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    SubjectRe: [PATCH] mm: compaction: Abort compaction if too many pages are isolated and caller is asynchronous
    On Tue, May 31, 2011 at 11:38:30PM +0900, Minchan Kim wrote:
    > > Yes. You find a new BUG.
    > > It seems to be related to this problem but it should be solved although
    >
    > typo : It doesn't seem to be.

    This should fix it, but I doubt it matters for this problem.

    ===
    Subject: mm: no page_count without a page pin

    From: Andrea Arcangeli <aarcange@redhat.com>

    It's unsafe to run page_count during the physical pfn scan.

    Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
    ---

    diff --git a/mm/vmscan.c b/mm/vmscan.c
    index faa0a08..e41e78a 100644
    --- a/mm/vmscan.c
    +++ b/mm/vmscan.c
    @@ -1124,8 +1124,18 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
    nr_lumpy_dirty++;
    scan++;
    } else {
    - /* the page is freed already. */
    - if (!page_count(cursor_page))
    + /*
    + * We can't use page_count() as that
    + * requires compound_head and we don't
    + * have a pin on the page here. If a
    + * page is tail, we may or may not
    + * have isolated the head, so assume
    + * it's not free, it'd be tricky to
    + * track the head status without a
    + * page pin.
    + */
    + if (!PageTail(cursor_page) &&
    + !atomic_read(&cursor_page->_count))
    continue;
    break;
    }

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