Re: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and check support for requests

From: Logan Gunthorpe
Date: Thu Aug 30 2018 - 15:17:57 EST

On 30/08/18 01:11 PM, Jens Axboe wrote:
> On 8/30/18 12:53 PM, Logan Gunthorpe wrote:
>> QUEUE_FLAG_PCI_P2P is introduced meaning a driver's request queue
>> supports targeting P2P memory.
>> When a request is submitted we check if PCI P2PDMA memory is assigned
>> to the first page in the bio. If it is, we ensure the queue it's
>> submitted to supports it, and enforce REQ_NOMERGE.
> I think this belongs in the caller - both the validity check, and
> passing in NOMERGE for this type of request. I don't want to impose
> this overhead on everything, for a pretty niche case.

Well, the point was to prevent driver writers from doing the wrong
thing. The WARN_ON would be a bit pointless in the driver if we rely on
the driver to either do the right thing or add the WARN_ON themselves.

If I'm going to change anything I'd drop the warning entirely and move
the NO_MERGE back into the caller...

Note: the check will be compiled out if the kernel does not support PCI P2P.