[PATCH 1/2 v2] mm/zsmalloc.c: Migration can leave pages in ZS_EMPTY indefinitely

[Henry Burns]

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.

Fixes: 48b4800a1c6a ("zsmalloc: page migration support")
Signed-off-by: Henry Burns <henrybu...@google.com>
---
 Changelog since v1:
 - Moved the comment from putback_zspage_deferred() to
   zs_page_putback().

 mm/zsmalloc.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 57fbb7ced69f..5105b9b66653 100644
--- a/mm/zsmalloc.c
+++ b/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);
 }
 
-- 
2.23.0.rc1.153.gdeed80330f-goog