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Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
To: Jason Gunthorpe <jgg@ziepe.ca>, Linus Torvalds
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From: John Hubbard <jhubbard@nvidia.com>
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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.

thanks,
-- 
John Hubbard
NVIDIA

> 
> Hmm, there are many users. I remember that the hfi1 folks really
> wanted the fast version for some reason..
> 
>> Because if we only start copying the page *iff* the vma is marked by
>> that "this vma had page pinning" _and_ the page count is bigger than
>> GUP_PIN_COUNTING_BIAS, than I think we can rest pretty easily knowing
>> that we aren't going to hit some regular old-fashioned UNIX server
>> cases with a lot of forks..
> 
> Agree
> 
> Given that this is a user visible regression, it is nearly rc6, what
> do you prefer for next steps?
> 
> Sorting out this for fork, especially if it has the vma change is
> probably more than a weeks time.
> 
> Revert this patch and try again next cycle?
> 
> Thanks,
> Jason
>