mm: zswap: add basic meminfo and vmstat coverage

Currently it requires poking at debugfs to figure out the size and
population of the zswap cache on a host.  There are no counters for reads
and writes against the cache.  As a result, it's difficult to understand
zswap behavior on production systems.

Print zswap memory consumption and how many pages are zswapped out in
/proc/meminfo.  Count zswapouts and zswapins in /proc/vmstat.

Link: https://lkml.kernel.org/r/20220510152847.230957-6-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/filesystems/proc.rst

@@ -964,6 +964,8 @@ Example output. You may not have all of these fields.
     Mlocked:               0 kB
     SwapTotal:             0 kB
     SwapFree:              0 kB
+    Zswap:              1904 kB
+    Zswapped:           7792 kB
     Dirty:                12 kB
     Writeback:             0 kB
     AnonPages:       4654780 kB
@@ -1055,6 +1057,10 @@ SwapTotal
 SwapFree
               Memory which has been evicted from RAM, and is temporarily
               on the disk
+Zswap
+              Memory consumed by the zswap backend (compressed size)
+Zswapped
+              Amount of anonymous memory stored in zswap (original size)
 Dirty
               Memory which is waiting to get written back to the disk
 Writeback

fs/proc/meminfo.c

@@ -86,6 +86,13 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
 
 	show_val_kb(m, "SwapTotal:      ", i.totalswap);
 	show_val_kb(m, "SwapFree:       ", i.freeswap);
+#ifdef CONFIG_ZSWAP
+	seq_printf(m,  "Zswap:          %8lu kB\n",
+		   (unsigned long)(zswap_pool_total_size >> 10));
+	seq_printf(m,  "Zswapped:       %8lu kB\n",
+		   (unsigned long)atomic_read(&zswap_stored_pages) <<
+		   (PAGE_SHIFT - 10));
+#endif
 	show_val_kb(m, "Dirty:          ",
 		    global_node_page_state(NR_FILE_DIRTY));
 	show_val_kb(m, "Writeback:      ",

include/linux/swap.h

@@ -620,6 +620,11 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
 }
 #endif
 
+#ifdef CONFIG_ZSWAP
+extern u64 zswap_pool_total_size;
+extern atomic_t zswap_stored_pages;
+#endif
+
 #if defined(CONFIG_SWAP) && defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
 extern void __cgroup_throttle_swaprate(struct page *page, gfp_t gfp_mask);
 static inline  void cgroup_throttle_swaprate(struct page *page, gfp_t gfp_mask)

include/linux/vm_event_item.h

@@ -136,6 +136,10 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 #ifdef CONFIG_KSM
 		COW_KSM,
 #endif
+#ifdef CONFIG_ZSWAP
+		ZSWPIN,
+		ZSWPOUT,
+#endif
 #ifdef CONFIG_X86
 		DIRECT_MAP_LEVEL2_SPLIT,
 		DIRECT_MAP_LEVEL3_SPLIT,

mm/vmstat.c

@@ -1396,6 +1396,10 @@ const char * const vmstat_text[] = {
 #ifdef CONFIG_KSM
 	"cow_ksm",
 #endif
+#ifdef CONFIG_ZSWAP
+	"zswpin",
+	"zswpout",
+#endif
 #ifdef CONFIG_X86
 	"direct_map_level2_splits",
 	"direct_map_level3_splits",

mm/zswap.c

@@ -42,9 +42,9 @@
 * statistics
 **********************************/
 /* Total bytes used by the compressed storage */
-static u64 zswap_pool_total_size;
+u64 zswap_pool_total_size;
 /* The number of compressed pages currently stored in zswap */
-static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
+atomic_t zswap_stored_pages = ATOMIC_INIT(0);
 /* The number of same-value filled pages currently stored in zswap */
 static atomic_t zswap_same_filled_pages = ATOMIC_INIT(0);
 
@@ -1243,6 +1243,7 @@ insert_entry:
 	/* update stats */
 	atomic_inc(&zswap_stored_pages);
 	zswap_update_total_size();
+	count_vm_event(ZSWPOUT);
 
 	return 0;
 
@@ -1285,11 +1286,10 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 		zswap_fill_page(dst, entry->value);
 		kunmap_atomic(dst);
 		ret = 0;
-		goto freeentry;
+		goto stats;
 	}
 
 	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
-
 		tmp = kmalloc(entry->length, GFP_ATOMIC);
 		if (!tmp) {
 			ret = -ENOMEM;
@@ -1304,10 +1304,8 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 		src += sizeof(struct zswap_header);
 
 	if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
-
 		memcpy(tmp, src, entry->length);
 		src = tmp;
-
 		zpool_unmap_handle(entry->pool->zpool, entry->handle);
 	}
 
@@ -1326,7 +1324,8 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 		kfree(tmp);
 
 	BUG_ON(ret);
-
+stats:
+	count_vm_event(ZSWPIN);
 freeentry:
 	spin_lock(&tree->lock);
 	zswap_entry_put(tree, entry);