Re: [RFC PATCH 2/2] mm, vmscan: do not loop on too_many_isolated for ever

[Mel Gorman]

On Wed, Jan 18, 2017 at 02:44:53PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Tetsuo Handa has reported [1] that direct reclaimers might get stuck in
> too_many_isolated loop basically for ever because the last few pages on
> the LRU lists are isolated by the kswapd which is stuck on fs locks when
> doing the pageout. This in turn means that there is nobody to actually
> trigger the oom killer and the system is basically unusable.
> 
> too_many_isolated has been introduced by 35cd78156c49 ("vmscan: throttle
> direct reclaim when too many pages are isolated already") to prevent
> from pre-mature oom killer invocations because back then no reclaim
> progress could indeed trigger the OOM killer too early. But since the
> oom detection rework 0a0337e0d1d1 ("mm, oom: rework oom detection")
> the allocation/reclaim retry loop considers all the reclaimable pages
> including those which are isolated - see 9f6c399ddc36 ("mm, vmscan:
> consider isolated pages in zone_reclaimable_pages") so we can loosen
> the direct reclaim throttling and instead rely on should_reclaim_retry
> logic which is the proper layer to control how to throttle and retry
> reclaim attempts.
> 
> Move the too_many_isolated check outside shrink_inactive_list because
> in fact active list might theoretically see too many isolated pages as
> well.
> 

No major objections in general. It's a bit odd you have a while loop for
something that will only loop once.

As for the TODO, one approach would be to use a waitqueue when too many
pages are isolated. Wake them one at a time when isolated pages drops
below the threshold.

-- 
Mel Gorman
SUSE Labs