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    SubjectRe: [patch 2/8] mm: vmscan: disregard swappiness shortly before going OOM
    On Thu, Dec 13, 2012 at 10:34:20AM +0000, Mel Gorman wrote:
    > On Wed, Dec 12, 2012 at 04:43:34PM -0500, Johannes Weiner wrote:
    > > When a reclaim scanner is doing its final scan before giving up and
    > > there is swap space available, pay no attention to swappiness
    > > preference anymore. Just swap.
    > >
    > > Note that this change won't make too big of a difference for general
    > > reclaim: anonymous pages are already force-scanned when there is only
    > > very little file cache left, and there very likely isn't when the
    > > reclaimer enters this final cycle.
    > >
    > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
    >
    > Ok, I see the motivation for your patch but is the block inside still
    > wrong for what you want? After your patch the block looks like this
    >
    > if (sc->priority || noswap) {
    > scan >>= sc->priority;
    > if (!scan && force_scan)
    > scan = SWAP_CLUSTER_MAX;
    > scan = div64_u64(scan * fraction[file], denominator);
    > }
    >
    > if sc->priority == 0 and swappiness==0 then you enter this block but
    > fraction[0] for anonymous pages will also be 0 and because of the ordering
    > of statements there, scan will be
    >
    > scan = scan * 0 / denominator
    >
    > so you are still not reclaiming anonymous pages in the swappiness=0
    > case. What did I miss?

    Don't get confused by noswap, it is only set when there physically is
    no swap space. If !sc->priority, that block is skipped and
    fraction[0] does not matter.


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