linux-kernel - Re: [RFC PATCH] mm: Fetch the dirty bit before we reset the pte

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Date:   Thu, 8 Oct 2020 10:02:42 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nick Piggin <npiggin@...il.com>, Peter Xu <peterx@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC PATCH] mm: Fetch the dirty bit before we reset the pte

On Thu, Oct 8, 2020 at 2:27 AM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> wrote:
>
> In copy_present_page, after we mark the pte non-writable, we should
> check for previous dirty bit updates and make sure we don't lose the dirty
> bit on reset.

No, we'll just remove that entirely.

Do you have a test-case that shows a problem? I have a patch that I
was going to delay until 5.10 because I didn't think it mattered in
practice..

The second part of this patch would be to add a sequence count
protection to fast-GUP pinning, so that GUP and fork() couldn't race,
but I haven't written that part.

Here's the first patch anyway. If you actually have a test-case where
this matters, I guess I need to apply it now..

                   Linus

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