mm: memcontrol: don't allocate cgroup swap arrays when memcg is disabled

Patch series "memcg swap fix & cleanups".


This patch (of 4):

Since commit 2d1c4980 ("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.

Link: https://lkml.kernel.org/r/20220926135704.400818-1-hannes@cmpxchg.org
Link: https://lkml.kernel.org/r/20220926135704.400818-2-hannes@cmpxchg.org
Fixes: 2d1c4980 ("mm: memcontrol: make swap tracking an integral part of memory control")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -7459,6 +7459,9 @@ 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);

mm/swap_cgroup.c

@@ -170,6 +170,9 @@ 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 = vcalloc(length, sizeof(void *));
@@ -204,6 +207,9 @@ 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;