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@xxxxxx
>> 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