_attrs(ibdev, &sgt, rq_dma_dir(rq), 0);
+		if (unlikely(ret))
 			goto out_free_pi_table;
-		}
+		*pi_count = sgt.nents;
 	}
 
 	return 0;
@@ -2026,8 +2029,6 @@ static blk_status_t nvme_rdma_queue_rq(struct blk_mq_hw_ctx *hctx,
 	if (ret)
 		goto unmap_qe;
 
-	nvme_start_request(rq);
-
 	if (IS_ENABLED(CONFIG_BLK_DEV_INTEGRITY) &&
 	    queue->pi_support &&
 	    (c->common.opcode == nvme_cmd_write ||
@@ -2039,11 +2040,13 @@ static blk_status_t nvme_rdma_queue_rq(struct blk_mq_hw_ctx *hctx,
 
 	err = nvme_rdma_map_data(queue, rq, c);
 	if (unlikely(err < 0)) {
-		dev_err(queue->ctrl->ctrl.device,
-			     "Failed to map data (%d)\n", err);
+		dev_err_ratelimited(queue->ctrl->ctrl.device,
+				    "Failed to map data (%d)\n", err);
 		goto err;
 	}
 
+	nvme_start_request(rq);
+
 	sqe->cqe.done = nvme_rdma_send_done;
 
 	ib_dma_sync_single_for_device(dev, sqe->dma,
@@ -2063,6 +2066,13 @@ static blk_status_t nvme_rdma_queue_rq(struct blk_mq_hw_ctx *hctx,
 		ret = nvme_host_path_error(rq);
 	else if (err == -ENOMEM || err == -EAGAIN)
 		ret = BLK_STS_RESOURCE;
+	/*
+	 * The DMA layer refused to map peer memory to this device: a
+	 * property of the pairing, not a path failure.  Match nvme-pci
+	 * and do not retry (see blk_path_error()).
+	 */
+	else if (err == -EREMOTEIO)
+		ret = BLK_STS_TARGET;
 	else
 		ret = BLK_STS_IOERR;
 	nvme_cleanup_cmd(rq);
-- 
2.43.0