[076/155] mm, memcg: prevent memory.low load/store tearing

Commit Message

Andrew Morton April 2, 2020, 4:07 a.m. UTC

From: Chris Down <[e-mail]>
Subject: mm, memcg: prevent memory.low load/store tearing

This can be set concurrently with reads, which may cause the wrong value
to be propagated.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Chris Down <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/page_counter.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

Patch

--- a/mm/page_counter.c~mm-memcg-prevent-memorylow-load-store-tearing
+++ a/mm/page_counter.c
@@ -17,6 +17,7 @@  static void propagate_protected_usage(st
 				      unsigned long usage)
 {
 	unsigned long protected, old_protected;
+	unsigned long low;
 	long delta;
 
 	if (!c->parent)
@@ -30,8 +31,9 @@  static void propagate_protected_usage(st
 			atomic_long_add(delta, &c->parent->children_min_usage);
 	}
 
-	if (c->low || atomic_long_read(&c->low_usage)) {
-		protected = min(usage, c->low);
+	low = READ_ONCE(c->low);
+	if (low || atomic_long_read(&c->low_usage)) {
+		protected = min(usage, low);
 		old_protected = atomic_long_xchg(&c->low_usage, protected);
 		delta = protected - old_protected;
 		if (delta)
@@ -222,7 +224,7 @@  void page_counter_set_low(struct page_co
 {
 	struct page_counter *c;
 
-	counter->low = nr_pages;
+	WRITE_ONCE(counter->low, nr_pages);
 
 	for (c = counter; c; c = c->parent)
 		propagate_protected_usage(c, atomic_long_read(&c->usage));