On Mon, Nov 25, 2013 at 03:26:07PM +0100, Vlastimil Babka wrote: > Compaction caches pfn's for its migrate and free scanners to avoid scanning > the whole zone each time. In compact_zone(), the cached values are read to > set up initial values for the scanners. There are several situations when > these cached pfn's are reset to the first and last pfn of the zone, > respectively. One of these situations is when a compaction has been deferred > for a zone and is now being restarted during a direct compaction, which is also > done in compact_zone(). > > However, compact_zone() currently reads the cached pfn's *before* resetting > them. This means the reset doesn't affect the compaction that performs it, and > with good chance also subsequent compactions, as update_pageblock_skip() is > likely to be called and update the cached pfn's to those being processed. > Another chance for a successful reset is when a direct compaction detects that > migration and free scanners meet (which has its own problems addressed by > another patch) and sets update_pageblock_skip flag which kswapd uses to do the > reset because it goes to sleep. > > This is clearly a bug that results in non-deterministic behavior, so this patch > moves the cached pfn reset to be performed *before* the values are read. > > Cc: Mel Gorman <mgorman@xxxxxxx> > Cc: Rik van Riel <riel@xxxxxxxxxx> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> Acked-by: Mel Gorman <mgorman@xxxxxxx> -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>