mm: zswap: invaldiate entry after writeback

When an entry started writeback, it used to be invalidated with ref count
logic alone, meaning that it would stay on the tree until all references
were put.  The problem with this behavior is that as soon as the writeback
started, the ownership of the data held by the entry is passed to the
swapcache and should not be left in zswap too.  Currently there are no
known issues because of this, but this change explicitly invalidates an
entry that started writeback to reduce opportunities for future bugs.

This patch is a follow up on the series titled "mm: zswap: move writeback
LRU from zpool to zswap" + commit f090b7949768("mm: zswap: support
exclusive loads").

Link: https://lkml.kernel.org/r/20230614143122.74471-1-cerasuolodomenico@gmail.com
Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/zswap.c b/mm/zswap.c
index 02d0a6f..c122f04 100644 (file)
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -599,6 +599,16 @@ static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
        return NULL;
 }
 
+static void zswap_invalidate_entry(struct zswap_tree *tree,
+                                  struct zswap_entry *entry)
+{
+       /* remove from rbtree */
+       zswap_rb_erase(&tree->rbroot, entry);
+
+       /* drop the initial reference from entry creation */
+       zswap_entry_put(tree, entry);
+}
+
 static int zswap_reclaim_entry(struct zswap_pool *pool)
 {
        struct zswap_entry *entry;
@@ -644,12 +654,13 @@ static int zswap_reclaim_entry(struct zswap_pool *pool)
                goto put_unlock;
        }
 
-       /* Check for invalidate() race */
-       if (entry != zswap_rb_search(&tree->rbroot, swpoffset))
-               goto put_unlock;
-
-       /* Drop base reference */
-       zswap_entry_put(tree, entry);
+       /*
+        * Writeback started successfully, the page now belongs to the
+        * swapcache. Drop the entry from zswap - unless invalidate already
+        * took it out while we had the tree->lock released for IO.
+        */
+       if (entry == zswap_rb_search(&tree->rbroot, swpoffset))
+               zswap_invalidate_entry(tree, entry);
 
 put_unlock:
        /* Drop local reference */
@@ -1376,16 +1387,6 @@ shrink:
        goto reject;
 }
 
-static void zswap_invalidate_entry(struct zswap_tree *tree,
-                                  struct zswap_entry *entry)
-{
-       /* remove from rbtree */
-       zswap_rb_erase(&tree->rbroot, entry);
-
-       /* drop the initial reference from entry creation */
-       zswap_entry_put(tree, entry);
-}
-
 /*
  * returns 0 if the page was successfully decompressed
  * return -1 on entry not found or error