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

Nhat Pham
2024-02-02T22:33:49Z

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