linux-kernel - Re: [PATCH 1/4] mm: Trial do_wp

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Message-ID: <24b8ad6b-4e4c-4651-86e6-bc2232fce772@nvidia.com>
Date:   Thu, 17 Sep 2020 13:19:02 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Peter Xu <peterx@...hat.com>, Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        "Yang Shi" <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        "Kirill Shutemov" <kirill@...temov.name>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Christoph Hellwig" <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

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
>