From 38448181459e24257b40d5258afdbaa3565e8cfc Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Wed, 16 Apr 2025 09:45:39 -0400 Subject: mm: vmscan: restore high-cpu watermark safety in kswapd Vlastimil points out that commit a211c6550efc ("mm: page_alloc: defrag_mode kswapd/kcompactd watermarks") switched kswapd from zone_watermark_ok_safe() to the standard, percpu-cached version of reading free pages, thus dropping the watermark safety precautions for systems with high CPU counts (e.g. >212 cpus on 64G). Restore them. Since zone_watermark_ok_safe() is no longer the right interface, and this was the last caller of the function anyway, open-code the zone_page_state_snapshot() conditional and delete the function. Link: https://lkml.kernel.org/r/20250416135142.778933-2-hannes@cmpxchg.org Fixes: a211c6550efc ("mm: page_alloc: defrag_mode kswapd/kcompactd watermarks") Signed-off-by: Johannes Weiner Reported-by: Vlastimil Babka Reviewed-by: Vlastimil Babka Cc: Brendan Jackman Signed-off-by: Andrew Morton --- mm/vmscan.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) (limited to 'mm/vmscan.c') diff --git a/mm/vmscan.c b/mm/vmscan.c index b620d74b0f66..cc422ad830d6 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -6736,6 +6736,7 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx) * meet watermarks. */ for_each_managed_zone_pgdat(zone, pgdat, i, highest_zoneidx) { + enum zone_stat_item item; unsigned long free_pages; if (sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING) @@ -6748,9 +6749,25 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int highest_zoneidx) * blocks to avoid polluting allocator fallbacks. */ if (defrag_mode) - free_pages = zone_page_state(zone, NR_FREE_PAGES_BLOCKS); + item = NR_FREE_PAGES_BLOCKS; else - free_pages = zone_page_state(zone, NR_FREE_PAGES); + item = NR_FREE_PAGES; + + /* + * When there is a high number of CPUs in the system, + * the cumulative error from the vmstat per-cpu cache + * can blur the line between the watermarks. In that + * case, be safe and get an accurate snapshot. + * + * TODO: NR_FREE_PAGES_BLOCKS moves in steps of + * pageblock_nr_pages, while the vmstat pcp threshold + * is limited to 125. On many configurations that + * counter won't actually be per-cpu cached. But keep + * things simple for now; revisit when somebody cares. + */ + free_pages = zone_page_state(zone, item); + if (zone->percpu_drift_mark && free_pages < zone->percpu_drift_mark) + free_pages = zone_page_state_snapshot(zone, item); if (__zone_watermark_ok(zone, order, mark, highest_zoneidx, 0, free_pages)) -- cgit v1.2.3