Re: [RFC PATCH v2 1/7] mm: zswap: add pool shrinking mechanism

Nhat Pham
2023-06-07T21:39:26Z

On Tue, Jun 6, 2023 at 7:56 AM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
> Each zpool driver (zbud, z3fold and zsmalloc) implements its own shrink
> function, which is called from zpool_shrink. However, with this commit,
> a unified shrink function is added to zswap. The ultimate goal is to
> eliminate the need for zpool_shrink once all zpool implementations have
> dropped their shrink code.
>
> To ensure the functionality of each commit, this change focuses solely
> on adding the mechanism itself. No modifications are made to
> the backends, meaning that functionally, there are no immediate changes.
> The zswap mechanism will only come into effect once the backends have
> removed their shrink code. The subsequent commits will address the
> modifications needed in the backends.
>
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> ---
> mm/zswap.c | 96 +++++++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 91 insertions(+), 5 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index bcb82e09eb64..c99bafcefecf 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -150,6 +150,12 @@ struct crypto_acomp_ctx {
> struct mutex *mutex;
> };
>
> +/*
> + * The lock ordering is zswap_tree.lock -> zswap_pool.lru_lock.
> + * The only case where lru_lock is not acquired while holding tree.lock is
> + * when a zswap_entry is taken off the lru for writeback, in that case it
> + * needs to be verified that it's still valid in the tree.
> + */
> struct zswap_pool {
> struct zpool *zpool;
> struct crypto_acomp_ctx __percpu *acomp_ctx;
> @@ -159,6 +165,8 @@ struct zswap_pool {
> struct work_struct shrink_work;
> struct hlist_node node;
> char tfm_name[CRYPTO_MAX_ALG_NAME];
> + struct list_head lru;
> + spinlock_t lru_lock;
> };
>
> /*
> @@ -176,10 +184,12 @@ struct zswap_pool {
> * 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.
> + * decompression. For a same value filled page length is 0, and both
> + * pool and lru are invalid and must be ignored.
> * pool - the zswap_pool the entry's data is in
> * handle - zpool allocation handle that stores the compressed page data
> * value - value of the same-value filled pages which have same content
> + * lru - handle to the pool's lru used to evict pages.
> */
> struct zswap_entry {
> struct rb_node rbnode;
> @@ -192,6 +202,7 @@ struct zswap_entry {
> unsigned long value;
> };
> struct obj_cgroup *objcg;
> + struct list_head lru;
> };
>
> struct zswap_header {
> @@ -364,6 +375,12 @@ static void zswap_free_entry(struct zswap_entry *entry)
> if (!entry->length)
> atomic_dec(&zswap_same_filled_pages);
> else {
> + /* zpool_evictable will be removed once all 3 backends have migrated */
> + if (!zpool_evictable(entry->pool->zpool)) {
> + spin_lock(&entry->pool->lru_lock);
> + list_del(&entry->lru);
> + spin_unlock(&entry->pool->lru_lock);
> + }
> zpool_free(entry->pool->zpool, entry->handle);
> zswap_pool_put(entry->pool);
> }
> @@ -584,14 +601,70 @@ static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
> return NULL;
> }
>
> +static int zswap_shrink(struct zswap_pool *pool)
> +{
> + struct zswap_entry *lru_entry, *tree_entry = NULL;
> + struct zswap_header *zhdr;
> + struct zswap_tree *tree;
> + int swpoffset;
> + int ret;
> +
> + /* get a reclaimable entry from LRU */
> + spin_lock(&pool->lru_lock);
> + if (list_empty(&pool->lru)) {
> + spin_unlock(&pool->lru_lock);
> + return -EINVAL;
> + }
> + lru_entry = list_last_entry(&pool->lru, struct zswap_entry, lru);
> + list_del_init(&lru_entry->lru);
> + zhdr = zpool_map_handle(pool->zpool, lru_entry->handle, ZPOOL_MM_RO);
> + tree = zswap_trees[swp_type(zhdr->swpentry)];
> + zpool_unmap_handle(pool->zpool, lru_entry->handle);
> + /*
> + * Once the pool lock is dropped, the lru_entry might get freed. The
> + * swpoffset is copied to the stack, and lru_entry isn't deref'd again
> + * until the entry is verified to still be alive in the tree.
> + */
> + swpoffset = swp_offset(zhdr->swpentry);
> + spin_unlock(&pool->lru_lock);
> +
> + /* hold a reference from tree so it won't be freed during writeback */
> + spin_lock(&tree->lock);
> + tree_entry = zswap_entry_find_get(&tree->rbroot, swpoffset);
> + if (tree_entry != lru_entry) {
> + if (tree_entry)
> + zswap_entry_put(tree, tree_entry);
> + spin_unlock(&tree->lock);
> + return -EAGAIN;
> + }
> + spin_unlock(&tree->lock);
> +
> + ret = zswap_writeback_entry(pool->zpool, lru_entry->handle);
> +
> + spin_lock(&tree->lock);
> + if (ret) {
> + spin_lock(&pool->lru_lock);
> + list_move(&lru_entry->lru, &pool->lru);
> + spin_unlock(&pool->lru_lock);
> + }
> + zswap_entry_put(tree, tree_entry);
> + spin_unlock(&tree->lock);
> +
> + return ret ? -EAGAIN : 0;
> +}
> +
> static void shrink_worker(struct work_struct *w)
> {
> struct zswap_pool *pool = container_of(w, typeof(*pool),
> shrink_work);
> int ret, failures = 0;
>
> + /* zpool_evictable will be removed once all 3 backends have migrated */
> do {
> - ret = zpool_shrink(pool->zpool, 1, NULL);
> + if (zpool_evictable(pool->zpool))
> + ret = zpool_shrink(pool->zpool, 1, NULL);
> + else
> + ret = zswap_shrink(pool);
> if (ret) {
> zswap_reject_reclaim_fail++;
> if (ret != -EAGAIN)
> @@ -655,6 +728,8 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> */
> kref_init(&pool->kref);
> INIT_LIST_HEAD(&pool->list);
> + INIT_LIST_HEAD(&pool->lru);
> + spin_lock_init(&pool->lru_lock);
> INIT_WORK(&pool->shrink_work, shrink_worker);
>
> zswap_pool_debug("created", pool);
> @@ -1270,7 +1345,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> }
>
> /* store */
> - hlen = zpool_evictable(entry->pool->zpool) ? sizeof(zhdr) : 0;
> + hlen = sizeof(zhdr);
> gfp = __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM;
> if (zpool_malloc_support_movable(entry->pool->zpool))
> gfp |= __GFP_HIGHMEM | __GFP_MOVABLE;
> @@ -1313,6 +1388,12 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> zswap_entry_put(tree, dupentry);
> }
> } while (ret == -EEXIST);
> + /* zpool_evictable will be removed once all 3 backends have migrated */
> + if (entry->length && !zpool_evictable(entry->pool->zpool)) {
> + spin_lock(&entry->pool->lru_lock);
> + list_add(&entry->lru, &entry->pool->lru);
> + spin_unlock(&entry->pool->lru_lock);
> + }
> spin_unlock(&tree->lock);
>
> /* update stats */
> @@ -1384,8 +1465,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> /* decompress */
> dlen = PAGE_SIZE;
> src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
> - if (zpool_evictable(entry->pool->zpool))
> - src += sizeof(struct zswap_header);
> + src += sizeof(struct zswap_header);
>
> if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> memcpy(tmp, src, entry->length);
> @@ -1415,6 +1495,12 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> freeentry:
> spin_lock(&tree->lock);
> zswap_entry_put(tree, entry);
> + /* zpool_evictable will be removed once all 3 backends have migrated */
> + if (entry->length && !zpool_evictable(entry->pool->zpool)) {
> + spin_lock(&entry->pool->lru_lock);
> + list_move(&entry->lru, &entry->pool->lru);
> + spin_unlock(&entry->pool->lru_lock);
> + }
> spin_unlock(&tree->lock);
>
> return ret;
> --
> 2.34.1
>

Looks real solid to me! Thanks, Domenico.
Acked-by: Nhat Pham <nphamcs@gmail.com>