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

From: Nhat Pham
Date: Wed Oct 18 2023 - 19:47:17 EST

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On Wed, Oct 18, 2023 at 4:20 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Tue, Oct 17, 2023 at 4:21 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> >
> > From: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
> >
> > 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@xxxxxxxxx/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?

The global limit hit reclaim behavior was previously asynchronous too.
We just added the round-robin part because now the zswap LRU is
cgroup-aware :)

For the cgroup limit hit, however, we cannot make it asynchronous,
as it is a bit hairy to add a per-cgroup shrink_work. So, we just
perform the reclaim synchronously.

The question is whether it makes sense to make the global limit
reclaim synchronous too. That is a task of its own IMO.

(FWIW, this somewhat mirrors the direct reclaimer v.s kswapd
story to me, but don't quote me too hard on this).

>
> >
> > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
> > Co-developed-by: Nhat Pham <nphamcs@xxxxxxxxx>
> > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> > ---
> > 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.

Regardless, thanks for the feedback, Yosry! Domenico definitely
knows more than me about this, but I'll respond with what I know,
and he can expand and/or fact-check me :)

>
> >
> > 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?

Ah, for some reason checkpatch did not pick up on this.
Weird.

>
> > /* 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()?

Domenico should know this better than me, but my understanding
is that moving it here protects concurrent modifications of
zswap_written_back_pages with the tree lock.

Is writeback single-threaded in the past? This counter is non-atomic,
and doesn't seem to be protected by any locks...

There definitely can be concurrent stores now though - with
a synchronous reclaim from cgroup-limit hit and another
from the old shrink worker.

(and with the new zswap shrinker, concurrent reclaim is
the expectation!)

zswap_reject_reclaim_fail was previously incremented in
shrink_worker I think. We need it to be incremented
for the shrinker as well, so might as well move it here.

>
> >
> > /*
> > * 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(..);

yeah might be a tad cleaner.

>
> > + }
> > +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

I don't have (strong) opinions or (educated) guesses
on the rest.

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