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Date:   Sat, 9 Jan 2021 21:54:05 -0500
From:   Andrea Arcangeli <>
To:     Andrew Morton <>,
Cc:, Yu Zhao <>,
        Andy Lutomirski <>,
        Peter Xu <>,
        Pavel Emelyanov <>,
        Mike Kravetz <>,
        Mike Rapoport <>,
        Minchan Kim <>,
        Will Deacon <>,
        Peter Zijlstra <>,
        Linus Torvalds <>,
        Hugh Dickins <>,
        "Kirill A. Shutemov" <>,
        Matthew Wilcox <>,
        Oleg Nesterov <>, Jann Horn <>,
        Kees Cook <>,
        John Hubbard <>,
        Leon Romanovsky <>,
        Jason Gunthorpe <>, Jan Kara <>,
        Kirill Tkhai <>,
        Nadav Amit <>, Jens Axboe <>
Subject: Re: [PATCH 1/1] mm: restore full accuracy in COW page reuse


On Sat, Jan 09, 2021 at 07:44:35PM -0500, Andrea Arcangeli wrote:
> allowing a child to corrupt memory in the parent. That's a problem
> that could happen not-maliciously too. So the scenario described

I updated the above partly quoted sentence since in the previous
version it didn't have full accuracy:

"However since a single transient GUP pin on a tail page, would elevate
the page_count for all other tail pages (unlike the mapcount which is
subpage granular), the COW page reuse inaccuracy would then cross
different vmas and the effect would happen at a distance in vma of
different processes. A single GUP pin taken on a subpage mapped in a
different process could trigger 511 false positive COWs copies in the
local process, after a fork()."

This a best effort to try to document all side effects, but it'd be
great to hear from Kirill too on the above detail to have

Thanks and have a great weekend everyone,

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