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

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 <redacted>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260719105327.864949-1-mykola@meshstor.io?part=2