[PATCH v2 4/5] mm/page_alloc: keep a zone's pagesets when it goes empty

["Gregory Price (Meta)" <gourry@gourry.net>]

When offlining empties a zone, zone_pcp_reset() frees its pagesets while
the vmstat worker can still hold one across cond_resched():

  CPU 0                             CPU 1
  refresh_cpu_vm_stats()
    pcp = zone->per_cpu_pageset
    cond_resched()
                                    offline_pages()
                                      zone_pcp_reset()
                                        free_percpu(zone->per_cpu_pageset)
    decay_pcp_high(zone, this_cpu_ptr(pcp))

decay_pcp_high() then reads, and can write, memory the percpu allocator
may already have handed out again. With kprobes holding the worker
across the offline, it rewrote a new allocation at the freed address in
2 of 3 runs.

Keep the pagesets for the life of the zone, as pgdat->per_cpu_nodestats
already is, and only fold the stats in zone_pcp_reset().

Commit 340175b7d14d ("mm/hotplug: free zone->pageset when a zone becomes
empty") freed them because onlining the zone again allocated a new set
and leaked the old one. setup_zone_pageset() now reuses the existing
set, and zone_pcp_init() keeps it when the node is re-added, so nothing
is allocated twice. The pageset address stayed the same across repeated
offline/online cycles and across removing and re-adding the node.

The pagesets take 320 bytes per possible CPU: 30KiB per zone with 96
CPUs, 160KiB with 512 and 2.5MiB with 8192. A populated zone already
holds this memory, we just no longer give it back on offline.

Fixes: 340175b7d14d ("mm/hotplug: free zone->pageset when a zone becomes empty")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 mm/page_alloc.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 75a440ebcead6..417c427b196d9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6356,6 +6356,12 @@ void __meminit setup_zone_pageset(struct zone *zone)
 {
 	int cpu;
 
+	/* Reuse the pagesets zone_pcp_reset() kept. */
+	if (zone->per_cpu_pageset != &boot_pageset) {
+		zone_set_pageset_high_and_batch(zone, 0);
+		return;
+	}
+
 	/* Size may be 0 on !SMP && !NUMA */
 	if (sizeof(struct per_cpu_zonestat) > 0)
 		zone->per_cpu_zonestats = alloc_percpu(struct per_cpu_zonestat);
@@ -6452,9 +6458,12 @@ __meminit void zone_pcp_init(struct zone *zone)
 	 * per cpu subsystem is not up at this point. The following code
 	 * relies on the ability of the linker to provide the
 	 * offset of a (static) per cpu variable into the per cpu area.
+	 * A re-added node keeps the pagesets zone_pcp_reset() kept.
 	 */
-	zone->per_cpu_pageset = &boot_pageset;
-	zone->per_cpu_zonestats = &boot_zonestats;
+	if (!zone->per_cpu_pageset || zone->per_cpu_pageset == &boot_pageset) {
+		zone->per_cpu_pageset = &boot_pageset;
+		zone->per_cpu_zonestats = &boot_zonestats;
+	}
 	zone->pageset_high_min = BOOT_PAGESET_HIGH;
 	zone->pageset_high_max = BOOT_PAGESET_HIGH;
 	zone->pageset_batch = BOOT_PAGESET_BATCH;
@@ -7728,6 +7737,10 @@ void zone_pcp_enable(struct zone *zone)
 	mutex_unlock(&pcp_batch_high_lock);
 }
 
+/*
+ * Keep the pagesets of an emptied zone: refresh_cpu_vm_stats() can hold
+ * one across cond_resched(), and nothing waits for it to let go.
+ */
 void zone_pcp_reset(struct zone *zone)
 {
 	int cpu;
@@ -7738,12 +7751,6 @@ void zone_pcp_reset(struct zone *zone)
 			pzstats = per_cpu_ptr(zone->per_cpu_zonestats, cpu);
 			drain_zonestat(zone, pzstats);
 		}
-		free_percpu(zone->per_cpu_pageset);
-		zone->per_cpu_pageset = &boot_pageset;
-		if (zone->per_cpu_zonestats != &boot_zonestats) {
-			free_percpu(zone->per_cpu_zonestats);
-			zone->per_cpu_zonestats = &boot_zonestats;
-		}
 	}
 }
 
-- 
2.53.0-Meta