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    SubjectRe: [PATCH] mm: reuse the unshared swapcache page in do_wp_page
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    On 2022/1/14 0:48, David Hildenbrand wrote:
    > On 13.01.22 17:37, Linus Torvalds wrote:
    >> On Thu, Jan 13, 2022 at 6:39 AM Matthew Wilcox <willy@infradead.org> wrote:
    >>>
    >>> Let's bring Linus in on this, but I think this reintroduces all of the
    >>> mapcount problems that we've been discussing recently.
    >>>
    >>> How about this as an alternative?
    >>
    >> No, at that point reuse_swap_page() is the better thing to do.
    >>
    >> Don't play games with page_count() (or even worse games with
    >> swap_count). The page count is only stable if it's 1. Any other value
    >> means that it can fluctuate due to concurrent lookups, some of which
    >> can be done locklessly under RCU.
    >
    > I'm pretty sure the patch as is will reintroduce the CVE. So I think in

    Actually, I wonder how reuse_swap_page() in this patch can reintroduce CVE,
    I think the invoking logic here is as same as that in do_swap_page().
    So, could you give me some hint about this? Thanks :)

    > addition to the reuse_swap_page() check we need more.
    >
    > I'm wondering if we can get rid of the mapcount checks in
    > reuse_swap_page() and instead check for page_count() and swapcount only.
    >
    > We don't care if it's unstable in a sense than it will be bigger than
    > what we expect. In that case we COW as we would already do.
    >
    > Thoughts?
    >

    --
    Best Regards,
    Liang Zhang

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