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@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.

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