Just to be clear, this is not fixed by Dave's patch to NR_FREE_PAGES accounting. I can still get 3.7-rc5 + Dave's fix to drop into an endless loop in kswapd within a couple of minutes on my test box. As described below, the bug comes from contradicting conditions in balance_pgdat(), not an accounting problem. On Tue, Nov 20, 2012 at 02:04:41PM -0500, Johannes Weiner wrote: > Hi guys, > > while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into > a busy spin state without doing reclaim. printk-style debugging told > me that this happens when the distance between a zone's high watermark > and its low watermark is less than two huge pages (DMA zone). > > 1. The first loop in balance_pgdat() over the zones finds all zones to > be above their high watermark and only does goto out (all_zones_ok). > > 2. pgdat_balanced() at the out: label also just checks the high > watermark, so the node is considered balanced and the order is not > reduced. > > 3. In the `if (order)' block after it, compaction_suitable() checks if > the zone's low watermark + twice the huge page size is okay, which > it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it > it go back to loop_again:. > > This will go on until somebody else allocates and breaches the high > watermark and then hopefully goes on to reclaim the zone above low > watermark + 2 * THP. > > I'm not really sure what the correct solution is. Should we modify > the zone_watermark_ok() checks in balance_pgdat() to take into account > the higher watermark requirements for reclaim on behalf of compaction? > Change the check in compaction_suitable() / not use it directly? > > Thanks, > Johannes > > -- > 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> -- 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>