Re: [patch 1/2]vmscan: correct all_unreclaimable for zone without lru pages

[Shaohua Li <shaohua.li@intel.com>]

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?

Thanks,
Shaohua

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