• Hugh Dickins's avatar
    [PATCH] can_share_swap_page: use page_mapcount · c475a8ab
    Hugh Dickins authored
    Remember that ironic get_user_pages race?  when the raised page_count on a
    page swapped out led do_wp_page to decide that it had to copy on write, so
    substituted a different page into userspace.  2.6.7 onwards have Andrea's
    solution, where try_to_unmap_one backs out if it finds page_count raised.
    
    Which works, but is unsatisfying (rmap.c has no other page_count heuristics),
    and was found a few months ago to hang an intensive page migration test.  A
    year ago I was hesitant to engage page_mapcount, now it seems the right fix.
    
    So remove the page_count hack from try_to_unmap_one; and use activate_page in
    unuse_mm when dropping lock, to replace its secondary effect of helping
    swapoff to make progress in that case.
    
    Simplify can_share_swap_page (now called only on anonymous pages) to check
    page_mapcount + page_swapcount == 1: still needs the page lock to stabilize
    their (pessimistic) sum, but does not need swapper_space.tree_lock for that.
    
    In do_swap_page, move swap_free and unlock_page below page_add_anon_rmap, to
    keep sum on the high side, and correct when can_share_swap_page called.
    Signed-off-by: default avatarHugh Dickins <hugh@veritas.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
    c475a8ab
swapfile.c 40.9 KB