mm/zsmalloc.c: migration can leave pages in ZS_EMPTY indefinitely

In zs_page_migrate() we call putback_zspage() after we have finished
migrating all pages in this zspage.  However, the return value is
ignored.  If a zs_free() races in between zs_page_isolate() and
zs_page_migrate(), freeing the last object in the zspage,
putback_zspage() will leave the page in ZS_EMPTY for potentially an
unbounded amount of time.

To fix this, we need to do the same thing as zs_page_putback() does:
schedule free_work to occur.

To avoid duplicated code, move the sequence to a new
putback_zspage_deferred() function which both zs_page_migrate() and
zs_page_putback() call.

Link: http://lkml.kernel.org/r/20190809181751.219326-1-henryburns@google.com
Fixes: 48b4800a ("zsmalloc: page migration support")
Signed-off-by: Henry Burns <henryburns@google.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Henry Burns <henrywolfeburns@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Jonathan Adams <jwadams@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -1862,6 +1862,18 @@ static void dec_zspage_isolation(struct zspage *zspage)
 	zspage->isolated--;
 }
 
+static void putback_zspage_deferred(struct zs_pool *pool,
+				    struct size_class *class,
+				    struct zspage *zspage)
+{
+	enum fullness_group fg;
+
+	fg = putback_zspage(class, zspage);
+	if (fg == ZS_EMPTY)
+		schedule_work(&pool->free_work);
+
+}
+
 static void replace_sub_page(struct size_class *class, struct zspage *zspage,
 				struct page *newpage, struct page *oldpage)
 {
@@ -2031,7 +2043,7 @@ static int zs_page_migrate(struct address_space *mapping, struct page *newpage,
 	 * the list if @page is final isolated subpage in the zspage.
 	 */
 	if (!is_zspage_isolated(zspage))
-		putback_zspage(class, zspage);
+		putback_zspage_deferred(pool, class, zspage);
 
 	reset_page(page);
 	put_page(page);
@@ -2077,14 +2089,13 @@ static void zs_page_putback(struct page *page)
 	spin_lock(&class->lock);
 	dec_zspage_isolation(zspage);
 	if (!is_zspage_isolated(zspage)) {
-		fg = putback_zspage(class, zspage);
 		/*
 		 * Due to page_lock, we cannot free zspage immediately
 		 * so let's defer.
 		 */
-		if (fg == ZS_EMPTY)
-			schedule_work(&pool->free_work);
+		putback_zspage_deferred(pool, class, zspage);
 	}
+
 	spin_unlock(&class->lock);
 }