Re: [patch 1/2]vmscan: correct all_unreclaimable for zone without lru pages
From: Shaohua Li <hidden>
Date: 2011-09-28 07:03:34
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:quoted
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!quoted
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@kvack.org. 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@kvack.org"> email@kvack.org </a>