Eric Lee / linux-smarc-t335x-v3.2

Download as
Browse Code ยป

Commit 7454f4ba40b419eb999a3c61a99da662bf1a2bb8

Authored by Mel Gorman
Committed by Linus Torvalds
1 parent a582a738c7
Exists in master and in 4 other branches v3.2_AM335xPSP_04.06.00.11, v3.2_NEWT335xPSP_04.06.00.11, v3.2_SBC_SMARTMEN, v3.2_SMARCT335xPSP_04.06.00.11

mm: compaction: ensure that the compaction free scanner does not move to the next zone 

Compaction works with two scanners, a migration and a free scanner.  When
the scanners crossover, migration within the zone is complete.  The
location of the scanner is recorded on each cycle to avoid excesive
scanning.

When a zone is small and mostly reserved, it's very easy for the migration
scanner to be close to the end of the zone.  Then the following situation
can occurs

  o migration scanner isolates some pages near the end of the zone
  o free scanner starts at the end of the zone but finds that the
    migration scanner is already there
  o free scanner gets reinitialised for the next cycle as
    cc->migrate_pfn + pageblock_nr_pages
    moving the free scanner into the next zone
  o migration scanner moves into the next zone

When this happens, NR_ISOLATED accounting goes haywire because some of the
accounting happens against the wrong zone.  One zones counter remains
positive while the other goes negative even though the overall global
count is accurate.  This was reported on X86-32 with !SMP because !SMP
allows the negative counters to be visible.  The fact that it is the bug
should theoritically be possible there.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Showing 1 changed file with 12 additions and 1 deletions Side-by-side Diff

... ... @@ -144,9 +144,20 @@
144 144 int nr_freepages = cc->nr_freepages;
145 145 struct list_head *freelist = &cc->freepages;
146 146  
  147 + /*
  148 + * Initialise the free scanner. The starting point is where we last
  149 + * scanned from (or the end of the zone if starting). The low point
  150 + * is the end of the pageblock the migration scanner is using.
  151 + */
147 152 pfn = cc->free_pfn;
148 153 low_pfn = cc->migrate_pfn + pageblock_nr_pages;
149   - high_pfn = low_pfn;
  154 +
  155 + /*
  156 + * Take care that if the migration scanner is at the end of the zone
  157 + * that the free scanner does not accidentally move to the next zone
  158 + * in the next isolation cycle.
  159 + */
  160 + high_pfn = min(low_pfn, pfn);
150 161  
151 162 /*
152 163 * Isolate free pages until enough are available to migrate the