zsmalloc: add a LRU to zs_pool to keep track of zspages in LRU order

This helps determines the coldest zspages as candidates for writeback.

Link: https://lkml.kernel.org/r/20221128191616.1261026-5-nphamcs@gmail.com
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zsmalloc.c

@@ -239,6 +239,11 @@ struct zs_pool {
 	/* Compact classes */
 	struct shrinker shrinker;
 
+#ifdef CONFIG_ZPOOL
+	/* List tracking the zspages in LRU order by most recently added object */
+	struct list_head lru;
+#endif
+
 #ifdef CONFIG_ZSMALLOC_STAT
 	struct dentry *stat_dentry;
 #endif
@@ -260,6 +265,12 @@ struct zspage {
 	unsigned int freeobj;
 	struct page *first_page;
 	struct list_head list; /* fullness list */
+
+#ifdef CONFIG_ZPOOL
+	/* links the zspage to the lru list in the pool */
+	struct list_head lru;
+#endif
+
 	struct zs_pool *pool;
 #ifdef CONFIG_COMPACTION
 	rwlock_t lock;
@@ -953,6 +964,9 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
 	}
 
 	remove_zspage(class, zspage, ZS_EMPTY);
+#ifdef CONFIG_ZPOOL
+	list_del(&zspage->lru);
+#endif
 	__free_zspage(pool, class, zspage);
 }
 
@@ -998,6 +1012,10 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
 		off %= PAGE_SIZE;
 	}
 
+#ifdef CONFIG_ZPOOL
+	INIT_LIST_HEAD(&zspage->lru);
+#endif
+
 	set_freeobj(zspage, 0);
 }
 
@@ -1270,6 +1288,31 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
 	obj_to_location(obj, &page, &obj_idx);
 	zspage = get_zspage(page);
 
+#ifdef CONFIG_ZPOOL
+	/*
+	 * Move the zspage to front of pool's LRU.
+	 *
+	 * Note that this is swap-specific, so by definition there are no ongoing
+	 * accesses to the memory while the page is swapped out that would make
+	 * it "hot". A new entry is hot, then ages to the tail until it gets either
+	 * written back or swaps back in.
+	 *
+	 * Furthermore, map is also called during writeback. We must not put an
+	 * isolated page on the LRU mid-reclaim.
+	 *
+	 * As a result, only update the LRU when the page is mapped for write
+	 * when it's first instantiated.
+	 *
+	 * This is a deviation from the other backends, which perform this update
+	 * in the allocation function (zbud_alloc, z3fold_alloc).
+	 */
+	if (mm == ZS_MM_WO) {
+		if (!list_empty(&zspage->lru))
+			list_del(&zspage->lru);
+		list_add(&zspage->lru, &pool->lru);
+	}
+#endif
+
 	/*
 	 * migration cannot move any zpages in this zspage. Here, pool->lock
 	 * is too heavy since callers would take some time until they calls
@@ -1988,6 +2031,9 @@ static void async_free_zspage(struct work_struct *work)
 		VM_BUG_ON(fullness != ZS_EMPTY);
 		class = pool->size_class[class_idx];
 		spin_lock(&pool->lock);
+#ifdef CONFIG_ZPOOL
+		list_del(&zspage->lru);
+#endif
 		__free_zspage(pool, class, zspage);
 		spin_unlock(&pool->lock);
 	}
@@ -2299,6 +2345,10 @@ struct zs_pool *zs_create_pool(const char *name)
 	 */
 	zs_register_shrinker(pool);
 
+#ifdef CONFIG_ZPOOL
+	INIT_LIST_HEAD(&pool->lru);
+#endif
+
 	return pool;
 
 err: