mm, memcg: prevent memory.low load/store tearing

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

Signed-off-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/448206f44b0fa7be9dad2ca2601d2bcb2c0b7844.1584034301.git.chris@chrisdown.name
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_counter.c b/mm/page_counter.c
index 75d53f1..509143f 100644 (file)
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -17,6 +17,7 @@ static void propagate_protected_usage(struct page_counter *c,
                                      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(struct page_counter *c,
                        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_counter *counter, unsigned long nr_pages)
 {
        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));