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    Subject[PATCH 1/6] mm, compaction: more robust check for scanners meeting
    Date
    Compaction should finish when the migration and free scanner meet, i.e. they
    reach the same pageblock. Currently however, the test in compact_finished()
    simply just compares the exact pfns, which may yield a false negative when the
    free scanner position is in the middle of a pageblock and the migration scanner
    reaches the beginning of the same pageblock.

    This hasn't been a problem until commit e14c720efdd7 ("mm, compaction: remember
    position within pageblock in free pages scanner") allowed the free scanner
    position to be in the middle of a pageblock between invocations. The hot-fix
    1d5bfe1ffb5b ("mm, compaction: prevent infinite loop in compact_zone")
    prevented the issue by adding a special check in the migration scanner to
    satisfy the current detection of scanners meeting.

    However, the proper fix is to make the detection more robust. This patch
    introduces the compact_scanners_met() function that returns true when the free
    scanner position is in the same or lower pageblock than the migration scanner.
    The special case in isolate_migratepages() introduced by 1d5bfe1ffb5b is
    removed.

    Suggested-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Michal Nazarewicz <mina86@mina86.com>
    Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: David Rientjes <rientjes@google.com>
    ---
    mm/compaction.c | 22 ++++++++++++++--------
    1 file changed, 14 insertions(+), 8 deletions(-)

    diff --git a/mm/compaction.c b/mm/compaction.c
    index 16e1b57..d46aaeb 100644
    --- a/mm/compaction.c
    +++ b/mm/compaction.c
    @@ -902,6 +902,16 @@ static bool suitable_migration_target(struct page *page)
    }

    /*
    + * Test whether the free scanner has reached the same or lower pageblock than
    + * the migration scanner, and compaction should thus terminate.
    + */
    +static inline bool compact_scanners_met(struct compact_control *cc)
    +{
    + return (cc->free_pfn >> pageblock_order)
    + <= (cc->migrate_pfn >> pageblock_order);
    +}
    +
    +/*
    * Based on information in the current compact_control, find blocks
    * suitable for isolating free pages from and then isolate them.
    */
    @@ -1131,12 +1141,8 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
    }

    acct_isolated(zone, cc);
    - /*
    - * Record where migration scanner will be restarted. If we end up in
    - * the same pageblock as the free scanner, make the scanners fully
    - * meet so that compact_finished() terminates compaction.
    - */
    - cc->migrate_pfn = (end_pfn <= cc->free_pfn) ? low_pfn : cc->free_pfn;
    + /* Record where migration scanner will be restarted. */
    + cc->migrate_pfn = low_pfn;

    return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
    }
    @@ -1151,7 +1157,7 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
    return COMPACT_PARTIAL;

    /* Compaction run completes if the migrate and free scanner meet */
    - if (cc->free_pfn <= cc->migrate_pfn) {
    + if (compact_scanners_met(cc)) {
    /* Let the next compaction start anew. */
    zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
    zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
    @@ -1380,7 +1386,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
    * migrate_pages() may return -ENOMEM when scanners meet
    * and we want compact_finished() to detect it
    */
    - if (err == -ENOMEM && cc->free_pfn > cc->migrate_pfn) {
    + if (err == -ENOMEM && !compact_scanners_met(cc)) {
    ret = COMPACT_PARTIAL;
    goto out;
    }
    --
    2.1.4


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