mm: memcg: add per-memcg zswap writeback stat

Since zswap now writes back pages from memcg-specific LRUs, we now need a
new stat to show writebacks count for each memcg.

[[e-mail]: rename ZSWP_WB to ZSWPWB]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Suggested-by: Nhat Pham <[e-mail]>
Signed-off-by: Domenico Cerasuolo <[e-mail]>
Signed-off-by: Nhat Pham <[e-mail]>
Tested-by: Bagas Sanjaya <[e-mail]>
Reviewed-by: Yosry Ahmed <[e-mail]>
Cc: Chris Li <[e-mail]>
Cc: Dan Streetman <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Seth Jennings <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Vitaly Wool <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/vm_event_item.h

···
 #ifdef CONFIG_ZSWAP
 		ZSWPIN,
 		ZSWPOUT,
+		ZSWPWB,
 #endif
 #ifdef CONFIG_X86
 		DIRECT_MAP_LEVEL2_SPLIT,

mm/memcontrol.c

···
 #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
 	ZSWPIN,
 	ZSWPOUT,
+	ZSWPWB,
 #endif
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	THP_FAULT_ALLOC,

mm/vmstat.c

···
 #ifdef CONFIG_ZSWAP
 	"zswpin",
 	"zswpout",
+	"zswpwb",
 #endif
 #ifdef CONFIG_X86
 	"direct_map_level2_splits",

mm/zswap.c

···
 	}
 	zswap_written_back_pages++;
 
+	if (entry->objcg)
+		count_objcg_event(entry->objcg, ZSWPWB);
+
+	count_vm_event(ZSWPWB);
 	/*
 	 * Writeback started successfully, the page now belongs to the
 	 * swapcache. Drop the entry from zswap - unless invalidate already