memcg: expose swapcache stat for memcg v1

Patch series "Expose swapcache stat for memcg v1", v2.

Since commit b6038942 ("mm: memcg: add swapcache stat for memcg v2")
adds swapcache stat for the cgroup v2, it seems there is no reason to hide
it in memcg v1.  Conversely, with swapcached it is more accurate to
evaluate the available memory for memcg.

Link: https://lkml.kernel.org/r/20230915105845.3199656-1-liushixin2@huawei.com
Link: https://lkml.kernel.org/r/20230915105845.3199656-2-liushixin2@huawei.comSigned-off-by: Liu Shixin <liushixin2@huawei.com>
Suggested-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/cgroup-v1/memory.rst

@@ -551,6 +551,7 @@ memory.stat file includes following statistics:
                     event happens each time a page is unaccounted from the
                     cgroup.
     swap            # of bytes of swap usage
+    swapcached      # of bytes of swap cached in memory
     dirty           # of bytes that are waiting to get written back to the disk.
     writeback       # of bytes of file/anon cache that are queued for syncing to
                     disk.

mm/memcontrol.c

@@ -4068,7 +4068,10 @@ static const unsigned int memcg1_stats[] = {
 	NR_WRITEBACK,
 	WORKINGSET_REFAULT_ANON,
 	WORKINGSET_REFAULT_FILE,
+#ifdef CONFIG_SWAP
 	MEMCG_SWAP,
+	NR_SWAPCACHE,
+#endif
 };
 
 static const char *const memcg1_stat_names[] = {
@@ -4083,7 +4086,10 @@ static const char *const memcg1_stat_names[] = {
 	"writeback",
 	"workingset_refault_anon",
 	"workingset_refault_file",
+#ifdef CONFIG_SWAP
 	"swap",
+	"swapcached",
+#endif
 };
 
 /* Universal VM events cgroup1 shows, original sort order */