On 9/5/18 2:09 PM, Logan Gunthorpe wrote:

On 05/09/18 02:11 PM, Christoph Hellwig wrote:

quoted

On Wed, Sep 05, 2018 at 01:54:31PM -0600, Jens Axboe wrote:

quoted

On 9/5/18 1:56 PM, Christoph Hellwig wrote:

quoted

On Wed, Sep 05, 2018 at 01:45:04PM -0600, Jens Axboe wrote:

quoted

The point is that the caller doesn't necessarily know where the bio
will end up, hence the caller can't fully check if the whole stack
supports P2P.

The caller must necessarily know where the bio will end up, as for P2P
support we need to query if the bio target is P2P capable vs the
source of the P2P memory.

Then what's the point of having the check at all?

Just an additional little safe guard.  If you think it isn't worth
it I guess we can just drop it for now.

Yes, the point is to prevent driver writers from doing the wrong thing
by not doing the necessary checks before submitting to the queue.

But if the caller must absolutely know where the bio will end up, then
it seems super redundant. So I'd vote for killing this check, it buys
us absolutely nothing and isn't even exhaustive in its current form.

-- 
Jens Axboe