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From:   Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Date:   Wed, 27 Sep 2023 21:48:10 +0200
Message-ID: <CA+CLi1httFOg4OM-0Hu3+fOvya4kpacCqN7A0upqOt4-YJiECg@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] zswap: make shrinking memcg-aware
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Nhat Pham <nphamcs@gmail.com>, akpm@linux-foundation.org,
        hannes@cmpxchg.org, sjenning@redhat.com, ddstreet@ieee.org,
        vitaly.wool@konsulko.com, mhocko@kernel.org,
        roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, linux-mm@kvack.org, kernel-team@meta.com,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Chris Li <chrisl@kernel.org>
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On Mon, Sep 25, 2023 at 10:17=E2=80=AFPM Yosry Ahmed <yosryahmed@google.com=
> wrote:
>
> +Chris Li
>
> On Tue, Sep 19, 2023 at 10:14=E2=80=AFAM Nhat Pham <nphamcs@gmail.com> wr=
ote:
> >
> > 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.
>
> Hey Nhat,
>
> I didn't take a very close look as I am currently swamped, but going
> through the patch I have some comments/questions below.
>
> I am not very familiar with list_lru, but it seems like the existing
> API derives the node and memcg from the list item itself. Seems like
> we can avoid a lot of changes if we allocate struct zswap_entry from
> the same node as the page, and account it to the same memcg. Would
> this be too much of a change or too strong of a restriction? It's a
> slab allocation and we will free memory on that node/memcg right
> after.
>
> >
> > 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/list_lru.h   |  39 +++++++
> >  include/linux/memcontrol.h |   5 +
> >  include/linux/zswap.h      |   9 ++
> >  mm/list_lru.c              |  46 ++++++--
> >  mm/swap_state.c            |  19 ++++
> >  mm/zswap.c                 | 221 +++++++++++++++++++++++++++++--------
> >  6 files changed, 287 insertions(+), 52 deletions(-)
> >
> > diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
> > index b35968ee9fb5..b517b4e2c7c4 100644
> > --- a/include/linux/list_lru.h
> > +++ b/include/linux/list_lru.h
> > @@ -89,6 +89,24 @@ void memcg_reparent_list_lrus(struct mem_cgroup *mem=
cg, struct mem_cgroup *paren
> >   */
> >  bool list_lru_add(struct list_lru *lru, struct list_head *item);
> >
> > +/**
> > + * __list_lru_add: add an element to a specific sublist.
> > + * @list_lru: the lru pointer
> > + * @item: the item to be added.
> > + * @memcg: the cgroup of the sublist to add the item to.
> > + * @nid: the node id of the sublist to add the item to.
> > + *
> > + * This function is similar to list_lru_add(), but it allows the calle=
r to
> > + * specify the sublist to which the item should be added. This can be =
useful
> > + * when the list_head node is not necessarily in the same cgroup and N=
UMA node
> > + * as the data it represents, such as zswap, where the list_head node =
could be
> > + * from kswapd and the data from a different cgroup altogether.
> > + *
> > + * Return value: true if the list was updated, false otherwise
> > + */
> > +bool __list_lru_add(struct list_lru *lru, struct list_head *item, int =
nid,
> > +               struct mem_cgroup *memcg);
> > +
> >  /**
> >   * list_lru_del: delete an element to the lru list
> >   * @list_lru: the lru pointer
> > @@ -102,6 +120,18 @@ bool list_lru_add(struct list_lru *lru, struct lis=
t_head *item);
> >   */
> >  bool list_lru_del(struct list_lru *lru, struct list_head *item);
> >
> > +/**
> > + * __list_lru_delete: delete an element from a specific sublist.
> > + * @list_lru: the lru pointer
> > + * @item: the item to be deleted.
> > + * @memcg: the cgroup of the sublist to delete the item from.
> > + * @nid: the node id of the sublist to delete the item from.
> > + *
> > + * Return value: true if the list was updated, false otherwise.
> > + */
> > +bool __list_lru_del(struct list_lru *lru, struct list_head *item, int =
nid,
> > +               struct mem_cgroup *memcg);
> > +
> >  /**
> >   * list_lru_count_one: return the number of objects currently held by =
@lru
> >   * @lru: the lru pointer.
> > @@ -137,6 +167,15 @@ void list_lru_isolate(struct list_lru_one *list, s=
truct list_head *item);
> >  void list_lru_isolate_move(struct list_lru_one *list, struct list_head=
 *item,
> >                            struct list_head *head);
> >
> > +/*
> > + * list_lru_putback: undo list_lru_isolate.
> > + *
> > + * Since we might have dropped the LRU lock in between, recompute list=
_lru_one
> > + * from the node's id and memcg.
> > + */
> > +void list_lru_putback(struct list_lru *lru, struct list_head *item, in=
t nid,
> > +               struct mem_cgroup *memcg);
> > +
> >  typedef enum lru_status (*list_lru_walk_cb)(struct list_head *item,
> >                 struct list_lru_one *list, spinlock_t *lock, void *cb_a=
rg);
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 67b823dfa47d..05d34b328d9d 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -1179,6 +1179,11 @@ static inline struct mem_cgroup *page_memcg_chec=
k(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/include/linux/zswap.h b/include/linux/zswap.h
> > index 2a60ce39cfde..04f80b64a09b 100644
> > --- a/include/linux/zswap.h
> > +++ b/include/linux/zswap.h
> > @@ -15,6 +15,8 @@ bool zswap_load(struct folio *folio);
> >  void zswap_invalidate(int type, pgoff_t offset);
> >  void zswap_swapon(int type);
> >  void zswap_swapoff(int type);
> > +bool zswap_remove_swpentry_from_lru(swp_entry_t swpentry);
> > +void zswap_insert_swpentry_into_lru(swp_entry_t swpentry);
> >
> >  #else
> >
> > @@ -32,6 +34,13 @@ static inline void zswap_invalidate(int type, pgoff_=
t offset) {}
> >  static inline void zswap_swapon(int type) {}
> >  static inline void zswap_swapoff(int type) {}
> >
> > +static inline bool zswap_remove_swpentry_from_lru(swp_entry_t swpentry=
)
> > +{
> > +       return false;
> > +}
> > +
> > +static inline void zswap_insert_swpentry_into_lru(swp_entry_t swpentry=
) {}
> > +
> >  #endif
> >
> >  #endif /* _LINUX_ZSWAP_H */
> > diff --git a/mm/list_lru.c b/mm/list_lru.c
> > index a05e5bef3b40..37c5c2ef6c0e 100644
> > --- a/mm/list_lru.c
> > +++ b/mm/list_lru.c
> > @@ -119,18 +119,26 @@ list_lru_from_kmem(struct list_lru *lru, int nid,=
 void *ptr,
> >  bool list_lru_add(struct list_lru *lru, struct list_head *item)
> >  {
> >         int nid =3D page_to_nid(virt_to_page(item));
> > +       struct mem_cgroup *memcg =3D list_lru_memcg_aware(lru) ?
> > +               mem_cgroup_from_slab_obj(item) : NULL;
> > +
> > +       return __list_lru_add(lru, item, nid, memcg);
> > +}
> > +EXPORT_SYMBOL_GPL(list_lru_add);
> > +
> > +bool __list_lru_add(struct list_lru *lru, struct list_head *item, int =
nid,
> > +               struct mem_cgroup *memcg)
> > +{
> >         struct list_lru_node *nlru =3D &lru->node[nid];
> > -       struct mem_cgroup *memcg;
> >         struct list_lru_one *l;
> >
> >         spin_lock(&nlru->lock);
> >         if (list_empty(item)) {
> > -               l =3D list_lru_from_kmem(lru, nid, item, &memcg);
> > +               l =3D list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(m=
emcg));
> >                 list_add_tail(item, &l->list);
> >                 /* Set shrinker bit if the first element was added */
> >                 if (!l->nr_items++)
> > -                       set_shrinker_bit(memcg, nid,
> > -                                        lru_shrinker_id(lru));
> > +                       set_shrinker_bit(memcg, nid, lru_shrinker_id(lr=
u));
>
> Unrelated diff.
>
> >                 nlru->nr_items++;
> >                 spin_unlock(&nlru->lock);
> >                 return true;
> > @@ -138,17 +146,27 @@ bool list_lru_add(struct list_lru *lru, struct li=
st_head *item)
> >         spin_unlock(&nlru->lock);
> >         return false;
> >  }
> > -EXPORT_SYMBOL_GPL(list_lru_add);
> > +EXPORT_SYMBOL_GPL(__list_lru_add);
> >
> >  bool list_lru_del(struct list_lru *lru, struct list_head *item)
> >  {
> >         int nid =3D page_to_nid(virt_to_page(item));
> > +       struct mem_cgroup *memcg =3D list_lru_memcg_aware(lru) ?
> > +               mem_cgroup_from_slab_obj(item) : NULL;
> > +
> > +       return __list_lru_del(lru, item, nid, memcg);
> > +}
> > +EXPORT_SYMBOL_GPL(list_lru_del);
> > +
> > +bool __list_lru_del(struct list_lru *lru, struct list_head *item, int =
nid,
> > +               struct mem_cgroup *memcg)
> > +{
> >         struct list_lru_node *nlru =3D &lru->node[nid];
> >         struct list_lru_one *l;
> >
> >         spin_lock(&nlru->lock);
> >         if (!list_empty(item)) {
> > -               l =3D list_lru_from_kmem(lru, nid, item, NULL);
>
> If we decide to keep the list_lru.c changes, do we have any other
> callers of list_lru_from_kmem()?

I see a commit already in mm-unstable that removes it.

>
> > +               l =3D list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(m=
emcg));
> >                 list_del_init(item);
> >                 l->nr_items--;
> >                 nlru->nr_items--;
> > @@ -158,7 +176,7 @@ bool list_lru_del(struct list_lru *lru, struct list=
_head *item)
> >         spin_unlock(&nlru->lock);
> >         return false;
> >  }
> > -EXPORT_SYMBOL_GPL(list_lru_del);
> > +EXPORT_SYMBOL_GPL(__list_lru_del);
> >
> >  void list_lru_isolate(struct list_lru_one *list, struct list_head *ite=
m)
> >  {
> > @@ -175,6 +193,20 @@ void list_lru_isolate_move(struct list_lru_one *li=
st, struct list_head *item,
> >  }
> >  EXPORT_SYMBOL_GPL(list_lru_isolate_move);
> >
> > +void list_lru_putback(struct list_lru *lru, struct list_head *item, in=
t nid,
> > +               struct mem_cgroup *memcg)
> > +{
> > +       struct list_lru_one *list =3D
> > +               list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg))=
;
> > +
> > +       if (list_empty(item)) {
> > +               list_add_tail(item, &list->list);
> > +               if (!list->nr_items++)
> > +                       set_shrinker_bit(memcg, nid, lru_shrinker_id(lr=
u));
> > +       }
> > +}
> > +EXPORT_SYMBOL_GPL(list_lru_putback);
> > +
> >  unsigned long list_lru_count_one(struct list_lru *lru,
> >                                  int nid, struct mem_cgroup *memcg)
> >  {
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index b3b14bd0dd64..1c826737aacb 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -21,6 +21,7 @@
> >  #include <linux/swap_slots.h>
> >  #include <linux/huge_mm.h>
> >  #include <linux/shmem_fs.h>
> > +#include <linux/zswap.h>
> >  #include "internal.h"
> >  #include "swap.h"
> >
> > @@ -417,6 +418,7 @@ struct page *__read_swap_cache_async(swp_entry_t en=
try, gfp_t gfp_mask,
> >         struct folio *folio;
> >         struct page *page;
> >         void *shadow =3D NULL;
> > +       bool zswap_lru_removed =3D false;
> >
> >         *new_page_allocated =3D false;
> >         si =3D get_swap_device(entry);
> > @@ -485,6 +487,17 @@ struct page *__read_swap_cache_async(swp_entry_t e=
ntry, gfp_t gfp_mask,
> >         __folio_set_locked(folio);
> >         __folio_set_swapbacked(folio);
> >
> > +       /*
> > +        * Page fault might itself trigger reclaim, on a zswap object t=
hat
> > +        * corresponds to the same swap entry. However, as the swap ent=
ry has
> > +        * previously been pinned, the task will run into an infinite l=
oop trying
> > +        * to pin the swap entry again.
> > +        *
> > +        * To prevent this from happening, we remove it from the zswap
> > +        * LRU to prevent its reclamation.
> > +        */
> > +       zswap_lru_removed =3D zswap_remove_swpentry_from_lru(entry);
> > +
>
> This will add a zswap lookup (and potentially an insertion below) in
> every single swap fault path, right?. Doesn't this introduce latency
> regressions? I am also not a fan of having zswap-specific details in
> this path.
>
> When you say "pinned", do you mean the call to swapcache_prepare()
> above (i.e. setting SWAP_HAS_CACHE)? IIUC, the scenario you are
> worried about is that the following call to charge the page may invoke
> reclaim, go into zswap, and try to writeback the same page we are
> swapping in here. The writeback call will recurse into
> __read_swap_cache_async(), call swapcache_prepare() and get EEXIST,
> and keep looping indefinitely. Is this correct?
>
> If yes, can we handle this by adding a flag to
> __read_swap_cache_async() that basically says "don't wait for
> SWAP_HAS_CACHE and the swapcache to be consistent, if
> swapcache_prepare() returns EEXIST just fail and return"? The zswap
> writeback path can pass in this flag and skip such pages. We might
> want to modify the writeback code to put back those pages at the end
> of the lru instead of in the beginning.

Thanks for the suggestion, this actually works and it seems cleaner so I th=
ink
we'll go for your solution.

>
> >         if (mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mask, entry=
))
> >                 goto fail_unlock;
> >
> > @@ -497,6 +510,9 @@ struct page *__read_swap_cache_async(swp_entry_t en=
try, gfp_t gfp_mask,
> >         if (shadow)
> >                 workingset_refault(folio, shadow);
> >
> > +       if (zswap_lru_removed)
> > +               zswap_insert_swpentry_into_lru(entry);
> > +
> >         /* Caller will initiate read into locked folio */
> >         folio_add_lru(folio);
> >         *new_page_allocated =3D true;
> > @@ -506,6 +522,9 @@ struct page *__read_swap_cache_async(swp_entry_t en=
try, gfp_t gfp_mask,
> >         return page;
> >
> >  fail_unlock:
> > +       if (zswap_lru_removed)
> > +               zswap_insert_swpentry_into_lru(entry);
> > +
> >         put_swap_folio(folio, entry);
> >         folio_unlock(folio);
> >         folio_put(folio);
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 412b1409a0d7..1a469e5d5197 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;
> >  };
> >
> >  /*
> > @@ -209,6 +210,7 @@ struct zswap_entry {
> >                 unsigned long value;
> >         };
> >         struct obj_cgroup *objcg;
> > +       int nid;
> >         struct list_head lru;
> >  };
>
> Ideally this can be avoided if we can allocate struct zswap_entry on
> the correct node.

We didn't consider allocating the entry on the node without charging it to =
the
memcg, we'll try it and if it's doable it would be a good compromise to avo=
id
adding the node id here.

>
> >
> > @@ -309,6 +311,29 @@ static void zswap_entry_cache_free(struct zswap_en=
try *entry)
> >         kmem_cache_free(zswap_entry_cache, entry);
> >  }
> >
> > +/*********************************
> > +* lru functions
> > +**********************************/
> > +static bool zswap_lru_add(struct list_lru *list_lru, struct zswap_entr=
y *entry)
> > +{
> > +       struct mem_cgroup *memcg =3D entry->objcg ?
> > +               get_mem_cgroup_from_objcg(entry->objcg) : NULL;
>
> This line is repeated at least 3 times, perhaps add a helper for it?
> get_mem_cgroup_from_zswap()?
>
> > +       bool added =3D __list_lru_add(list_lru, &entry->lru, entry->nid=
, memcg);
> > +
> > +       mem_cgroup_put(memcg);
> > +       return added;
> > +}
> > +
> > +static bool zswap_lru_del(struct list_lru *list_lru, struct zswap_entr=
y *entry)
> > +{
> > +       struct mem_cgroup *memcg =3D entry->objcg ?
> > +               get_mem_cgroup_from_objcg(entry->objcg) : NULL;
> > +       bool removed =3D __list_lru_del(list_lru, &entry->lru, entry->n=
id, memcg);
> > +
> > +       mem_cgroup_put(memcg);
> > +       return removed;
> > +}
> > +
> >  /*********************************
> >  * rbtree functions
> >  **********************************/
> > @@ -393,9 +418,7 @@ static void zswap_free_entry(struct zswap_entry *en=
try)
> >         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 +652,16 @@ static void zswap_invalidate_entry(struct zswap_t=
ree *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 =3D container_of(item, struct zswap_e=
ntry, lru);
> > +       struct mem_cgroup *memcg;
> >         struct zswap_tree *tree;
> >         pgoff_t swpoffset;
> > -       int ret;
> > +       enum lru_status ret =3D 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 =3D 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 ag=
ain
> > @@ -651,26 +669,35 @@ static int zswap_reclaim_entry(struct zswap_pool =
*pool)
> >          */
> >         swpoffset =3D swp_offset(entry->swpentry);
> >         tree =3D zswap_trees[swp_type(entry->swpentry)];
> > -       spin_unlock(&pool->lru_lock);
> > +       list_lru_isolate(l, item);
> > +       spin_unlock(lock);
> >
> >         /* Check for invalidate() race */
> >         spin_lock(&tree->lock);
> >         if (entry !=3D zswap_rb_search(&tree->rbroot, swpoffset)) {
> > -               ret =3D -EAGAIN;
> >                 goto unlock;
> >         }
> >         /* Hold a reference to prevent a free during writeback */
> >         zswap_entry_get(entry);
> >         spin_unlock(&tree->lock);
> >
> > -       ret =3D zswap_writeback_entry(entry, tree);
> > +       writeback_result =3D 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++;
> > +
> > +               /* Check for invalidate() race */
> > +               if (entry !=3D zswap_rb_search(&tree->rbroot, swpoffset=
))
> > +                       goto put_unlock;
> > +
> > +               memcg =3D entry->objcg ? get_mem_cgroup_from_objcg(entr=
y->objcg) : NULL;
> > +               spin_lock(lock);
> > +               /* we cannot use zswap_lru_add here, because it increme=
nts node's lru count */
> > +               list_lru_putback(&entry->pool->list_lru, item, entry->n=
id, memcg);
> > +               spin_unlock(lock);
> > +               mem_cgroup_put(memcg);
> > +               ret =3D LRU_RETRY;
> >                 goto put_unlock;
> >         }
> >
> > @@ -686,19 +713,63 @@ 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 =3D 0;
> > +       bool is_empty =3D true;
> > +
> > +       pool =3D zswap_pool_current_get();
> > +       if (!pool)
> > +               return -EINVAL;
> > +
> > +       for_each_node_state(nid, N_NORMAL_MEMORY) {
> > +               unsigned long nr_to_walk =3D 1;
> > +
> > +               if (list_lru_walk_one(&pool->list_lru, nid, memcg, &shr=
ink_memcg_cb,
> > +                                     NULL, &nr_to_walk))
> > +                       shrunk++;
> > +               if (!nr_to_walk)
>
> nr_to_walk will be 0 if we shrunk 1 page, so it's the same condition
> as the above, right?
>
> is_empty seems to be shrunk =3D=3D 0 if I understand correctly, seems lik=
e
> there is no need for both.

It is indeed 0 when we shrunk 1 page, but it could also be 0 also when the
reclaim failed and the list was not empty.

>
> > +                       is_empty =3D false;
> > +       }
> > +       zswap_pool_put(pool);
> > +
> > +       if (is_empty)
> > +               return -EINVAL;
> > +       if (shrunk)
> > +               return 0;
> > +       return -EAGAIN;
> >  }
> >
> >  static void shrink_worker(struct work_struct *w)
> >  {
> >         struct zswap_pool *pool =3D container_of(w, typeof(*pool),
> >                                                 shrink_work);
> > -       int ret, failures =3D 0;
> > +       int ret, failures =3D 0, memcg_selection_failures =3D 0;
> >
> > +       /* global reclaim will select cgroup in a round-robin fashion. =
*/
> >         do {
> > -               ret =3D zswap_reclaim_entry(pool);
> > +               /* previous next_shrink has become a zombie - restart f=
rom the top */
>
> Do we skip zombies because all zswap entries are reparented with the objc=
g?
>
> If yes, why do we restart from the top instead of just skipping them?
> memcgs after a zombie will not be reachable now IIUC.
>
> Also, why explicitly check for zombies instead of having
> shrink_memcg() just skip memcgs with no zswap entries? The logic is
> slightly complicated.

I think you have a point here, I'm not sure if the iteration can go on once=
 we
get a zombie, if it can, we'll just skip it.

>
> > +               if (pool->next_shrink && !mem_cgroup_online(pool->next_=
shrink)) {
> > +                       mem_cgroup_put(pool->next_shrink);
> > +                       pool->next_shrink =3D NULL;
> > +               }
> > +               pool->next_shrink =3D mem_cgroup_iter(NULL, pool->next_=
shrink, NULL);
> > +
> > +               /* fails to find a suitable cgroup - give the worker an=
other chance. */
> > +               if (!pool->next_shrink) {
> > +                       if (++memcg_selection_failures =3D=3D 2)
> > +                               break;
> > +                       continue;
> > +               }
> > +
> > +               ret =3D shrink_memcg(pool->next_shrink);
> > +
> >                 if (ret) {
> > -                       zswap_reject_reclaim_fail++;
> >                         if (ret !=3D -EAGAIN)
> >                                 break;
> >                         if (++failures =3D=3D MAX_RECLAIM_RETRIES)
> > @@ -764,9 +835,8 @@ static struct zswap_pool *zswap_pool_create(char *t=
ype, 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);
> > +       list_lru_init_memcg(&pool->list_lru, NULL);
> >
> >         zswap_pool_debug("created", pool);
> >
> > @@ -831,6 +901,9 @@ static void zswap_pool_destroy(struct zswap_pool *p=
ool)
> >
> >         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 =3D 0; i < ZSWAP_NR_ZPOOLS; i++)
> >                 zpool_destroy_pool(pool->zpools[i]);
> >         kfree(pool);
> > @@ -1199,8 +1272,10 @@ bool zswap_store(struct folio *folio)
> >         struct scatterlist input, output;
> >         struct crypto_acomp_ctx *acomp_ctx;
> >         struct obj_cgroup *objcg =3D NULL;
> > +       struct mem_cgroup *memcg =3D NULL;
> >         struct zswap_pool *pool;
> >         struct zpool *zpool;
> > +       int lru_alloc_ret;
> >         unsigned int dlen =3D PAGE_SIZE;
> >         unsigned long handle, value;
> >         char *buf;
> > @@ -1218,14 +1293,15 @@ bool zswap_store(struct folio *folio)
> >         if (!zswap_enabled || !tree)
> >                 return false;
> >
> > -       /*
> > -        * 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 =3D get_obj_cgroup_from_folio(folio);
> > -       if (objcg && !obj_cgroup_may_zswap(objcg))
> > -               goto reject;
> > +       if (objcg && !obj_cgroup_may_zswap(objcg)) {
> > +               memcg =3D 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()) {
> > @@ -1240,7 +1316,11 @@ bool zswap_store(struct folio *folio)
> >                 else
> >                         zswap_pool_reached_full =3D false;
> >         }
> > -
> > +       pool =3D zswap_pool_current_get();
> > +       if (!pool) {
> > +               ret =3D -EINVAL;
> > +               goto reject;
> > +       }
> >         /* allocate entry */
> >         entry =3D zswap_entry_cache_alloc(GFP_KERNEL);
> >         if (!entry) {
> > @@ -1256,6 +1336,7 @@ bool zswap_store(struct folio *folio)
> >                         entry->length =3D 0;
> >                         entry->value =3D value;
> >                         atomic_inc(&zswap_same_filled_pages);
> > +                       zswap_pool_put(pool);
> >                         goto insert_entry;
> >                 }
> >                 kunmap_atomic(src);
> > @@ -1264,6 +1345,15 @@ bool zswap_store(struct folio *folio)
> >         if (!zswap_non_same_filled_pages_enabled)
> >                 goto freepage;
> >
> > +       if (objcg) {
> > +               memcg =3D get_mem_cgroup_from_objcg(objcg);
> > +               lru_alloc_ret =3D memcg_list_lru_alloc(memcg, &pool->li=
st_lru, GFP_KERNEL);
> > +               mem_cgroup_put(memcg);
> > +
> > +               if (lru_alloc_ret)
> > +                       goto freepage;
> > +       }
> > +
> >         /* if entry is successfully added, it keeps the reference */
> >         entry->pool =3D zswap_pool_current_get();
> >         if (!entry->pool)
> > @@ -1325,6 +1415,7 @@ bool zswap_store(struct folio *folio)
> >
> >  insert_entry:
> >         entry->objcg =3D objcg;
> > +       entry->nid =3D page_to_nid(page);
> >         if (objcg) {
> >                 obj_cgroup_charge_zswap(objcg, entry->length);
> >                 /* Account before objcg ref is moved to tree */
> > @@ -1338,9 +1429,8 @@ 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);
> >
> > @@ -1447,9 +1537,8 @@ 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);
> > @@ -1507,6 +1596,48 @@ void zswap_swapoff(int type)
> >         zswap_trees[type] =3D NULL;
> >  }
> >
> > +bool zswap_remove_swpentry_from_lru(swp_entry_t swpentry)
> > +{
> > +       struct zswap_tree *tree =3D zswap_trees[swp_type(swpentry)];
> > +       struct zswap_entry *entry;
> > +       struct zswap_pool *pool;
> > +       bool removed =3D false;
> > +
> > +       /* get the zswap entry and prevent it from being freed */
> > +       spin_lock(&tree->lock);
> > +       entry =3D zswap_rb_search(&tree->rbroot, swp_offset(swpentry));
> > +       /* skip if the entry is already written back or is a same fille=
d page */
> > +       if (!entry || !entry->length)
> > +               goto tree_unlock;
> > +
> > +       pool =3D entry->pool;
> > +       removed =3D zswap_lru_del(&pool->list_lru, entry);
> > +
> > +tree_unlock:
> > +       spin_unlock(&tree->lock);
> > +       return removed;
> > +}
> > +
> > +void zswap_insert_swpentry_into_lru(swp_entry_t swpentry)
> > +{
> > +       struct zswap_tree *tree =3D zswap_trees[swp_type(swpentry)];
> > +       struct zswap_entry *entry;
> > +       struct zswap_pool *pool;
> > +
> > +       /* get the zswap entry and prevent it from being freed */
> > +       spin_lock(&tree->lock);
> > +       entry =3D zswap_rb_search(&tree->rbroot, swp_offset(swpentry));
> > +       /* skip if the entry is already written back or is a same fille=
d page */
> > +       if (!entry || !entry->length)
> > +               goto tree_unlock;
> > +
> > +       pool =3D entry->pool;
> > +       zswap_lru_add(&pool->list_lru, entry);
> > +
> > +tree_unlock:
> > +       spin_unlock(&tree->lock);
> > +}
> > +
> >  /*********************************
> >  * debugfs functions
> >  **********************************/
> > @@ -1560,7 +1691,7 @@ static int zswap_setup(void)
> >         struct zswap_pool *pool;
> >         int ret;
> >
> > -       zswap_entry_cache =3D KMEM_CACHE(zswap_entry, 0);
> > +       zswap_entry_cache =3D KMEM_CACHE(zswap_entry, SLAB_ACCOUNT);
> >         if (!zswap_entry_cache) {
> >                 pr_err("entry cache creation failed\n");
> >                 goto cache_fail;
> > --
> > 2.34.1