Re: [PATCH] mm: zswap: fix double invalidate with exclusive loads

[Domenico Cerasuolo <cerasuolodomenico@gmail.com>]

On Wed, Jun 21, 2023 at 11:30 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> If exclusive loads are enabled for zswap, we invalidate the entry before
> returning from zswap_frontswap_load(), after dropping the local
> reference. However, the tree lock is dropped during decompression after
> the local reference is acquired, so the entry could be invalidated
> before we drop the local ref. If this happens, the entry is freed once
> we drop the local ref, and zswap_invalidate_entry() tries to invalidate
> an already freed entry.
>
> Fix this by:
> (a) Making sure zswap_invalidate_entry() is always called with a local
>     ref held, to avoid being called on a freed entry.
> (b) Making sure zswap_invalidate_entry() only drops the ref if the entry
>     was actually on the rbtree. Otherwise, another invalidation could
>     have already happened, and the initial ref is already dropped.
>
> With these changes, there is no need to check that there is no need to
> make sure the entry still exists in the tree in zswap_reclaim_entry()
> before invalidating it, as zswap_reclaim_entry() will make this check
> internally.
>
> Fixes: b9c91c43412f ("mm: zswap: support exclusive loads")
> Reported-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  mm/zswap.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 87b204233115..62195f72bf56 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -355,12 +355,14 @@ static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
>         return 0;
>  }
>
> -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> +static bool zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
>  {
>         if (!RB_EMPTY_NODE(&entry->rbnode)) {
>                 rb_erase(&entry->rbnode, root);
>                 RB_CLEAR_NODE(&entry->rbnode);
> +               return true;
>         }
> +       return false;
>  }
>
>  /*
> @@ -599,14 +601,16 @@ static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
>         return NULL;
>  }
>
> +/*
> + * If the entry is still valid in the tree, drop the initial ref and remove it
> + * from the tree. This function must be called with an additional ref held,
> + * otherwise it may race with another invalidation freeing the entry.
> + */

On re-reading this comment there's one thing I'm not sure I get, do we
really need to hold an additional local ref to call this? As far as I
understood, once we check that the entry was in the tree before putting
the initial ref, there's no need for an additional local one.

>  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);
> +       if (zswap_rb_erase(&tree->rbroot, entry))
> +               zswap_entry_put(tree, entry);
>  }
>
>  static int zswap_reclaim_entry(struct zswap_pool *pool)
> @@ -659,8 +663,7 @@ static int zswap_reclaim_entry(struct zswap_pool *pool)
>          * 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);
> +       zswap_invalidate_entry(tree, entry);
>
>  put_unlock:
>         /* Drop local reference */
> @@ -1466,7 +1469,6 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>                 count_objcg_event(entry->objcg, ZSWPIN);
>  freeentry:
>         spin_lock(&tree->lock);
> -       zswap_entry_put(tree, entry);
>         if (!ret && zswap_exclusive_loads_enabled) {
>                 zswap_invalidate_entry(tree, entry);
>                 *exclusive = true;
> @@ -1475,6 +1477,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>                 list_move(&entry->lru, &entry->pool->lru);
>                 spin_unlock(&entry->pool->lru_lock);
>         }
> +       zswap_entry_put(tree, entry);
>         spin_unlock(&tree->lock);
>
>         return ret;
> --
> 2.41.0.162.gfafddb0af9-goog
>