On Thu 02-02-17 11:44:22, Michal Hocko wrote: > On Tue 31-01-17 14:32:08, Vinayak Menon wrote: > > During global reclaim, the nr_reclaimed passed to vmpressure > > includes the pages reclaimed from slab. But the corresponding > > scanned slab pages is not passed. This can cause total reclaimed > > pages to be greater than scanned, causing an unsigned underflow > > in vmpressure resulting in a critical event being sent to root > > cgroup. So do not consider reclaimed slab pages for vmpressure > > calculation. The reclaimed pages from slab can be excluded because > > the freeing of a page by slab shrinking depends on each slab's > > object population, making the cost model (i.e. scan:free) different > > from that of LRU. > > This might be true but what happens if the slab reclaim contributes > significantly to the overal reclaim? This would be quite rare but not > impossible. > > I am wondering why we cannot simply make cap nr_reclaimed to nr_scanned > and be done with this all? Sure it will be imprecise but the same will > be true with this approach. In other words something as "beautiful" as the following: diff --git a/mm/vmpressure.c b/mm/vmpressure.c index 149fdf6c5c56..abea42817dd0 100644 --- a/mm/vmpressure.c +++ b/mm/vmpressure.c @@ -236,6 +236,15 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree, return; /* + * Due to accounting issues - e.g. THP contributing 1 to scanned but + * potentially much more to reclaimed or SLAB pages not contributing + * to scanned at all - we have to skew reclaimed to prevent from + * wrong pressure levels due to overflows. + */ + if (reclaimed > scanned) + reclaimed = scanned; + + /* * If we got here with no pages scanned, then that is an indicator * that reclaimer was unable to find any shrinkable LRUs at the * current scanning depth. But it does not mean that we should -- Michal Hocko SUSE Labs -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
