Re: [PATCH v3 2/5] zswap: make shrinking memcg-aware

Yosry Ahmed
2023-10-18T23:21:49Z

On Tue, Oct 17, 2023 at 4:21 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> From: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
>
> Currently, we only have a single global LRU for zswap. This makes it
> impossible to perform worload-specific shrinking - an memcg cannot
> determine which pages in the pool it owns, and often ends up writing
> pages from other memcgs. This issue has been previously observed in
> practice and mitigated by simply disabling memcg-initiated shrinking:
>
> https://lore.kernel.org/all/20230530232435.3097106-1-nphamcs@gmail.com/T/#u
>
> This patch fully resolves the issue by replacing the global zswap LRU
> with memcg- and NUMA-specific LRUs, and modify the reclaim logic:
>
> a) When a store attempt hits an memcg limit, it now triggers a
> synchronous reclaim attempt that, if successful, allows the new
> hotter page to be accepted by zswap.
> b) If the store attempt instead hits the global zswap limit, it will
> trigger an asynchronous reclaim attempt, in which an memcg is
> selected for reclaim in a round-robin-like fashion.

Could you explain the rationale behind the difference in behavior here
between the global limit and the memcg limit?

>
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> Co-developed-by: Nhat Pham <nphamcs@gmail.com>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> ---
> include/linux/memcontrol.h | 5 ++
> mm/swap.h | 3 +-
> mm/swap_state.c | 17 +++-
> mm/zswap.c | 179 ++++++++++++++++++++++++++-----------
> 4 files changed, 147 insertions(+), 57 deletions(-)

This is a dense patch, I haven't absorbed all of it yet, but the first
round of comments below.

>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 031102ac9311..3de10fabea0f 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1179,6 +1179,11 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
> return NULL;
> }
>
> +static inline struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
> +{
> + return NULL;
> +}
> +
> static inline bool folio_memcg_kmem(struct folio *folio)
> {
> return false;
> diff --git a/mm/swap.h b/mm/swap.h
> index 8a3c7a0ace4f..bbd6ce661a20 100644
> --- a/mm/swap.h
> +++ b/mm/swap.h
> @@ -50,7 +50,8 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> struct vm_area_struct *vma,
> unsigned long addr,
> - bool *new_page_allocated);
> + bool *new_page_allocated,
> + bool fail_if_exists);
> struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
> struct vm_fault *vmf);
> struct page *swapin_readahead(swp_entry_t entry, gfp_t flag,
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index b3b14bd0dd64..0356df52b06a 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -411,7 +411,7 @@ struct folio *filemap_get_incore_folio(struct address_space *mapping,
>
> struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> struct vm_area_struct *vma, unsigned long addr,
> - bool *new_page_allocated)
> + bool *new_page_allocated, bool fail_if_exists)

nit: I don't feel like "fail" is the correct word here. Perhaps "skip"?

> {
> struct swap_info_struct *si;
> struct folio *folio;
> @@ -468,6 +468,15 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> if (err != -EEXIST)
> goto fail_put_swap;
>
> + /*
> + * This check guards against a state that happens if a call
> + * to __read_swap_cache_async triggers a reclaim, if the
> + * reclaimer (zswap's writeback as of now) then decides to
> + * reclaim that same entry, then the subsequent call to
> + * __read_swap_cache_async would get stuck in this loop.

I think this comment needs to first state that it is protecting
against a recursive call in general, not necessarily in reclaim, as
__read_swap_cache_async() is not usually called in the context of
reclaim so this can be confusing. Then it can give the exact example
we have today. Perhaps something like:

Protect against a recursive call to __read_swap_cache_async() on the
same entry waiting forever here because SWAP_HAS_CACHE is set but the
folio is not the swap cache yet. This can happen today if
mem_cgroup_swapin_charge_folio() below triggers reclaim through zswap,
which may call __read_swap_cache_async() in the writeback path.

> + */
> + if (fail_if_exists && err == -EEXIST)

We already made sure in the preceding condition that err is -EEXIST.

> + goto fail_put_swap;
> /*
> * We might race against __delete_from_swap_cache(), and
> * stumble across a swap_map entry whose SWAP_HAS_CACHE
> @@ -530,7 +539,7 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> {
> bool page_was_allocated;
> struct page *retpage = __read_swap_cache_async(entry, gfp_mask,
> - vma, addr, &page_was_allocated);
> + vma, addr, &page_was_allocated, false);
>
> if (page_was_allocated)
> swap_readpage(retpage, false, plug);
> @@ -649,7 +658,7 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> /* Ok, do the async read-ahead now */
> page = __read_swap_cache_async(
> swp_entry(swp_type(entry), offset),
> - gfp_mask, vma, addr, &page_allocated);
> + gfp_mask, vma, addr, &page_allocated, false);
> if (!page)
> continue;
> if (page_allocated) {
> @@ -815,7 +824,7 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
> pte_unmap(pte);
> pte = NULL;
> page = __read_swap_cache_async(entry, gfp_mask, vma,
> - addr, &page_allocated);
> + addr, &page_allocated, false);
> if (!page)
> continue;
> if (page_allocated) {
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 083c693602b8..d2989ad11814 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -34,6 +34,7 @@
> #include <linux/writeback.h>
> #include <linux/pagemap.h>
> #include <linux/workqueue.h>
> +#include <linux/list_lru.h>
>
> #include "swap.h"
> #include "internal.h"
> @@ -171,8 +172,8 @@ 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;
> + struct list_lru list_lru;
> + struct mem_cgroup *next_shrink;
> };
>
> /*
> @@ -288,15 +289,25 @@ static void zswap_update_total_size(void)
> zswap_pool_total_size = total;
> }
>
> +static inline struct mem_cgroup *get_mem_cgroup_from_entry(struct zswap_entry *entry)
> +{
> + return entry->objcg ? get_mem_cgroup_from_objcg(entry->objcg) : NULL;
> +}
> +
> +static inline int entry_to_nid(struct zswap_entry *entry)
> +{
> + return page_to_nid(virt_to_page(entry));
> +}
> +
> /*********************************
> * zswap entry functions
> **********************************/
> static struct kmem_cache *zswap_entry_cache;
>
> -static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
> +static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp, int nid)
> {
> struct zswap_entry *entry;
> - entry = kmem_cache_alloc(zswap_entry_cache, gfp);
> + entry = kmem_cache_alloc_node(zswap_entry_cache, gfp, nid);
> if (!entry)
> return NULL;
> entry->refcount = 1;
> @@ -309,6 +320,27 @@ static void zswap_entry_cache_free(struct zswap_entry *entry)
> kmem_cache_free(zswap_entry_cache, entry);
> }
>
> +/*********************************
> +* lru functions
> +**********************************/
> +static bool zswap_lru_add(struct list_lru *list_lru, struct zswap_entry *entry)
> +{
> + struct mem_cgroup *memcg = get_mem_cgroup_from_entry(entry);

Could we avoid the need for get/put with an rcu_read_lock() instead?

> + bool added = __list_lru_add(list_lru, &entry->lru, entry_to_nid(entry), memcg);
> +
> + mem_cgroup_put(memcg);
> + return added;
> +}
> +
> +static bool zswap_lru_del(struct list_lru *list_lru, struct zswap_entry *entry)
> +{
> + struct mem_cgroup *memcg = get_mem_cgroup_from_entry(entry);
> + bool removed = __list_lru_del(list_lru, &entry->lru, entry_to_nid(entry), memcg);
> +
> + mem_cgroup_put(memcg);
> + return removed;
> +}
> +
> /*********************************
> * rbtree functions
> **********************************/
> @@ -393,9 +425,7 @@ static void zswap_free_entry(struct zswap_entry *entry)
> if (!entry->length)
> atomic_dec(&zswap_same_filled_pages);
> else {
> - spin_lock(&entry->pool->lru_lock);
> - list_del(&entry->lru);
> - spin_unlock(&entry->pool->lru_lock);
> + zswap_lru_del(&entry->pool->list_lru, entry);
> zpool_free(zswap_find_zpool(entry), entry->handle);
> zswap_pool_put(entry->pool);
> }
> @@ -629,21 +659,16 @@ static void zswap_invalidate_entry(struct zswap_tree *tree,
> zswap_entry_put(tree, entry);
> }
>
> -static int zswap_reclaim_entry(struct zswap_pool *pool)
> +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,
> + spinlock_t *lock, void *arg)
> {
> - struct zswap_entry *entry;
> + struct zswap_entry *entry = container_of(item, struct zswap_entry, lru);
> + struct mem_cgroup *memcg;
> struct zswap_tree *tree;
> pgoff_t swpoffset;
> - int ret;
> + enum lru_status ret = LRU_REMOVED_RETRY;
> + int writeback_result;
>
> - /* Get an entry off the LRU */
> - spin_lock(&pool->lru_lock);
> - if (list_empty(&pool->lru)) {
> - spin_unlock(&pool->lru_lock);
> - return -EINVAL;
> - }
> - entry = list_last_entry(&pool->lru, struct zswap_entry, lru);
> - list_del_init(&entry->lru);
> /*
> * Once the lru lock is dropped, the entry might get freed. The
> * swpoffset is copied to the stack, and entry isn't deref'd again
> @@ -651,28 +676,33 @@ static int zswap_reclaim_entry(struct zswap_pool *pool)
> */
> swpoffset = swp_offset(entry->swpentry);
> tree = zswap_trees[swp_type(entry->swpentry)];
> - spin_unlock(&pool->lru_lock);
> + list_lru_isolate(l, item);
> + spin_unlock(lock);

Perhaps a comment somewhere stating that we only return either
LRU_REMOVED_RETRY or LRU_RETRY, so it's fine to drop and reacquire the
lock.

>
> /* Check for invalidate() race */
> spin_lock(&tree->lock);
> if (entry != zswap_rb_search(&tree->rbroot, swpoffset)) {
> - ret = -EAGAIN;
> goto unlock;
> }

nit: braces no longer needed?

> /* Hold a reference to prevent a free during writeback */
> zswap_entry_get(entry);
> spin_unlock(&tree->lock);
>
> - ret = zswap_writeback_entry(entry, tree);
> + writeback_result = zswap_writeback_entry(entry, tree);
>
> spin_lock(&tree->lock);
> - if (ret) {
> - /* Writeback failed, put entry back on LRU */
> - spin_lock(&pool->lru_lock);
> - list_move(&entry->lru, &pool->lru);
> - spin_unlock(&pool->lru_lock);
> + if (writeback_result) {
> + zswap_reject_reclaim_fail++;
> + memcg = get_mem_cgroup_from_entry(entry);
> + spin_lock(lock);
> + /* we cannot use zswap_lru_add here, because it increments node's lru count */
> + list_lru_putback(&entry->pool->list_lru, item, entry_to_nid(entry), memcg);
> + spin_unlock(lock);
> + mem_cgroup_put(memcg);
> + ret = LRU_RETRY;
> goto put_unlock;
> }
> + zswap_written_back_pages++;

Why is this moved here from zswap_writeback_entry()? Also why is
zswap_reject_reclaim_fail incremented here instead of inside
zswap_writeback_entry()?

>
> /*
> * Writeback started successfully, the page now belongs to the
> @@ -686,7 +716,36 @@ static int zswap_reclaim_entry(struct zswap_pool *pool)
> zswap_entry_put(tree, entry);
> unlock:
> spin_unlock(&tree->lock);
> - return ret ? -EAGAIN : 0;
> + spin_lock(lock);
> + return ret;
> +}
> +
> +static int shrink_memcg(struct mem_cgroup *memcg)
> +{
> + struct zswap_pool *pool;
> + int nid, shrunk = 0;
> +
> + pool = zswap_pool_current_get();
> + if (!pool)
> + return -EINVAL;
> +
> + /*
> + * Skip zombies because their LRUs are reparented and we would be
> + * reclaiming from the parent instead of the dead memcgroup.

nit: s/memcgroup/memcg.

> + */
> + if (memcg && !mem_cgroup_online(memcg))
> + goto out;

If we move this above zswap_pool_current_get(), we can return directly
and remove the label. I noticed we will return -EAGAIN if memcg is
offline. IIUC -EAGAIN for the caller will move on to the next memcg,
but I am wondering if a different errno would be clearer here.

> +
> + for_each_node_state(nid, N_NORMAL_MEMORY) {
> + unsigned long nr_to_walk = 1;
> +
> + if (list_lru_walk_one(&pool->list_lru, nid, memcg, &shrink_memcg_cb,
> + NULL, &nr_to_walk))
> + shrunk++;

nit:
shrunk += list_lru_walk_one(..);

> + }
> +out:
> + zswap_pool_put(pool);
> + return shrunk ? 0 : -EAGAIN;
> }
>
> static void shrink_worker(struct work_struct *w)
> @@ -695,10 +754,13 @@ static void shrink_worker(struct work_struct *w)
> shrink_work);
> int ret, failures = 0;
>
> + /* global reclaim will select cgroup in a round-robin fashion. */
> do {
> - ret = zswap_reclaim_entry(pool);
> + pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);

Perhaps next_shrink_memcg is a better name here?

> +
> + ret = shrink_memcg(pool->next_shrink);
> +
> if (ret) {
> - zswap_reject_reclaim_fail++;
> if (ret != -EAGAIN)
> break;
> if (++failures == MAX_RECLAIM_RETRIES)
> @@ -764,8 +826,7 @@ 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);
> + list_lru_init_memcg(&pool->list_lru, NULL);
> INIT_WORK(&pool->shrink_work, shrink_worker);
>
> zswap_pool_debug("created", pool);
> @@ -831,6 +892,9 @@ static void zswap_pool_destroy(struct zswap_pool *pool)
>
> cpuhp_state_remove_instance(CPUHP_MM_ZSWP_POOL_PREPARE, &pool->node);
> free_percpu(pool->acomp_ctx);
> + list_lru_destroy(&pool->list_lru);
> + if (pool->next_shrink)
> + mem_cgroup_put(pool->next_shrink);
> for (i = 0; i < ZSWAP_NR_ZPOOLS; i++)
> zpool_destroy_pool(pool->zpools[i]);
> kfree(pool);
> @@ -1076,7 +1140,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>
> /* try to allocate swap cache page */
> page = __read_swap_cache_async(swpentry, GFP_KERNEL, NULL, 0,
> - &page_was_allocated);
> + &page_was_allocated, true);
> if (!page) {
> ret = -ENOMEM;
> goto fail;
> @@ -1142,7 +1206,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
> /* start writeback */
> __swap_writepage(page, &wbc);
> put_page(page);
> - zswap_written_back_pages++;
>
> return ret;
>
> @@ -1199,8 +1262,10 @@ bool zswap_store(struct folio *folio)
> struct scatterlist input, output;
> struct crypto_acomp_ctx *acomp_ctx;
> struct obj_cgroup *objcg = NULL;
> + struct mem_cgroup *memcg = NULL;
> struct zswap_pool *pool;
> struct zpool *zpool;
> + int lru_alloc_ret;
> unsigned int dlen = PAGE_SIZE;
> unsigned long handle, value;
> char *buf;
> @@ -1230,15 +1295,15 @@ bool zswap_store(struct folio *folio)
> zswap_invalidate_entry(tree, dupentry);
> }
> spin_unlock(&tree->lock);
> -
> - /*
> - * XXX: zswap reclaim does not work with cgroups yet. Without a
> - * cgroup-aware entry LRU, we will push out entries system-wide based on
> - * local cgroup limits.
> - */
> objcg = get_obj_cgroup_from_folio(folio);
> - if (objcg && !obj_cgroup_may_zswap(objcg))
> - goto reject;
> + if (objcg && !obj_cgroup_may_zswap(objcg)) {
> + memcg = get_mem_cgroup_from_objcg(objcg);
> + if (shrink_memcg(memcg)) {
> + mem_cgroup_put(memcg);
> + goto reject;
> + }
> + mem_cgroup_put(memcg);
> + }
>
> /* reclaim space if needed */
> if (zswap_is_full()) {
> @@ -1254,10 +1319,15 @@ bool zswap_store(struct folio *folio)
> zswap_pool_reached_full = false;
> }
>
> + pool = zswap_pool_current_get();
> + if (!pool)
> + goto reject;
> +

Why do we need to move zswap_pool_current_get() up here?

> /* allocate entry */
> - entry = zswap_entry_cache_alloc(GFP_KERNEL);
> + entry = zswap_entry_cache_alloc(GFP_KERNEL, page_to_nid(page));
> if (!entry) {
> zswap_reject_kmemcache_fail++;
> + zswap_pool_put(pool);
> goto reject;
> }
>
> @@ -1269,6 +1339,7 @@ bool zswap_store(struct folio *folio)
> entry->length = 0;
> entry->value = value;
> atomic_inc(&zswap_same_filled_pages);
> + zswap_pool_put(pool);
> goto insert_entry;
> }
> kunmap_atomic(src);
> @@ -1278,9 +1349,15 @@ bool zswap_store(struct folio *folio)
> goto freepage;
>
> /* if entry is successfully added, it keeps the reference */
> - entry->pool = zswap_pool_current_get();
> - if (!entry->pool)
> - goto freepage;
> + entry->pool = pool;
> + if (objcg) {
> + memcg = get_mem_cgroup_from_objcg(objcg);
> + lru_alloc_ret = memcg_list_lru_alloc(memcg, &pool->list_lru, GFP_KERNEL);
> + mem_cgroup_put(memcg);
> +
> + if (lru_alloc_ret)
> + goto freepage;
> + }
>
> /* compress */
> acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> @@ -1358,9 +1435,8 @@ bool zswap_store(struct folio *folio)
> zswap_invalidate_entry(tree, dupentry);
> }
> if (entry->length) {
> - spin_lock(&entry->pool->lru_lock);
> - list_add(&entry->lru, &entry->pool->lru);
> - spin_unlock(&entry->pool->lru_lock);
> + INIT_LIST_HEAD(&entry->lru);
> + zswap_lru_add(&pool->list_lru, entry);
> }
> spin_unlock(&tree->lock);
>
> @@ -1373,8 +1449,8 @@ bool zswap_store(struct folio *folio)
>
> put_dstmem:
> mutex_unlock(acomp_ctx->mutex);
> - zswap_pool_put(entry->pool);
> freepage:
> + zswap_pool_put(entry->pool);
> zswap_entry_cache_free(entry);
> reject:
> if (objcg)
> @@ -1467,9 +1543,8 @@ bool zswap_load(struct folio *folio)
> zswap_invalidate_entry(tree, entry);
> folio_mark_dirty(folio);
> } else if (entry->length) {
> - spin_lock(&entry->pool->lru_lock);
> - list_move(&entry->lru, &entry->pool->lru);
> - spin_unlock(&entry->pool->lru_lock);
> + zswap_lru_del(&entry->pool->list_lru, entry);
> + zswap_lru_add(&entry->pool->list_lru, entry);
> }
> zswap_entry_put(tree, entry);
> spin_unlock(&tree->lock);
> --
> 2.34.1