• Michel Lespinasse's avatar
    mm anon rmap: replace same_anon_vma linked list with an interval tree. · bf181b9f
    Michel Lespinasse authored
    When a large VMA (anon or private file mapping) is first touched, which
    will populate its anon_vma field, and then split into many regions through
    the use of mprotect(), the original anon_vma ends up linking all of the
    vmas on a linked list.  This can cause rmap to become inefficient, as we
    have to walk potentially thousands of irrelevent vmas before finding the
    one a given anon page might fall into.
    
    By replacing the same_anon_vma linked list with an interval tree (where
    each avc's interval is determined by its vma's start and last pgoffs), we
    can make rmap efficient for this use case again.
    
    While the change is large, all of its pieces are fairly simple.
    
    Most places that were walking the same_anon_vma list were looking for a
    known pgoff, so they can just use the anon_vma_interval_tree_foreach()
    interval tree iterator instead.  The exception here is ksm, where the
    page's index is not known.  It would probably be possible to rework ksm so
    that the index would be known, but for now I have decided to keep things
    simple and just walk the entirety of the interval tree there.
    
    When updating vma's that already have an anon_vma assigned, we must take
    care to re-index the corresponding avc's on their interval tree.  This is
    done through the use of anon_vma_interval_tree_pre_update_vma() and
    anon_vma_interval_tree_post_update_vma(), which remove the avc's from
    their interval tree before the update and re-insert them after the update.
     The anon_vma stays locked during the update, so there is no chance that
    rmap would miss the vmas that are being updated.
    Signed-off-by: default avatarMichel Lespinasse <walken@google.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Daniel Santos <daniel.santos@pobox.com>
    Cc: Hugh Dickins <hughd@google.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    bf181b9f
interval_tree.c 2.2 KB