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Date:   Sat, 9 Jan 2021 17:37:09 -0800
From:   Linus Torvalds <>
To:     Andrea Arcangeli <>
Cc:     Andrew Morton <>,
        Linux-MM <>,
        Linux Kernel Mailing List <>,
        Yu Zhao <>, Andy Lutomirski <>,
        Peter Xu <>,
        Pavel Emelyanov <>,
        Mike Kravetz <>,
        Mike Rapoport <>,
        Minchan Kim <>,
        Will Deacon <>,
        Peter Zijlstra <>,
        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 0/1] mm: restore full accuracy in COW page reuse

On Sat, Jan 9, 2021 at 5:19 PM Linus Torvalds
<> wrote:
> And no, I didn't make the UFFDIO_WRITEPROTECT code take the mmap_sem
> for writing. For whoever wants to look at that, it's
> mwriteprotect_range() in mm/userfaultfd.c and the fix is literally to
> turn the read-lock (and unlock) into a write-lock (and unlock).

Oh, and if it wasn't obvious, we'll have to debate what to do with
trying to mprotect() a pinned page. Do we just ignore the pinned page
(the way my clear_refs patch did)? Or do we turn it into -EBUSY? Or

So it's not *just* the locking that needs to be fixed. But just take a
look at that suggested clear_refs patch of mine - it sure isn't


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