On Thu, 2011-09-29 at 17:18 +0800, Minchan Kim wrote: > 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. it does something, but might not reclaim any pages, for example, it starts page write, but page isn't in disk yet in DEF_PRIORITY and it skip further reclaiming 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? The zone could have lru pages. Let's take an example, allocation from ZONE_HIGHMEM, then kswapd runs, ZONE_NORMAL gets all_unreclaimable set even it has free pages. Then we do write blkdev device, which use ZONE_NORMAL for page cache. Some pages in ZONE_NORMAL are in lru, then we run into direct page reclaim for ZONE_NORMAL. Since all_unreclaimable is set and pages in ZONE_NORMAL lru are dirty, direct reclaim could fail. But I'd agree this is a corner case. Besides when I saw ZONE_DMA has a lot of free pages and all_unreclaimable is set, it's really confusing. 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>