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    SubjectRe: [PATCH] mm: munlock use mapcount to avoid terrible overhead
    On Tue, 18 Oct 2011 17:02:56 -0700 (PDT)
    Hugh Dickins <hughd@google.com> wrote:

    > A process spent 30 minutes exiting, just munlocking the pages of a large
    > anonymous area that had been alternately mprotected into page-sized vmas:
    > for every single page there's an anon_vma walk through all the other
    > little vmas to find the right one.
    >
    > A general fix to that would be a lot more complicated (use prio_tree on
    > anon_vma?), but there's one very simple thing we can do to speed up the
    > common case: if a page to be munlocked is mapped only once, then it is
    > our vma that it is mapped into, and there's no need whatever to walk
    > through all the others.
    >
    > Okay, there is a very remote race in munlock_vma_pages_range(), if
    > between its follow_page() and lock_page(), another process were to
    > munlock the same page, then page reclaim remove it from our vma, then
    > another process mlock it again. We would find it with page_mapcount
    > 1, yet it's still mlocked in another process. But never mind, that's
    > much less likely than the down_read_trylock() failure which munlocking
    > already tolerates (in try_to_unmap_one()): in due course page reclaim
    > will discover and move the page to unevictable instead.
    >

    And how long did the test case take with the patch applied?

    > ---
    > mm/mlock.c | 5 ++++-
    > 1 file changed, 4 insertions(+), 1 deletion(-)
    >
    > --- 3.1-rc10/mm/mlock.c 2011-07-21 19:17:23.000000000 -0700
    > +++ linux/mm/mlock.c 2011-10-06 12:47:54.670436979 -0700
    > @@ -110,7 +110,10 @@ void munlock_vma_page(struct page *page)
    > if (TestClearPageMlocked(page)) {
    > dec_zone_page_state(page, NR_MLOCK);
    > if (!isolate_lru_page(page)) {
    > - int ret = try_to_munlock(page);
    > + int ret = SWAP_AGAIN;
    > +
    > + if (page_mapcount(page) > 1)
    > + ret = try_to_munlock(page);
    > /*
    > * did try_to_unlock() succeed or punt?
    > */

    tsk.

    --- a/mm/mlock.c~mm-munlock-use-mapcount-to-avoid-terrible-overhead-fix
    +++ a/mm/mlock.c
    @@ -112,6 +112,11 @@ void munlock_vma_page(struct page *page)
    if (!isolate_lru_page(page)) {
    int ret = SWAP_AGAIN;

    + /*
    + * Optimization: if the page was mapped just once,
    + * that's our mapping and we don't need to check all the
    + * other vmas.
    + */
    if (page_mapcount(page) > 1)
    ret = try_to_munlock(page);
    /*
    _


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