Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

[Jason Gunthorpe]

On Thu, Sep 17, 2020 at 01:19:02PM -0700, John Hubbard wrote:
> On 9/17/20 1:06 PM, Jason Gunthorpe wrote:
> > On Thu, Sep 17, 2020 at 12:42:11PM -0700, Linus Torvalds wrote:
> ...
> > > Is there possibly somethign else we can filter on than just
> > > GUP_PIN_COUNTING_BIAS? Because it could be as simple as just marking
> > > the vma itself and saying "this vma has had a page pinning event done
> > > on it".
> > 
> > We'd have to give up pin_user_pages_fast() to do that as we can't fast
> > walk and get vmas?
> 
> oops, yes. I'd forgotten about that point. Basically all of the O_DIRECT
> callers need _fast. This is a big problem.

What about an atomic counter in the mm_struct? 

# of pages currently under pin.

The use case Linus is worried about will never have any pins, so it
would be 0 and it could skip this entire test.

Jason