• Mel Gorman's avatar
    mm: compaction: partially revert capture of suitable high-order page · 8fb74b9f
    Mel Gorman authored
    Eric Wong reported on 3.7 and 3.8-rc2 that ppoll() got stuck when
    waiting for POLLIN on a local TCP socket.  It was easier to trigger if
    there was disk IO and dirty pages at the same time and he bisected it to
    commit 1fb3f8ca ("mm: compaction: capture a suitable high-order page
    immediately when it is made available").
    
    The intention of that patch was to improve high-order allocations under
    memory pressure after changes made to reclaim in 3.6 drastically hurt
    THP allocations but the approach was flawed.  For Eric, the problem was
    that page->pfmemalloc was not being cleared for captured pages leading
    to a poor interaction with swap-over-NFS support causing the packets to
    be dropped.  However, I identified a few more problems with the patch
    including the fact that it can increase contention on zone->lock in some
    cases which could result in async direct compaction being aborted early.
    
    In retrospect the capture patch took the wrong approach.  What it should
    have done is mark the pageblock being migrated as MIGRATE_ISOLATE if it
    was allocating for THP and avoided races that way.  While the patch was
    showing to improve allocation success rates at the time, the benefit is
    marginal given the relative complexity and it should be revisited from
    scratch in the context of the other reclaim-related changes that have
    taken place since the patch was first written and tested.  This patch
    partially reverts commit 1fb3f8ca ("mm: compaction: capture a
    suitable high-order page immediately when it is made available").
    Reported-and-tested-by: default avatarEric Wong <normalperson@yhbt.net>
    Tested-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
    Cc: David Miller <davem@davemloft.net>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    8fb74b9f
compaction.c 32.4 KB