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    SubjectRe: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount
    On Tue, Jan 24, 2023 at 5:58 PM Zach O'Keefe <zokeefe@google.com> wrote:
    >
    > During collapse, in a few places we check to see if a given small page
    > has any unaccounted references. If the refcount on the page doesn't
    > match our expectations, it must be there is an unknown user concurrently
    > interested in the page, and so it's not safe to move the contents
    > elsewhere. However, the unaccounted pins are likely an ephemeral state.
    >
    > In such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that
    > collapse may succeed on retry.

    The page may be DMA pinned (for example, pin_user_pages()), it is not
    worth retrying for such pages. But it may also not be worth optimizing
    for this case at this point.

    So the patch looks good to me. Reviewed-by: Yang Shi <shy828301@gmail.com>

    >
    > Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
    > Reported-by: Hugh Dickins <hughd@google.com>
    > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
    >
    > ---
    > mm/khugepaged.c | 1 +
    > 1 file changed, 1 insertion(+)
    >
    > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
    > index e23619bfecc4..fa38cae240b9 100644
    > --- a/mm/khugepaged.c
    > +++ b/mm/khugepaged.c
    > @@ -2712,6 +2712,7 @@ static int madvise_collapse_errno(enum scan_result r)
    > case SCAN_CGROUP_CHARGE_FAIL:
    > return -EBUSY;
    > /* Resource temporary unavailable - trying again might succeed */
    > + case SCAN_PAGE_COUNT:
    > case SCAN_PAGE_LOCK:
    > case SCAN_PAGE_LRU:
    > case SCAN_DEL_PAGE_LRU:
    > --
    > 2.39.1.405.gd4c25cc71f-goog
    >

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