Re: [PATCH 5/6 mm] memcgroup: fix zone isolation OOM

[Balbir Singh <balbir-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

Hugh Dickins wrote:
> mem_cgroup_charge_common shows a tendency to OOM without good reason,
> when a memhog goes well beyond its rss limit but with plenty of swap
> available.  Seen on x86 but not on PowerPC; seen when the next patch
> omits swapcache from memcgroup, but we presume it can happen without.
> 
> mem_cgroup_isolate_pages is not quite satisfying reclaim's criteria
> for OOM avoidance.  Already it has to scan beyond the nr_to_scan limit
> when it finds a !LRU page or an active page when handling inactive or
> an inactive page when handling active.  It needs to do exactly the same
> when it finds a page from the wrong zone (the x86 tests had two zones,
> the PowerPC tests had only one).
> 
> Don't increment scan and then decrement it in these cases, just move
> the incrementation down.  Fix recent off-by-one when checking against
> nr_to_scan.  Cut out "Check if the meta page went away from under us",
> presumably left over from early debugging: no amount of such checks
> could save us if this list really were being updated without locking.
> 

It's a spill over from the old code, we do all operations under
the mem_cont's lru_lock.

> This change does make the unlimited scan while holding two spinlocks
> even worse - bad for latency and bad for containment; but that's a
> separate issue which is better left to be fixed a little later.
> 
> Signed-off-by: Hugh Dickins <hugh-DTz5qymZ9yRBDgjK7y7TUQ@public.gmane.org>

For the swapout test case scenario sent by Hugh

Tested-by: Balbir Singh <balbir-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL