On Wed, 2011-09-28 at 14:57 +0800, Minchan Kim wrote: > On Tue, Sep 27, 2011 at 03:23:04PM +0800, Shaohua Li wrote: > > I saw DMA zone always has ->all_unreclaimable set. The reason is the high zones > > are big, so zone_watermark_ok/_safe() will always return false with a high > > classzone_idx for DMA zone, because DMA zone's lowmem_reserve is big for a high > > classzone_idx. When kswapd runs into DMA zone, it doesn't scan/reclaim any > > pages(no pages in lru), but mark the zone as all_unreclaimable. This can > > happen in other low zones too. > > Good catch! > > > This is confusing and can potentially cause oom. Say a low zone has > > all_unreclaimable when high zone hasn't enough memory. Then allocating > > some pages in low zone(for example reading blkdev with highmem support), > > then run into direct reclaim. Since the zone has all_unreclaimable set, > > direct reclaim might reclaim nothing and an oom reported. If > > all_unreclaimable is unset, the zone can actually reclaim some pages. > > If all_unreclaimable is unset, in the inner loop of balance_pgdat we always have > > all_zones_ok 0 when checking a low zone's watermark. If high zone watermark isn't > > good, there is no problem. Otherwise, we might loop one more time in the outer > > loop, but since high zone watermark is ok, the end_zone will be lower, then low > > zone's watermark check will be ok and the outer loop will break. So looks this > > doesn't bring any problem. > > I think it would be better to correct zone_reclaimable. > My point is zone_reclaimable should consider zone->pages_scanned. > The point of the function is how many pages scanned VS how many pages remained in LRU. > If reclaimer doesn't scan the zone at all because of no lru pages, it shouldn't tell > the zone is all_unreclaimable. actually this is exact my first version of the patch. The problem is if a zone is true unreclaimable (used by kenrel pages or whatever), we will have zone->pages_scanned 0 too. I thought we should set all_unreclaimable in this case. Thanks, Shaohua -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>