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    SubjectRe: [PATCH 04/21] mm: zap_pte_range dont dirty anon
    Hugh Dickins <hugh@veritas.com> wrote:
    >
    > > What is the page is (for example) clean swapcache, having been recently
    > > faulted in. If this pte indicates that this process has modified the page
    > > and we don't run set_page_dirty(), the page could be reclaimed and the
    > > change is lost.
    >
    > Absolutely. But either the page is unique to this mm, shared only with
    > swapcache: in which case we're about to do a free_swap_cache on it (that
    > may be delayed in actually freeing the swap because of not getting page
    > lock, presumably because vmscan just got to it, but no matter), and we
    > don't care at all that the page no longer represents what's on swap disk.
    >
    > Or, the page is shared with another mm. But it's an anonymous page
    > (a private page), so it's shared via fork, and COW applies to it.

    mmap(MAP_ANONYMOUS|MAP_SHARED)
    fork()
    swapout
    swapin
    swapoff

    Now we have two mm's sharing a clean, non-cowable, non-swapcache anonymous
    page, no?
    -
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