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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Mon, 25 Sep 2023 16:59:47 -0700
Message-ID: <CAJD7tkaQtqZ-J8Ey+zZk=c3GKpvvR4i3zmT6HFvbCewMeQmeTA@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] zswap: shrinks zswap pool based on memory pressure
To:     Nhat Pham <nphamcs@gmail.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org,
        cerasuolodomenico@gmail.com, 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
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On Mon, Sep 25, 2023 at 4:29=E2=80=AFPM Nhat Pham <nphamcs@gmail.com> wrote=
:
>
> On Mon, Sep 25, 2023 at 1:38=E2=80=AFPM Yosry Ahmed <yosryahmed@google.co=
m> wrote:
> >
> > On Tue, Sep 19, 2023 at 10:14=E2=80=AFAM Nhat Pham <nphamcs@gmail.com> =
wrote:
> > >
> > > Currently, we only shrink the zswap pool when the user-defined limit =
is
> > > hit. This means that if we set the limit too high, cold data that are
> > > unlikely to be used again will reside in the pool, wasting precious
> > > memory. It is hard to predict how much zswap space will be needed ahe=
ad
> > > of time, as this depends on the workload (specifically, on factors su=
ch
> > > as memory access patterns and compressibility of the memory pages).
> > >
> > > This patch implements a memcg- and NUMA-aware shrinker for zswap, tha=
t
> > > is initiated when there is memory pressure. The shrinker does not
> > > have any parameter that must be tuned by the user, and can be opted i=
n
> > > or out on a per-memcg basis.
> >
> > What's the use case for having per-memcg opt-in/out?
> >
> > If there is memory pressure, reclaiming swap-backed pages will push
> > pages out of zswap anyway, regardless of this patch. With this patch,
> > any sort of reclaim can push pages out of zswap. Wouldn't that be
> > preferable to reclaiming memory that is currently resident in memory
> > (so arguably hotter than the pages in zswap)? Why would this decision
> > be different per-memcg?
> I'm not quite following your argument here. The point of having this
> be done on a per-memcg basis is that we have different workloads
> with different memory access pattern (and as a result, different memory
> coldness distribution).
>
> In a workload where there is a lot of cold data, we can really benefit
> from reclaiming all of those pages and repurpose the memory reclaimed
> (for e.g for filecache).
>
> On the other hand, in a workload where there aren't a lot of cold data,
> reclaiming its zswapped pages will at best do nothing (wasting CPU
> cycles on compression/decompression), and at worst hurt performance
> (due to increased IO when we need those writtenback pages again).
>
> Such different workloads could co-exist in the same system, and having
> a per-memcg knob allows us to crank on the shrinker only on workloads
> where it makes sense.

I am not sure we are on the same page here.

What you're describing sounds more like proactive reclaim, which we
wouldn't invoke unless the workload has cold data anyway.

IIUC, outside of that, this shrinker will run when there is memory
pressure. This means that we need to free memory anyway, regardless of
its absolute coldness. We want to evict the colder pages in the memcg.
It seems to be that in ~all cases, evicting pages in zswap will be
better than evicting pages in memory, as the pages in memory are
arguably hotter (since they weren't reclaimed first). This seems to be
something that would be true for all workloads.

What am I missing?

> >
> > >
> > > Furthermore, to make it more robust for many workloads and prevent
> > > overshrinking (i.e evicting warm pages that might be refaulted into
> > > memory), we build in the following heuristics:
> > >
> > > * Estimate the number of warm pages residing in zswap, and attempt to
> > >   protect this region of the zswap LRU.
> > > * Scale the number of freeable objects by an estimate of the memory
> > >   saving factor. The better zswap compresses the data, the fewer page=
s
> > >   we will evict to swap (as we will otherwise incur IO for relatively
> > >   small memory saving).
> > > * During reclaim, if the shrinker encounters a page that is also bein=
g
> > >   brought into memory, the shrinker will cautiously terminate its
> > >   shrinking action, as this is a sign that it is touching the warmer
> > >   region of the zswap LRU.
> >
> > I don't have an opinion about the reclaim heuristics here, I will let
> > reclaim experts chip in.
> >
> > >
> > > On a benchmark that we have run:
> >
> > Please add more details (as much as possible) about the benchmarks used=
 here.
> Sure! I built the kernel in a memory-limited cgroup a couple times,
> then measured the build time.
>
> To simulate conditions where there are cold, unused data, I
> also generated a bunch of data in tmpfs (and never touch them
> again).

Please include such details in the commit message, there is also
another reference below to "another" benchmark.


> >
> > >
> > > (without the shrinker)
> > > real -- mean: 153.27s, median: 153.199s
> > > sys -- mean: 541.652s, median: 541.903s
> > > user -- mean: 4384.9673999999995s, median: 4385.471s
> > >
> > > (with the shrinker)
> > > real -- mean: 151.4956s, median: 151.456s
> > > sys -- mean: 461.14639999999997s, median: 465.656s
> > > user -- mean: 4384.7118s, median: 4384.675s
> > >
> > > We observed a 14-15% reduction in kernel CPU time, which translated t=
o
> > > over 1% reduction in real time.
> > >
> > > On another benchmark, where there was a lot more cold memory residing=
 in
> > > zswap, we observed even more pronounced gains:
> > >
> > > (without the shrinker)
> > > real -- mean: 157.52519999999998s, median: 157.281s
> > > sys -- mean: 769.3082s, median: 780.545s
> > > user -- mean: 4378.1622s, median: 4378.286s
> > >
> > > (with the shrinker)
> > > real -- mean: 152.9608s, median: 152.845s
> > > sys -- mean: 517.4446s, median: 506.749s
> > > user -- mean: 4387.694s, median: 4387.935s
> > >
> > > Here, we saw around 32-35% reduction in kernel CPU time, which
> > > translated to 2.8% reduction in real time. These results confirm our
> > > hypothesis that the shrinker is more helpful the more cold memory we
> > > have.
> > >
> > > Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> > > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > > ---
> > >  Documentation/admin-guide/mm/zswap.rst |  12 ++
> > >  include/linux/memcontrol.h             |   1 +
> > >  include/linux/mmzone.h                 |  14 ++
> > >  mm/memcontrol.c                        |  33 +++++
> > >  mm/swap_state.c                        |  31 ++++-
> > >  mm/zswap.c                             | 180 +++++++++++++++++++++++=
+-
> > >  6 files changed, 263 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/a=
dmin-guide/mm/zswap.rst
> > > index 45b98390e938..ae8597a67804 100644
> > > --- a/Documentation/admin-guide/mm/zswap.rst
> > > +++ b/Documentation/admin-guide/mm/zswap.rst
> > > @@ -153,6 +153,18 @@ attribute, e. g.::
> > >
> > >  Setting this parameter to 100 will disable the hysteresis.
> > >
> > > +When there is a sizable amount of cold memory residing in the zswap =
pool, it
> > > +can be advantageous to proactively write these cold pages to swap an=
d reclaim
> > > +the memory for other use cases. By default, the zswap shrinker is di=
sabled.
> > > +User can enable it by first switching on the global knob:
> > > +
> > > +  echo Y > /sys/module/zswap/par meters/shrinker_enabled
> > > +
> > > +When the kernel is compiled with CONFIG_MEMCG_KMEM, user needs to fu=
rther turn
> > > +it on for each cgroup that the shrinker should target:
> > > +
> > > +  echo 1 > /sys/fs/cgroup/<cgroup-name>/memory.zswap.shrinker.enable=
d
> > > +
> > >  A debugfs interface is provided for various statistic about pool siz=
e, number
> > >  of pages stored, same-value filled pages and various counters for th=
e reasons
> > >  pages are rejected.
> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index 05d34b328d9d..f005ea667863 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -219,6 +219,7 @@ struct mem_cgroup {
> > >
> > >  #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
> > >         unsigned long zswap_max;
> > > +       atomic_t zswap_shrinker_enabled;
> > >  #endif
> > >
> > >         unsigned long soft_limit;
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index 4106fbc5b4b3..81f4c5ea3e16 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -637,6 +637,20 @@ struct lruvec {
> > >  #ifdef CONFIG_MEMCG
> > >         struct pglist_data *pgdat;
> > >  #endif
> > > +#ifdef CONFIG_ZSWAP
> > > +       /*
> > > +        * Number of pages in zswap that should be protected from the=
 shrinker.
> > > +        * This number is an estimate of the following counts:
> > > +        *
> > > +        * a) Recent page faults.
> > > +        * b) Recent insertion to the zswap LRU. This includes new zs=
wap stores,
> > > +        *    as well as recent zswap LRU rotations.
> > > +        *
> > > +        * These pages are likely to be warm, and might incur IO if t=
he are written
> > > +        * to swap.
> > > +        */
> > > +       unsigned long nr_zswap_protected;
> > > +#endif
> >
> > Would this be better abstracted in a zswap lruvec struct?
> There is just one field, so that sounds like overkill to me.
> But if we need to store more data (for smarter heuristics),
> that'll be a good idea. I'll keep this in mind. Thanks for the
> suggestion, Yosry!

(A space between the quoted text and the reply usually helps visually :)

It wasn't really about the number of fields, but rather place this
struct in zswap.h (with the long comment explaining what it's doing),
and adding an abstracted struct member here. The comment will live in
an appropriate file, further modifications don't need to touch
mmzone.h, and struct lruvec is less cluttered for readers that don't
care about zswap (and we can avoid the ifdef).

Anyway, this is all mostly aesthetic so I don't feel strongly.

> >
> > >  };
> > >
> > >  /* Isolate unmapped pages */
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 9f84b3f7b469..1a2c97cf396f 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -5352,6 +5352,8 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state=
 *parent_css)
> > >         WRITE_ONCE(memcg->soft_limit, PAGE_COUNTER_MAX);
> > >  #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
> > >         memcg->zswap_max =3D PAGE_COUNTER_MAX;
> > > +       /* Disable the shrinker by default */
> > > +       atomic_set(&memcg->zswap_shrinker_enabled, 0);
> > >  #endif
> > >         page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
> > >         if (parent) {
> > > @@ -7877,6 +7879,31 @@ static ssize_t zswap_max_write(struct kernfs_o=
pen_file *of,
> > >         return nbytes;
> > >  }
> > >
> > > +static int zswap_shrinker_enabled_show(struct seq_file *m, void *v)
> > > +{
> > > +       struct mem_cgroup *memcg =3D mem_cgroup_from_seq(m);
> > > +
> > > +       seq_printf(m, "%d\n", atomic_read(&memcg->zswap_shrinker_enab=
led));
> > > +       return 0;
> > > +}
> > > +
> > > +static ssize_t zswap_shrinker_enabled_write(struct kernfs_open_file =
*of,
> > > +                              char *buf, size_t nbytes, loff_t off)
> > > +{
> > > +       struct mem_cgroup *memcg =3D mem_cgroup_from_css(of_css(of));
> > > +       int zswap_shrinker_enabled;
> > > +       ssize_t parse_ret =3D kstrtoint(strstrip(buf), 0, &zswap_shri=
nker_enabled);
> > > +
> > > +       if (parse_ret)
> > > +               return parse_ret;
> > > +
> > > +       if (zswap_shrinker_enabled < 0 || zswap_shrinker_enabled > 1)
> > > +               return -ERANGE;
> > > +
> > > +       atomic_set(&memcg->zswap_shrinker_enabled, zswap_shrinker_ena=
bled);
> > > +       return nbytes;
> > > +}
> > > +
> > >  static struct cftype zswap_files[] =3D {
> > >         {
> > >                 .name =3D "zswap.current",
> > > @@ -7889,6 +7916,12 @@ static struct cftype zswap_files[] =3D {
> > >                 .seq_show =3D zswap_max_show,
> > >                 .write =3D zswap_max_write,
> > >         },
> > > +       {
> > > +               .name =3D "zswap.shrinker.enabled",
> > > +               .flags =3D CFTYPE_NOT_ON_ROOT,
> > > +               .seq_show =3D zswap_shrinker_enabled_show,
> > > +               .write =3D zswap_shrinker_enabled_write,
> > > +       },
> > >         { }     /* terminate */
> > >  };
> > >  #endif /* CONFIG_MEMCG_KMEM && CONFIG_ZSWAP */
> > > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > > index 1c826737aacb..788e36a06c34 100644
> > > --- a/mm/swap_state.c
> > > +++ b/mm/swap_state.c
> > > @@ -618,6 +618,22 @@ static unsigned long swapin_nr_pages(unsigned lo=
ng offset)
> > >         return pages;
> > >  }
> > >
> > > +#ifdef CONFIG_ZSWAP
> > > +/*
> > > + * Refault is an indication that warmer pages are not resident in me=
mory.
> > > + * Increase the size of zswap's protected area.
> > > + */
> > > +static void inc_nr_protected(struct page *page)
> > > +{
> > > +       struct lruvec *lruvec =3D folio_lruvec(page_folio(page));
> > > +       unsigned long flags;
> > > +
> > > +       spin_lock_irqsave(&lruvec->lru_lock, flags);
> > > +       lruvec->nr_zswap_protected++;
> > > +       spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> > > +}
> > > +#endif
> > > +
> >
> > A few questions:
> > - Why is this function named in such a generic way?
> Perhaps inc_nr_zswap_protected would be better? :)

If we use an atomic, the function can go away anyway. See below.

> > - Why is this function here instead of in mm/zswap.c?
> No particular reason :) It's not being used anywhere else,
> so I just put it as a static function here.

It is inline in mm/zswap.c in one place. I personally would have
preferred nr_zswap_protected and the helper to be defined in
zswap.h/zswap.c as I mentioned below. Anyway, this function can go
away.

> > - Why is this protected by the heavily contested lruvec lock instead
> > of being an atomic?
> nr_zswap_protected can be decayed (see zswap_lru_add), which
> I don't think it can be implemented with atomics :( It'd be much
> cleaner indeed.

I think a cmpxchg (or a try_cmpxchg) loop can be used in this case to
implement it using an atomic?

See https://docs.kernel.org/core-api/wrappers/atomic_t.html.

> > > +               lruvec->nr_zswap_protected++;
> > >
> > > +               /*
> > > +                * Decay to avoid overflow and adapt to changing work=
loads.
> > > +                * This is based on LRU reclaim cost decaying heurist=
ics.
> > > +                */
> > > +               if (lruvec->nr_zswap_protected > lru_size / 4)
> > > +                       lruvec->nr_zswap_protected /=3D 2;

>
> I'm wary of adding new locks, so I just re-use this existing lock.
> But if lruvec lock is heavily congested (I'm not aware/familar with
> this issue), then perhaps a new, dedicated lock would help?
> >
> > >  /**
> > >   * swap_cluster_readahead - swap in pages in hope we need them soon
> > >   * @entry: swap entry of this memory
> > > @@ -686,7 +702,12 @@ struct page *swap_cluster_readahead(swp_entry_t =
entry, gfp_t gfp_mask,
> > >         lru_add_drain();        /* Push any new pages onto the LRU no=
w */
> > >  skip:
> > >         /* The page was likely read above, so no need for plugging he=
re */
> > > -       return read_swap_cache_async(entry, gfp_mask, vma, addr, NULL=
);
> > > +       page =3D read_swap_cache_async(entry, gfp_mask, vma, addr, NU=
LL);
> > > +#ifdef CONFIG_ZSWAP
> > > +       if (page)
> > > +               inc_nr_protected(page);
> > > +#endif
> > > +       return page;
> > >  }
> > >
> > >  int init_swap_address_space(unsigned int type, unsigned long nr_page=
s)
> > > @@ -853,8 +874,12 @@ static struct page *swap_vma_readahead(swp_entry=
_t fentry, gfp_t gfp_mask,
> > >         lru_add_drain();
> > >  skip:
> > >         /* The page was likely read above, so no need for plugging he=
re */
> > > -       return read_swap_cache_async(fentry, gfp_mask, vma, vmf->addr=
ess,
> > > -                                    NULL);
> > > +       page =3D read_swap_cache_async(fentry, gfp_mask, vma, vmf->ad=
dress, NULL);
> > > +#ifdef CONFIG_ZSWAP
> > > +       if (page)
> > > +               inc_nr_protected(page);
> > > +#endif
> > > +       return page;
> > >  }
> > >
> > >  /**
> > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > index 1a469e5d5197..79cb18eeb8bf 100644
> > > --- a/mm/zswap.c
> > > +++ b/mm/zswap.c
> > > @@ -145,6 +145,26 @@ module_param_named(exclusive_loads, zswap_exclus=
ive_loads_enabled, bool, 0644);
> > >  /* Number of zpools in zswap_pool (empirically determined for scalab=
ility) */
> > >  #define ZSWAP_NR_ZPOOLS 32
> > >
> > > +/*
> > > + * Global flag to enable/disable memory pressure-based shrinker for =
all memcgs.
> > > + * If CONFIG_MEMCG_KMEM is on, we can further selectively disable
> > > + * the shrinker for each memcg.
> > > + */
> > > +static bool zswap_shrinker_enabled;
> > > +module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0=
644);
> > > +#ifdef CONFIG_MEMCG_KMEM
> > > +static bool is_shrinker_enabled(struct mem_cgroup *memcg)
> > > +{
> > > +       return zswap_shrinker_enabled &&
> > > +               atomic_read(&memcg->zswap_shrinker_enabled);
> > > +}
> > > +#else
> > > +static bool is_shrinker_enabled(struct mem_cgroup *memcg)
> > > +{
> > > +       return zswap_shrinker_enabled;
> > > +}
> > > +#endif
> > > +
> > >  /*********************************
> > >  * data structures
> > >  **********************************/
> > > @@ -174,6 +194,8 @@ struct zswap_pool {
> > >         char tfm_name[CRYPTO_MAX_ALG_NAME];
> > >         struct list_lru list_lru;
> > >         struct mem_cgroup *next_shrink;
> > > +       struct shrinker *shrinker;
> > > +       atomic_t nr_stored;
> > >  };
> > >
> > >  /*
> > > @@ -273,17 +295,26 @@ static bool zswap_can_accept(void)
> > >                         DIV_ROUND_UP(zswap_pool_total_size, PAGE_SIZE=
);
> > >  }
> > >
> > > +static u64 get_zswap_pool_size(struct zswap_pool *pool)
> > > +{
> > > +       u64 pool_size =3D 0;
> > > +       int i;
> > > +
> > > +       for (i =3D 0; i < ZSWAP_NR_ZPOOLS; i++)
> > > +               pool_size +=3D zpool_get_total_size(pool->zpools[i]);
> > > +
> > > +       return pool_size;
> > > +}
> > > +
> > >  static void zswap_update_total_size(void)
> > >  {
> > >         struct zswap_pool *pool;
> > >         u64 total =3D 0;
> > > -       int i;
> > >
> > >         rcu_read_lock();
> > >
> > >         list_for_each_entry_rcu(pool, &zswap_pools, list)
> > > -               for (i =3D 0; i < ZSWAP_NR_ZPOOLS; i++)
> > > -                       total +=3D zpool_get_total_size(pool->zpools[=
i]);
> > > +               total +=3D get_zswap_pool_size(pool);
> > >
> > >         rcu_read_unlock();
> > >
> > > @@ -318,8 +349,23 @@ static bool zswap_lru_add(struct list_lru *list_=
lru, struct zswap_entry *entry)
> > >  {
> > >         struct mem_cgroup *memcg =3D entry->objcg ?
> > >                 get_mem_cgroup_from_objcg(entry->objcg) : NULL;
> > > +       struct lruvec *lruvec =3D mem_cgroup_lruvec(memcg, NODE_DATA(=
entry->nid));
> > >         bool added =3D __list_lru_add(list_lru, &entry->lru, entry->n=
id, memcg);
> > > +       unsigned long flags, lru_size;
> > > +
> > > +       if (added) {
> > > +               lru_size =3D list_lru_count_one(list_lru, entry->nid,=
 memcg);
> > > +               spin_lock_irqsave(&lruvec->lru_lock, flags);
> > > +               lruvec->nr_zswap_protected++;
> > >
> > > +               /*
> > > +                * Decay to avoid overflow and adapt to changing work=
loads.
> > > +                * This is based on LRU reclaim cost decaying heurist=
ics.
> > > +                */
> > > +               if (lruvec->nr_zswap_protected > lru_size / 4)
> > > +                       lruvec->nr_zswap_protected /=3D 2;
> > > +               spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> > > +       }
> > >         mem_cgroup_put(memcg);
> > >         return added;
> > >  }
> > > @@ -420,6 +466,7 @@ static void zswap_free_entry(struct zswap_entry *=
entry)
> > >         else {
> > >                 zswap_lru_del(&entry->pool->list_lru, entry);
> > >                 zpool_free(zswap_find_zpool(entry), entry->handle);
> > > +               atomic_dec(&entry->pool->nr_stored);
> > >                 zswap_pool_put(entry->pool);
> > >         }
> > >         zswap_entry_cache_free(entry);
> > > @@ -461,6 +508,98 @@ static struct zswap_entry *zswap_entry_find_get(=
struct rb_root *root,
> > >         return entry;
> > >  }
> > >
> > > +/*********************************
> > > +* shrinker functions
> > > +**********************************/
> > > +static enum lru_status shrink_memcg_cb(struct list_head *item, struc=
t list_lru_one *l,
> > > +                                      spinlock_t *lock, void *arg);
> > > +
> > > +static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> > > +               struct shrink_control *sc)
> > > +{
> > > +       struct zswap_pool *pool =3D shrinker->private_data;
> > > +       unsigned long shrink_ret, nr_zswap_protected, flags,
> > > +               lru_size =3D list_lru_shrink_count(&pool->list_lru, s=
c);
> > > +       struct lruvec *lruvec =3D mem_cgroup_lruvec(sc->memcg, NODE_D=
ATA(sc->nid));
> > > +       bool encountered_page_in_swapcache =3D false;
> > > +
> > > +       spin_lock_irqsave(&lruvec->lru_lock, flags);
> > > +       nr_zswap_protected =3D lruvec->nr_zswap_protected;
> > > +       spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> > > +
> > > +       /*
> > > +        * Abort if the shrinker is disabled or if we are shrinking i=
nto the
> > > +        * protected region.
> > > +        */
> > > +       if (!is_shrinker_enabled(sc->memcg) ||
> > > +                       nr_zswap_protected >=3D lru_size - sc->nr_to_=
scan) {
> > > +               sc->nr_scanned =3D 0;
> > > +               return SHRINK_STOP;
> > > +       }
> > > +
> > > +       shrink_ret =3D list_lru_shrink_walk(&pool->list_lru, sc, &shr=
ink_memcg_cb,
> > > +               &encountered_page_in_swapcache);
> > > +
> > > +       if (encountered_page_in_swapcache)
> > > +               return SHRINK_STOP;
> > > +
> > > +       return shrink_ret ? shrink_ret : SHRINK_STOP;
> > > +}
> > > +
> > > +static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> > > +               struct shrink_control *sc)
> > > +{
> > > +       struct zswap_pool *pool =3D shrinker->private_data;
> > > +       struct mem_cgroup *memcg =3D sc->memcg;
> > > +       struct lruvec *lruvec =3D mem_cgroup_lruvec(memcg, NODE_DATA(=
sc->nid));
> > > +       unsigned long nr_backing, nr_stored, nr_freeable, flags;
> > > +
> > > +#ifdef CONFIG_MEMCG_KMEM
> > > +       cgroup_rstat_flush(memcg->css.cgroup);
> > > +       nr_backing =3D memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE=
_SHIFT;
> > > +       nr_stored =3D memcg_page_state(memcg, MEMCG_ZSWAPPED);
> > > +#else
> > > +       /* use pool stats instead of memcg stats */
> > > +       nr_backing =3D get_zswap_pool_size(pool) >> PAGE_SHIFT;
> > > +       nr_stored =3D atomic_read(&pool->nr_stored);
> > > +#endif
> > > +
> > > +       if (!is_shrinker_enabled(memcg) || !nr_stored)
> > > +               return 0;
> > > +
> > > +       nr_freeable =3D list_lru_shrink_count(&pool->list_lru, sc);
> > > +       /*
> > > +        * Subtract the lru size by an estimate of the number of page=
s
> > > +        * that should be protected.
> > > +        */
> > > +       spin_lock_irqsave(&lruvec->lru_lock, flags);
> > > +       nr_freeable =3D nr_freeable > lruvec->nr_zswap_protected ?
> > > +               nr_freeable - lruvec->nr_zswap_protected : 0;
> > > +       spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> > > +
> > > +       /*
> > > +        * Scale the number of freeable pages by the memory saving fa=
ctor.
> > > +        * This ensures that the better zswap compresses memory, the =
fewer
> > > +        * pages we will evict to swap (as it will otherwise incur IO=
 for
> > > +        * relatively small memory saving).
> > > +        */
> > > +       return mult_frac(nr_freeable, nr_backing, nr_stored);
> > > +}
> > > +
> > > +static void zswap_alloc_shrinker(struct zswap_pool *pool)
> > > +{
> > > +       pool->shrinker =3D
> > > +               shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_A=
WARE, "mm-zswap");
> > > +       if (!pool->shrinker)
> > > +               return;
> > > +
> > > +       pool->shrinker->private_data =3D pool;
> > > +       pool->shrinker->scan_objects =3D zswap_shrinker_scan;
> > > +       pool->shrinker->count_objects =3D zswap_shrinker_count;
> > > +       pool->shrinker->batch =3D 0;
> > > +       pool->shrinker->seeks =3D DEFAULT_SEEKS;
> > > +}
> > > +
> > >  /*********************************
> > >  * per-cpu code
> > >  **********************************/
> > > @@ -656,11 +795,14 @@ static enum lru_status shrink_memcg_cb(struct l=
ist_head *item, struct list_lru_o
> > >                                        spinlock_t *lock, void *arg)
> > >  {
> > >         struct zswap_entry *entry =3D container_of(item, struct zswap=
_entry, lru);
> > > +       bool *encountered_page_in_swapcache =3D (bool *)arg;
> > >         struct mem_cgroup *memcg;
> > >         struct zswap_tree *tree;
> > > +       struct lruvec *lruvec;
> > >         pgoff_t swpoffset;
> > >         enum lru_status ret =3D LRU_REMOVED_RETRY;
> > >         int writeback_result;
> > > +       unsigned long flags;
> > >
> > >         /*
> > >          * Once the lru lock is dropped, the entry might get freed. T=
he
> > > @@ -696,8 +838,24 @@ static enum lru_status shrink_memcg_cb(struct li=
st_head *item, struct list_lru_o
> > >                 /* we cannot use zswap_lru_add here, because it incre=
ments node's lru count */
> > >                 list_lru_putback(&entry->pool->list_lru, item, entry-=
>nid, memcg);
> > >                 spin_unlock(lock);
> > > -               mem_cgroup_put(memcg);
> > >                 ret =3D LRU_RETRY;
> > > +
> > > +               /*
> > > +                * Encountering a page already in swap cache is a sig=
n that we are shrinking
> > > +                * into the warmer region. We should terminate shrink=
ing (if we're in the dynamic
> > > +                * shrinker context).
> > > +                */
> > > +               if (writeback_result =3D=3D -EEXIST && encountered_pa=
ge_in_swapcache) {
> > > +                       ret =3D LRU_SKIP;
> > > +                       *encountered_page_in_swapcache =3D true;
> > > +               }
> > > +               lruvec =3D mem_cgroup_lruvec(memcg, NODE_DATA(entry->=
nid));
> > > +               spin_lock_irqsave(&lruvec->lru_lock, flags);
> > > +               /* Increment the protection area to account for the L=
RU rotation. */
> > > +               lruvec->nr_zswap_protected++;
> > > +               spin_unlock_irqrestore(&lruvec->lru_lock, flags);
> > > +
> > > +               mem_cgroup_put(memcg);
> > >                 goto put_unlock;
> > >         }
> > >
> > > @@ -828,6 +986,11 @@ static struct zswap_pool *zswap_pool_create(char=
 *type, char *compressor)
> > >                                        &pool->node);
> > >         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
> > > @@ -836,12 +999,17 @@ static struct zswap_pool *zswap_pool_create(cha=
r *type, char *compressor)
> > >         kref_init(&pool->kref);
> > >         INIT_LIST_HEAD(&pool->list);
> > >         INIT_WORK(&pool->shrink_work, shrink_worker);
> > > -       list_lru_init_memcg(&pool->list_lru, NULL);
> > > +       if (list_lru_init_memcg(&pool->list_lru, pool->shrinker))
> > > +               goto lru_fail;
> > > +       shrinker_register(pool->shrinker);
> > >
> > >         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);
> > > @@ -899,6 +1067,7 @@ 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);
> > > @@ -1431,6 +1600,7 @@ bool zswap_store(struct folio *folio)
> > >         if (entry->length) {
> > >                 INIT_LIST_HEAD(&entry->lru);
> > >                 zswap_lru_add(&pool->list_lru, entry);
> > > +               atomic_inc(&pool->nr_stored);
> > >         }
> > >         spin_unlock(&tree->lock);
> > >
> > > --
> > > 2.34.1
> Thanks for the comments/suggestion, Yosry!