On Tue 07-06-11 16:07:02, Mel Gorman wrote: > 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 > difficult to reproduce on X86-64 is probably just a co-incidence as > the bug should theoritically be possible there. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx> > Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx> Reviewed-by: Michal Hocko <mhocko@xxxxxxx> -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>