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    Su bject: Re: [PATCH] mm: fix special case -1 order check in compact_finished
    Reply-To:
    In-Reply-To: <20110530123831.GG20166@tiehlicka.suse.cz>

    Hi Michal,

    On Mon, May 30, 2011 at 02:38:31PM +0200, Michal Hocko wrote:
    > 56de7263 (mm: compaction: direct compact when a high-order allocation
    > fails) introduced a check for cc->order == -1 in compact_finished. We
    > should continue compacting in that case because the request came from
    > userspace and there is no particular order to compact for.
    >
    > The check is, however, done after zone_watermark_ok which uses order as
    > a right hand argument for shifts. Not only watermark check is pointless
    > if we can break out without it but it also uses 1 << -1 which is not
    > well defined (at least from C standard). Let's move the -1 check above
    > zone_watermark_ok.
    >
    > Signed-off-by: Michal Hocko <mhocko@suse.cz>
    > Cc: Mel Gorman <mgorman@suse.de>
    > ---
    > compaction.c | 14 +++++++-------
    > 1 file changed, 7 insertions(+), 7 deletions(-)
    > Index: linus_tree/mm/compaction.c
    > ===================================================================
    > --- linus_tree.orig/mm/compaction.c 2011-05-30 14:19:58.000000000 +0200
    > +++ linus_tree/mm/compaction.c 2011-05-30 14:20:40.000000000 +0200
    > @@ -420,13 +420,6 @@ static int compact_finished(struct zone
    > if (cc->free_pfn <= cc->migrate_pfn)
    > return COMPACT_COMPLETE;
    >
    > - /* Compaction run is not finished if the watermark is not met */
    > - watermark = low_wmark_pages(zone);
    > - watermark += (1 << cc->order);
    > -
    > - if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
    > - return COMPACT_CONTINUE;
    > -
    > /*
    > * order == -1 is expected when compacting via
    > * /proc/sys/vm/compact_memory
    > @@ -434,6 +427,13 @@ static int compact_finished(struct zone
    > if (cc->order == -1)
    > return COMPACT_CONTINUE;
    >
    > + /* Compaction run is not finished if the watermark is not met */
    > + watermark = low_wmark_pages(zone);
    > watermark += (1 << cc->order);
    > +
    > + if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
    > + return COMPACT_CONTINUE;
    > +
    > /* Direct compactor: Is a suitable page free? */
    > for (order = cc->order; order < MAX_ORDER; order++) {
    > /* Job done if page is free of the right migratetype */

    It looks good to me.
    Let's think about another place, compaction_suitable.
    It has same problem so we can move the check right before zone_watermark_ok.
    As I look it more, I thought we need free pages for compaction so we would
    be better to give up early if we can't get enough free pages. But I changed
    my mind. It's a totally user request and we can get free pages in migration
    progress(ex, other big memory hogger might free his big rss).
    So my conclusion is that we should do *best effort* than early give up.
    If you agree with me, how about resending patch with compaction_suitable fix?

    > --
    > Michal Hocko
    > SUSE Labs
    > SUSE LINUX s.r.o.
    > Lihovarska 1060/12
    > 190 00 Praha 9
    > Czech Republic
    >
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