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    SubjectRe: [syzbot] kernel BUG in __page_mapcount
    On Tue, Dec 21, 2021 at 10:40 AM Matthew Wilcox <willy@infradead.org> wrote:
    >
    > On Tue, Dec 21, 2021 at 10:24:27AM -0800, Yang Shi wrote:
    > > It seems the THP is split during smaps walk. The reproducer does call
    > > MADV_FREE on partial THP which may split the huge page.
    > >
    > > The below fix (untested) should be able to fix it.
    >
    > Did you read the rest of the thread on this? If the page is being

    I just revisited this. Now I see what you mean about "the rest of the
    thread". My gmail client doesn't put them in the same thread, sigh...

    Yeah, try_get_compound_head() seems like the right way.

    Or we just simply treat migration entries as mapcount == 1 as Kirill
    suggested or just skip migration entries since they are transient or
    show migration entries separately.


    > migrated, we should still account it ... also, you've changed the
    > refcount, so this:
    >
    > if (page_count(page) == 1) {
    > smaps_page_accumulate(mss, page, size, size << PSS_SHIFT, dirty,
    > locked, true);
    > return;
    > }
    >
    > will never trigger.

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