[PATCH 2/6] mm, memcg: Prevent memory.max load tearing

[Chris Down]

This one is a bit more nuanced because we have memcg_max_mutex, which is
mostly just used for enforcing invariants, but we still need to
READ_ONCE since (despite its name) it doesn't really protect memory.max
access.

On write (page_counter_set_max() and memory_max_write()) we use xchg(),
which uses smp_mb(), so that's already fine.

Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Roman Gushchin <guro@xxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: cgroups@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxx
---
 mm/memcontrol.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d32d3c0a16d4..aca2964ea494 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1507,7 +1507,7 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
 
 	pr_info("memory: usage %llukB, limit %llukB, failcnt %lu\n",
 		K((u64)page_counter_read(&memcg->memory)),
-		K((u64)memcg->memory.max), memcg->memory.failcnt);
+		K((u64)READ_ONCE(memcg->memory.max)), memcg->memory.failcnt);
 	if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		pr_info("swap: usage %llukB, limit %llukB, failcnt %lu\n",
 			K((u64)page_counter_read(&memcg->swap)),
@@ -1538,7 +1538,7 @@ unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
 {
 	unsigned long max;
 
-	max = memcg->memory.max;
+	max = READ_ONCE(memcg->memory.max);
 	if (mem_cgroup_swappiness(memcg)) {
 		unsigned long memsw_max;
 		unsigned long swap_max;
@@ -3006,7 +3006,7 @@ static int mem_cgroup_resize_max(struct mem_cgroup *memcg,
 		 * Make sure that the new limit (memsw or memory limit) doesn't
 		 * break our basic invariant rule memory.max <= memsw.max.
 		 */
-		limits_invariant = memsw ? max >= memcg->memory.max :
+		limits_invariant = memsw ? max >= READ_ONCE(memcg->memory.max) :
 					   max <= memcg->memsw.max;
 		if (!limits_invariant) {
 			mutex_unlock(&memcg_max_mutex);
@@ -3753,8 +3753,8 @@ static int memcg_stat_show(struct seq_file *m, void *v)
 	/* Hierarchical information */
 	memory = memsw = PAGE_COUNTER_MAX;
 	for (mi = memcg; mi; mi = parent_mem_cgroup(mi)) {
-		memory = min(memory, mi->memory.max);
-		memsw = min(memsw, mi->memsw.max);
+		memory = min(memory, READ_ONCE(mi->memory.max));
+		memsw = min(memsw, READ_ONCE(mi->memsw.max));
 	}
 	seq_printf(m, "hierarchical_memory_limit %llu\n",
 		   (u64)memory * PAGE_SIZE);
@@ -4257,7 +4257,7 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 	*pheadroom = PAGE_COUNTER_MAX;
 
 	while ((parent = parent_mem_cgroup(memcg))) {
-		unsigned long ceiling = min(memcg->memory.max,
+		unsigned long ceiling = min(READ_ONCE(memcg->memory.max),
 					    READ_ONCE(memcg->high));
 		unsigned long used = page_counter_read(&memcg->memory);
 
-- 
2.25.1