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@xxxxxxxxxxx>

Looks correct to me.

Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>