On Tue, Oct 29, 2024 at 10:46 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Mon, Oct 28, 2024 at 12:26:53PM -0600, Yu Zhao wrote: > > OOM kills due to vastly overestimated free highatomic reserves were > > observed: > > > > ... invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0 ... > > Node 0 Normal free:1482936kB boost:0kB min:410416kB low:739404kB high:1068392kB reserved_highatomic:1073152KB ... > > Node 0 Normal: 1292*4kB (ME) 1920*8kB (E) 383*16kB (UE) 220*32kB (ME) 340*64kB (E) 2155*128kB (UE) 3243*256kB (UE) 615*512kB (U) 1*1024kB (M) 0*2048kB 0*4096kB = 1477408kB > > > > The second line above shows that the OOM kill was due to the following > > condition: > > > > free (1482936kB) - reserved_highatomic (1073152kB) = 409784KB < min (410416kB) > > > > And the third line shows there were no free pages in any > > MIGRATE_HIGHATOMIC pageblocks, which otherwise would show up as type > > 'H'. Therefore __zone_watermark_unusable_free() underestimated the > > usable free memory by over 1GB, which resulted in the unnecessary OOM > > kill above. > > > > The comments in __zone_watermark_unusable_free() warns about the > > potential risk, i.e., > > > > If the caller does not have rights to reserves below the min > > watermark then subtract the high-atomic reserves. This will > > over-estimate the size of the atomic reserve but it avoids a search. > > > > However, it is possible to keep track of free pages in reserved > > highatomic pageblocks with a new per-zone counter nr_free_highatomic > > protected by the zone lock, to avoid a search when calculating the > > usable free memory. And the cost would be minimal, i.e., simple > > arithmetics in the highatomic alloc/free/move paths. > > > > Note that since nr_free_highatomic can be relatively small, using a > > per-cpu counter might cause too much drift and defeat its purpose, > > in addition to the extra memory overhead. > > > > Reported-by: Link Lin <linkl@xxxxxxxxxx> > > Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx> > > Acked-by: David Rientjes <rientjes@xxxxxxxxxx> > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > > @@ -642,6 +644,9 @@ static inline void account_freepages(struct zone *zone, int nr_pages, > > > > if (is_migrate_cma(migratetype)) > > __mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages); > > + > > + if (is_migrate_highatomic(migratetype)) > > + WRITE_ONCE(zone->nr_free_highatomic, zone->nr_free_highatomic + nr_pages); > > Minor nit, the page can only be of one migratetype, so `else if' would > be better. Right (copied and pasted without thinking). Andrew, could you please fix this up in place? Thank you!
