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Date: Sun, 11 Feb 2024 13:04:48 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>,
Yosry Ahmed <yosryahmed@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools
On Sun, Feb 11, 2024 at 5:57 AM Chengming Zhou
<zhouchengming@...edance.com> wrote:
>
> Dynamic zswap_pool creation may create/reuse to have multiple
> zswap_pools in a list, only the first will be current used.
>
> Each zswap_pool has its own lru and shrinker, which is not
> necessary and has its problem:
>
> 1. When memory has pressure, all shrinker of zswap_pools will
> try to shrink its own lru, there is no order between them.
>
> 2. When zswap limit hit, only the last zswap_pool's shrink_work
> will try to shrink its lru, which is inefficient.
>
> Anyway, having a global lru and shrinker shared by all zswap_pools
> is better and efficient.
>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
I'll do a careful review later, but IMO this is a good idea :)
Chris pointed out when he reviewed the zswap shrinker patch series
that the reclaim algorithm has to decide which pool to reclaim from,
and I have always thought that it was a bit weird that we have to do
it at all. We should reclaim stored objects by access ordering,
irregardless of which pool it belongs to. Having a shared LRU and
other associated reclaim structures is sound, and saves a bit of space
too while we're at it.
> ---
> mm/zswap.c | 153 ++++++++++++++++++++++---------------------------------------
> 1 file changed, 55 insertions(+), 98 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 62fe307521c9..7668db8c10e3 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -176,14 +176,17 @@ struct zswap_pool {
> struct kref kref;
> struct list_head list;
> struct work_struct release_work;
> - struct work_struct shrink_work;
> struct hlist_node node;
> char tfm_name[CRYPTO_MAX_ALG_NAME];
> +};
> +
> +struct {
> struct list_lru list_lru;
> - struct mem_cgroup *next_shrink;
> - struct shrinker *shrinker;
> atomic_t nr_stored;
> -};
> + struct shrinker *shrinker;
> + struct work_struct shrink_work;
> + struct mem_cgroup *next_shrink;
> +} zswap;
>
> /*
> * struct zswap_entry
> @@ -301,9 +304,6 @@ static void zswap_update_total_size(void)
> * pool functions
> **********************************/
>
> -static void zswap_alloc_shrinker(struct zswap_pool *pool);
> -static void shrink_worker(struct work_struct *w);
> -
> static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> {
> int i;
> @@ -353,30 +353,16 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
> if (ret)
> goto error;
>
> - zswap_alloc_shrinker(pool);
> - if (!pool->shrinker)
> - goto error;
> -
> - pr_debug("using %s compressor\n", pool->tfm_name);
> -
> /* being the current pool takes 1 ref; this func expects the
> * caller to always add the new pool as the current pool
> */
> kref_init(&pool->kref);
> INIT_LIST_HEAD(&pool->list);
> - if (list_lru_init_memcg(&pool->list_lru, pool->shrinker))
> - goto lru_fail;
> - shrinker_register(pool->shrinker);
> - INIT_WORK(&pool->shrink_work, shrink_worker);
> - atomic_set(&pool->nr_stored, 0);
>
> zswap_pool_debug("created", pool);
>
> return pool;
>
> -lru_fail:
> - list_lru_destroy(&pool->list_lru);
> - shrinker_free(pool->shrinker);
> error:
> if (pool->acomp_ctx)
> free_percpu(pool->acomp_ctx);
> @@ -434,15 +420,8 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
>
> zswap_pool_debug("destroying", pool);
>
> - shrinker_free(pool->shrinker);
> cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
> free_percpu(pool->acomp_ctx);
> - list_lru_destroy(&pool->list_lru);
> -
> - spin_lock(&zswap_pools_lock);
> - mem_cgroup_iter_break(NULL, pool->next_shrink);
> - pool->next_shrink = NULL;
> - spin_unlock(&zswap_pools_lock);
>
> for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
> zpool_destroy_pool(pool->zpools[i]);
> @@ -529,24 +508,6 @@ static struct zswap_pool *zswap_pool_current_get(void)
> return pool;
> }
>
> -static struct zswap_pool *zswap_pool_last_get(void)
> -{
> - struct zswap_pool *pool, *last = NULL;
> -
> - rcu_read_lock();
> -
> - list_for_each_entry_rcu(pool, &zswap_pools, list)
> - last = pool;
> - WARN_ONCE(!last && zswap_has_pool,
> - "%s: no page storage pool!\n", __func__);
> - if (!zswap_pool_get(last))
> - last = NULL;
> -
> - rcu_read_unlock();
> -
> - return last;
> -}
> -
> /* type and compressor must be null-terminated */
> static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
> {
> @@ -816,14 +777,10 @@ void zswap_folio_swapin(struct folio *folio)
>
> void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
> {
> - struct zswap_pool *pool;
> -
> - /* lock out zswap pools list modification */
> + /* lock out zswap shrinker walking memcg tree */
> spin_lock(&zswap_pools_lock);
> - list_for_each_entry(pool, &zswap_pools, list) {
> - if (pool->next_shrink == memcg)
> - pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
> - }
> + if (zswap.next_shrink == memcg)
> + zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);
> spin_unlock(&zswap_pools_lock);
> }
>
> @@ -923,9 +880,9 @@ static void zswap_entry_free(struct zswap_entry *entry)
> if (!entry->length)
> atomic_dec(&zswap_same_filled_pages);
> else {
> - zswap_lru_del(&entry->pool->list_lru, entry);
> + zswap_lru_del(&zswap.list_lru, entry);
> zpool_free(zswap_find_zpool(entry), entry->handle);
> - atomic_dec(&entry->pool->nr_stored);
> + atomic_dec(&zswap.nr_stored);
> zswap_pool_put(entry->pool);
> }
> if (entry->objcg) {
> @@ -1288,7 +1245,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> {
> struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
> unsigned long shrink_ret, nr_protected, lru_size;
> - struct zswap_pool *pool = shrinker->private_data;
> bool encountered_page_in_swapcache = false;
>
> if (!zswap_shrinker_enabled ||
> @@ -1299,7 +1255,7 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>
> nr_protected =
> atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> - lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> + lru_size = list_lru_shrink_count(&zswap.list_lru, sc);
>
> /*
> * Abort if we are shrinking into the protected region.
> @@ -1316,7 +1272,7 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> return SHRINK_STOP;
> }
>
> - shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
> + shrink_ret = list_lru_shrink_walk(&zswap.list_lru, sc, &shrink_memcg_cb,
> &encountered_page_in_swapcache);
>
> if (encountered_page_in_swapcache)
> @@ -1328,7 +1284,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> struct shrink_control *sc)
> {
> - struct zswap_pool *pool = shrinker->private_data;
> struct mem_cgroup *memcg = sc->memcg;
> struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
> unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> @@ -1343,7 +1298,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> #else
> /* use pool stats instead of memcg stats */
> nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
> - nr_stored = atomic_read(&pool->nr_stored);
> + nr_stored = atomic_read(&zswap.nr_stored);
> #endif
>
> if (!nr_stored)
> @@ -1351,7 +1306,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>
> nr_protected =
> atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> - nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
> + nr_freeable = list_lru_shrink_count(&zswap.list_lru, sc);
> /*
> * Subtract the lru size by an estimate of the number of pages
> * that should be protected.
> @@ -1367,23 +1322,24 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> return mult_frac(nr_freeable, nr_backing, nr_stored);
> }
>
> -static void zswap_alloc_shrinker(struct zswap_pool *pool)
> +static struct shrinker *zswap_alloc_shrinker(void)
> {
> - pool->shrinker =
> + struct shrinker *shrinker;
> +
> + shrinker =
> shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
> - if (!pool->shrinker)
> - return;
> + if (!shrinker)
> + return NULL;
>
> - pool->shrinker->private_data = pool;
> - pool->shrinker->scan_objects = zswap_shrinker_scan;
> - pool->shrinker->count_objects = zswap_shrinker_count;
> - pool->shrinker->batch = 0;
> - pool->shrinker->seeks = DEFAULT_SEEKS;
> + shrinker->scan_objects = zswap_shrinker_scan;
> + shrinker->count_objects = zswap_shrinker_count;
> + shrinker->batch = 0;
> + shrinker->seeks = DEFAULT_SEEKS;
> + return shrinker;
> }
>
> static int shrink_memcg(struct mem_cgroup *memcg)
> {
> - struct zswap_pool *pool;
> int nid, shrunk = 0;
>
> if (!mem_cgroup_zswap_writeback_enabled(memcg))
> @@ -1396,32 +1352,25 @@ static int shrink_memcg(struct mem_cgroup *memcg)
> if (memcg && !mem_cgroup_online(memcg))
> return -ENOENT;
>
> - pool = zswap_pool_current_get();
> - if (!pool)
> - return -EINVAL;
> -
> for_each_node_state(nid, N_NORMAL_MEMORY) {
> unsigned long nr_to_walk = 1;
>
> - shrunk += list_lru_walk_one(&pool->list_lru, nid, memcg,
> + shrunk += list_lru_walk_one(&zswap.list_lru, nid, memcg,
> &shrink_memcg_cb, NULL, &nr_to_walk);
> }
> - zswap_pool_put(pool);
> return shrunk ? 0 : -EAGAIN;
> }
>
> static void shrink_worker(struct work_struct *w)
> {
> - struct zswap_pool *pool = container_of(w, typeof(*pool),
> - shrink_work);
> struct mem_cgroup *memcg;
> int ret, failures = 0;
>
> /* global reclaim will select cgroup in a round-robin fashion. */
> do {
> spin_lock(&zswap_pools_lock);
> - pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
> - memcg = pool->next_shrink;
> + zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);
> + memcg = zswap.next_shrink;
>
> /*
> * We need to retry if we have gone through a full round trip, or if we
> @@ -1445,7 +1394,7 @@ static void shrink_worker(struct work_struct *w)
> if (!mem_cgroup_tryget_online(memcg)) {
> /* drop the reference from mem_cgroup_iter() */
> mem_cgroup_iter_break(NULL, memcg);
> - pool->next_shrink = NULL;
> + zswap.next_shrink = NULL;
> spin_unlock(&zswap_pools_lock);
>
> if (++failures == MAX_RECLAIM_RETRIES)
> @@ -1467,7 +1416,6 @@ static void shrink_worker(struct work_struct *w)
> resched:
> cond_resched();
> } while (!zswap_can_accept());
> - zswap_pool_put(pool);
> }
>
> static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
> @@ -1508,7 +1456,6 @@ bool zswap_store(struct folio *folio)
> struct zswap_entry *entry, *dupentry;
> struct obj_cgroup *objcg = NULL;
> struct mem_cgroup *memcg = NULL;
> - struct zswap_pool *shrink_pool;
>
> VM_WARN_ON_ONCE(!folio_test_locked(folio));
> VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> @@ -1576,7 +1523,7 @@ bool zswap_store(struct folio *folio)
>
> if (objcg) {
> memcg = get_mem_cgroup_from_objcg(objcg);
> - if (memcg_list_lru_alloc(memcg, &entry->pool->list_lru, GFP_KERNEL)) {
> + if (memcg_list_lru_alloc(memcg, &zswap.list_lru, GFP_KERNEL)) {
> mem_cgroup_put(memcg);
> goto put_pool;
> }
> @@ -1607,8 +1554,8 @@ bool zswap_store(struct folio *folio)
> }
> if (entry->length) {
> INIT_LIST_HEAD(&entry->lru);
> - zswap_lru_add(&entry->pool->list_lru, entry);
> - atomic_inc(&entry->pool->nr_stored);
> + zswap_lru_add(&zswap.list_lru, entry);
> + atomic_inc(&zswap.nr_stored);
> }
> spin_unlock(&tree->lock);
>
> @@ -1640,9 +1587,7 @@ bool zswap_store(struct folio *folio)
> return false;
>
> shrink:
> - shrink_pool = zswap_pool_last_get();
> - if (shrink_pool && !queue_work(shrink_wq, &shrink_pool->shrink_work))
> - zswap_pool_put(shrink_pool);
> + queue_work(shrink_wq, &zswap.shrink_work);
> goto reject;
> }
>
> @@ -1804,6 +1749,21 @@ static int zswap_setup(void)
> if (ret)
> goto hp_fail;
>
> + shrink_wq = alloc_workqueue("zswap-shrink",
> + WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> + if (!shrink_wq)
> + goto hp_fail;
> +
> + zswap.shrinker = zswap_alloc_shrinker();
> + if (!zswap.shrinker)
> + goto shrinker_fail;
> + if (list_lru_init_memcg(&zswap.list_lru, zswap.shrinker))
> + goto lru_fail;
> + shrinker_register(zswap.shrinker);
> +
> + INIT_WORK(&zswap.shrink_work, shrink_worker);
> + atomic_set(&zswap.nr_stored, 0);
> +
> pool = __zswap_pool_create_fallback();
> if (pool) {
> pr_info("loaded using pool %s/%s\n", pool->tfm_name,
> @@ -1815,19 +1775,16 @@ static int zswap_setup(void)
> zswap_enabled = false;
> }
>
> - shrink_wq = alloc_workqueue("zswap-shrink",
> - WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> - if (!shrink_wq)
> - goto fallback_fail;
> -
> if (zswap_debugfs_init())
> pr_warn("debugfs initialization failed\n");
> zswap_init_state = ZSWAP_INIT_SUCCEED;
> return 0;
>
> -fallback_fail:
> - if (pool)
> - zswap_pool_destroy(pool);
> +lru_fail:
> + list_lru_destroy(&zswap.list_lru);
> + shrinker_free(zswap.shrinker);
> +shrinker_fail:
> + destroy_workqueue(shrink_wq);
> hp_fail:
> kmem_cache_destroy(zswap_entry_cache);
> cache_fail:
>
> --
> b4 0.10.1
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