Re: [PATCH 2/6] md: ensure REQ_NOMERGE is set on P2PDMA bios

[Logan Gunthorpe]

On 2026-07-18 10:25, Mykola Marzhan wrote:
> 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@xxxxxxxxxxx>

Makes sense to me. Matches the similar check in nvmet_bdev_execute_rw().

Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>