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

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Message-ID: <29a76f98-ddc5-4fc1-92ed-3eff01130b11@nvidia.com>
Date:   Thu, 17 Sep 2020 12:55:34 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>
CC:     Jason Gunthorpe <jgg@...pe.ca>,
        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 12:42 PM, Linus Torvalds wrote:
> On Thu, Sep 17, 2020 at 12:03 PM Peter Xu <peterx@...hat.com> 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".
> 
> 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..
> 
>                         Linus

OK, so this sounds like an improvement that would be desirable in the
core API:

1) Have all the pin_user_pages*() functions reach up and mark the vma(s)
as FOLL_PIN_HAPPENED. I'm assuming that the vma can live it's full life
without the need to ever clear that flag. That *seems* reasonable, based
on the use cases for these kinds of pages.

2) And then rename:

    page_maybe_dma_pinned() --> page_likely_dma_pinned()

:)


thanks,
-- 
John Hubbard
NVIDIA