From 38448181459e24257b40d5258afdbaa3565e8cfc Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Wed, 16 Apr 2025 09:45:39 -0400 Subject: [PATCH] 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 --- include/linux/mmzone.h | 2 -- mm/page_alloc.c | 12 ------------ mm/vmscan.c | 21 +++++++++++++++++++-- 3 files changed, 19 insertions(+), 16 deletions(-) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 4c95fcc9e9df..6ccec1bf2896 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -1502,8 +1502,6 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark, bool zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark, int highest_zoneidx, unsigned int alloc_flags); -bool zone_watermark_ok_safe(struct zone *z, unsigned int order, - unsigned long mark, int highest_zoneidx); /* * Memory initialization context, use to differentiate memory added by * the platform statically or via memory hotplug interface. diff --git a/mm/page_alloc.c b/mm/page_alloc.c index e506e365d6f1..5669baf2a6fe 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3470,18 +3470,6 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order, return false; } -bool zone_watermark_ok_safe(struct zone *z, unsigned int order, - unsigned long mark, int highest_zoneidx) -{ - long free_pages = zone_page_state(z, NR_FREE_PAGES); - - if (z->percpu_drift_mark && free_pages < z->percpu_drift_mark) - free_pages = zone_page_state_snapshot(z, NR_FREE_PAGES); - - return __zone_watermark_ok(z, order, mark, highest_zoneidx, 0, - free_pages); -} - #ifdef CONFIG_NUMA int __read_mostly node_reclaim_distance = RECLAIM_DISTANCE; 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)) -- 2.49.0