Re: [PATCH 6/6] mm/zswap: zswap entry doesn't need refcount anymore

[Nhat Pham <nphamcs@gmail.com>]

On Thu, Feb 1, 2024 at 7:50 AM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> Since we don't need to leave zswap entry on the zswap tree anymore,
> we should remove it from tree once we find it from the tree.
>
> Then after using it, we can directly free it, no concurrent path
> can find it from tree. Only the shrinker can see it from lru list,
> which will also double check under tree lock, so no race problem.
>
> So we don't need refcount in zswap entry anymore and don't need to
> take the spinlock for the second time to invalidate it.
>
> The side effect is that zswap_entry_free() maybe not happen in tree
> spinlock, but it's ok since nothing need to be protected by the lock.
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>

Oh this is sweet! Fewer things to keep in mind.
Reviewed-by: Nhat Pham <nphamcs@gmail.com>

> ---
>  mm/zswap.c | 63 +++++++++++---------------------------------------------------
>  1 file changed, 11 insertions(+), 52 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index cbf379abb6c7..cd67f7f6b302 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -193,12 +193,6 @@ struct zswap_pool {
>   *
>   * rbnode - links the entry into red-black tree for the appropriate swap type
>   * swpentry - associated swap entry, the offset indexes into the red-black tree
> - * refcount - the number of outstanding reference to the entry. This is needed
> - *            to protect against premature freeing of the entry by code
> - *            concurrent calls to load, invalidate, and writeback.  The lock
> - *            for the zswap_tree structure that contains the entry must
> - *            be held while changing the refcount.  Since the lock must
> - *            be held, there is no reason to also make refcount atomic.
>   * length - the length in bytes of the compressed page data.  Needed during
>   *          decompression. For a same value filled page length is 0, and both
>   *          pool and lru are invalid and must be ignored.
> @@ -211,7 +205,6 @@ struct zswap_pool {
>  struct zswap_entry {
>         struct rb_node rbnode;
>         swp_entry_t swpentry;
> -       int refcount;

Hah this should even make zswap a bit more space-efficient. IIRC Yosry
has some analysis regarding how much less efficient zswap will be
every time we add a new field to zswap entry - this should go in the
opposite direction :)

>         unsigned int length;
>         struct zswap_pool *pool;
>         union {
> @@ -222,11 +215,6 @@ struct zswap_entry {
>         struct list_head lru;
>  };
>
> -/*
> - * The tree lock in the zswap_tree struct protects a few things:
> - * - the rbtree
> - * - the refcount field of each entry in the tree
> - */
>  struct zswap_tree {
>         struct rb_root rbroot;
>         spinlock_t lock;
> @@ -890,14 +878,10 @@ static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
>         return 0;
>  }
>
> -static bool zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
> +static void 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;
> +       rb_erase(&entry->rbnode, root);
> +       RB_CLEAR_NODE(&entry->rbnode);
>  }
>
>  /*********************************
> @@ -911,7 +895,6 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp, int nid)
>         entry = kmem_cache_alloc_node(zswap_entry_cache, gfp, nid);
>         if (!entry)
>                 return NULL;
> -       entry->refcount = 1;
>         RB_CLEAR_NODE(&entry->rbnode);
>         return entry;
>  }
> @@ -954,33 +937,15 @@ static void zswap_entry_free(struct zswap_entry *entry)
>         zswap_update_total_size();
>  }
>
> -/* caller must hold the tree lock */
> -static void zswap_entry_get(struct zswap_entry *entry)
> -{
> -       WARN_ON_ONCE(!entry->refcount);
> -       entry->refcount++;
> -}
> -
> -/* caller must hold the tree lock */
> -static void zswap_entry_put(struct zswap_entry *entry)
> -{
> -       WARN_ON_ONCE(!entry->refcount);
> -       if (--entry->refcount == 0) {
> -               WARN_ON_ONCE(!RB_EMPTY_NODE(&entry->rbnode));
> -               zswap_entry_free(entry);
> -       }
> -}
> -
>  /*
> - * 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.
> + * The caller hold the tree lock and search the entry from the tree,
> + * so it must be on the tree, remove it from the tree and free it.
>   */
>  static void zswap_invalidate_entry(struct zswap_tree *tree,
>                                    struct zswap_entry *entry)
>  {
> -       if (zswap_rb_erase(&tree->rbroot, entry))
> -               zswap_entry_put(entry);
> +       zswap_rb_erase(&tree->rbroot, entry);
> +       zswap_entry_free(entry);
>  }
>
>  /*********************************
> @@ -1219,7 +1184,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>         }
>
>         /* Safe to deref entry after the entry is verified above. */
> -       zswap_entry_get(entry);
> +       zswap_rb_erase(&tree->rbroot, entry);
>         spin_unlock(&tree->lock);
>
>         zswap_decompress(entry, &folio->page);
> @@ -1228,10 +1193,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>         if (entry->objcg)
>                 count_objcg_event(entry->objcg, ZSWPWB);
>
> -       spin_lock(&tree->lock);
> -       zswap_invalidate_entry(tree, entry);
> -       zswap_entry_put(entry);
> -       spin_unlock(&tree->lock);
> +       zswap_entry_free(entry);
>
>         /* folio is up to date */
>         folio_mark_uptodate(folio);
> @@ -1702,7 +1664,7 @@ bool zswap_load(struct folio *folio)
>                 spin_unlock(&tree->lock);
>                 return false;
>         }
> -       zswap_entry_get(entry);
> +       zswap_rb_erase(&tree->rbroot, entry);
>         spin_unlock(&tree->lock);
>
>         if (entry->length)
> @@ -1717,10 +1679,7 @@ bool zswap_load(struct folio *folio)
>         if (entry->objcg)
>                 count_objcg_event(entry->objcg, ZSWPIN);
>
> -       spin_lock(&tree->lock);
> -       zswap_invalidate_entry(tree, entry);
> -       zswap_entry_put(entry);
> -       spin_unlock(&tree->lock);
> +       zswap_entry_free(entry);
>
>         folio_mark_dirty(folio);
>
>
> --
> b4 0.10.1