[PATCH 1/2] mm: memcontrol: don't allocate cgroup swap arrays when memcg is disabled

[Johannes Weiner]

Since commit 2d1c498072de ("mm: memcontrol: make swap tracking an
integral part of memory control"), the cgroup swap arrays are used to
track memory ownership at the time of swap readahead and swapoff, even
if swap space *accounting* has been turned off by the user via
swapaccount=0 (which sets cgroup_memory_noswap).

However, the patch was overzealous: by simply dropping the
cgroup_memory_noswap conditionals in the swapon, swapoff and uncharge
path, it caused the cgroup arrays being allocated even when the memory
controller as a whole is disabled. This is a waste of that memory.

Restore mem_cgroup_disabled() checks, implied previously by
cgroup_memory_noswap, in the swapon, swapoff, and swap_entry_free
callbacks.

Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/memcontrol.c  | 3 +++
 mm/swap_cgroup.c | 6 ++++++
 2 files changed, 9 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 668d1d7c2645..49bdcf603af1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7101,6 +7101,9 @@ void mem_cgroup_uncharge_swap(swp_entry_t entry, unsigned int nr_pages)
 	struct mem_cgroup *memcg;
 	unsigned short id;
 
+	if (mem_cgroup_disabled())
+		return;
+
 	id = swap_cgroup_record(entry, 0, nr_pages);
 	rcu_read_lock();
 	memcg = mem_cgroup_from_id(id);
diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
index 7f34343c075a..08c3246f9269 100644
--- a/mm/swap_cgroup.c
+++ b/mm/swap_cgroup.c
@@ -171,6 +171,9 @@ int swap_cgroup_swapon(int type, unsigned long max_pages)
 	unsigned long length;
 	struct swap_cgroup_ctrl *ctrl;
 
+	if (mem_cgroup_disabled())
+		return 0;
+
 	length = DIV_ROUND_UP(max_pages, SC_PER_PAGE);
 	array_size = length * sizeof(void *);
 
@@ -206,6 +209,9 @@ void swap_cgroup_swapoff(int type)
 	unsigned long i, length;
 	struct swap_cgroup_ctrl *ctrl;
 
+	if (mem_cgroup_disabled())
+		return;
+
 	mutex_lock(&swap_cgroup_mutex);
 	ctrl = &swap_cgroup_ctrl[type];
 	map = ctrl->map;
-- 
2.30.1