On Thu, Sep 29, 2011 at 09:14:51AM +0800, Shaohua Li wrote: > On Thu, 2011-09-29 at 01:57 +0800, Minchan Kim wrote: > > On Wed, Sep 28, 2011 at 03:08:31PM +0800, Shaohua Li wrote: > > > 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. > > > > Let's think the problem again. > > Fundamental problem is that why the lower zone's lowmem_reserve for higher zone is huge big > > that might be bigger than the zone's size. > > I think we need the boundary for limiting lowmem_reseve. > > So how about this? > I didn't see a reason why high zone allocation should fallback to low > zone if high zone is big. Changing the lowmem_reserve can cause the > fallback. Has any rationale here? I try to think better solution than yours but I got failed. :( The why I try to avoid your patch is that kswapd is very complicated these days so I wanted to not add more logic for handling corner cases if we can solve it other ways. But as I said, but I got failed. It seems that it doesn't make sense that previous my patch that limit lowmem_reserve. Because we can have higher zone which is very big size so that lowmem_zone[higher_zone] of low zone could be bigger freely than the lowmem itself size. It implies the low zone should be not used for higher allocation. It has no reason to limit it. My brain was broken. :( But I have a question about your patch still. What happens if DMA zone sets zone->all_unreclaimable with 1? You said as follows, > 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, If low zone has enough pages for allocation, it cannot have entered reclaim. It means now low zone doesn't have enough free pages for the order allocation. So it's natural to enter reclaim path. > direct reclaim might reclaim nothing and an oom reported. If It's not correct "nothing". At least, it will do something in DEF_PRIORITY. > all_unreclaimable is unset, the zone can actually reclaim some pages. The reason of this problem is that the zone has no lru page, you said. Then how could we reclaim some pages in the zone even if the zone's all_unreclaimable is unset? You expect slab pages? But our heuristic of setting all_unreclaimable in kswapd is that we consider we can't reclaim any slab pages any more(ie, nr_slab == 0) as well as too many lru scanning. So I think we should not depend on some luck which can reclaim some slab pages. If I misunderstood your point, could you elaborate more? The reason I am very picky about this is that I's really like to avoid omplicating kswapd without any real problem. > > Thanks, > Shaohua > -- Kind regards, Minchan Kim -- 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/ . 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