Date:   Thu, 16 Nov 2023 16:39:26 -0800
From:   Chris Li <chrisl@...nel.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Nhat Pham <nphamcs@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, tj@...nel.org,
        lizefan.x@...edance.com, Johannes Weiner <hannes@...xchg.org>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>, mhocko@...nel.org,
        roman.gushchin@...ux.dev, Shakeel Butt <shakeelb@...gle.com>,
        muchun.song@...ux.dev, Hugh Dickins <hughd@...gle.com>,
        corbet@....net, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        senozhatsky@...omium.org, rppt@...nel.org,
        linux-mm <linux-mm@...ck.org>, kernel-team@...a.com,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        david@...t.cz
Subject: Re: [PATCH v5] zswap: memcontrol: implement zswap writeback disabling

Hi Yosry,

I think we are in agreement here. I am already Acked on the previous
version of this patch.
Consider this version as well because the change suggestion is from me.
I am fine with merging the zswap.writeback.

It is also good to get some discussion on the more general ABI as
well. Just like you said, we can also explore the zswap + swapfile
using the more general API. Because obsoleting user-visible ABI is
much harder.

Chris

On Thu, Nov 16, 2023 at 1:01 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Thu, Nov 16, 2023 at 12:53 PM Chris Li <chrisl@...nel.org> wrote:
> >
> > Hi Nhat,
> >
> > As we discussed, I just want to bounce some alternative ideas related
> > to this write back disabled feature.
> >
> > Currently, the common usage case is zswap alone or zswap + SSD. We
> > treat zswap and
> > SSD as two different tiers with different swap in performance characteristics.
> > If we make it more generic, we can also have more than two swap tiers.
> > e.g. we can have zswap, SSD, network swap or HDD swap.
> > The disable flag is just one bit of information, it can't describe
> > what is the next tier.if it is not the current swap file
> > implementation .
> >
> > One idea is that we can have a more general swap_tier_list object to
> > describe the order of the swap device, The system can have more than
> > one such list to describe different combinations of the tier
> > selection.
> >
> > Each memcg can have a pointer point to one of such swap_tier_list
> > objects, replacing the disabled write back flag in this patch.
> > When you swap out, it will just go through each tier in the list, try
> > to swap it out.
> >  This has some implications for the zswap shrink as well. It becomes a
> > more generic "move swap out data to another tier". Generally need to
> > load into the swap cache then write to another tier.
> >
> > Open question is how we deal with the swap cache index across
> > different tiers. The zswap.writeback_disable will not be needed if we
> > have a more generic swap tier framework.
>
> I agree that this should be the long-term goal, and I suggested that
> we make the interface more future-proof by making it more generic to
> accept the types or tiers of swap allowed by the memcg:
>
> https://lore.kernel.org/lkml/CAJD7tkY8iPBo99+1gdsSRMNDu4jkVKz8rb=W+xk9=GE0y=kSuw@mail.gmail.com/
>
> Since we only have swap and zswap now, the implementation can be
> similar to this code and basically just disable writeback if zswap is
> the only allowed swapping mechanism. So we don't necessarily need to
> have a full swap tiering implementation, but I agree with Chris that
> at least having a future-proof interface to work with generic swap
> tiering is preferrable.
>
>
>
> >
> > Chris
> >
> >
> > On Wed, Nov 15, 2023 at 9:24 AM Nhat Pham <nphamcs@...il.com> wrote:
> > >
> > > During our experiment with zswap, we sometimes observe swap IOs due to
> > > occasional zswap store failures and writebacks-to-swap. These swapping
> > > IOs prevent many users who cannot tolerate swapping from adopting zswap
> > > to save memory and improve performance where possible.
> > >
> > > This patch adds the option to disable this behavior entirely: do not
> > > writeback to backing swapping device when a zswap store attempt fail,
> > > and do not write pages in the zswap pool back to the backing swap
> > > device (both when the pool is full, and when the new zswap shrinker is
> > > called).
> > >
> > > This new behavior can be opted-in/out on a per-cgroup basis via a new
> > > cgroup file. By default, writebacks to swap device is enabled, which is
> > > the previous behavior. Initially, writeback is enabled for the root
> > > cgroup, and a newly created cgroup will inherit the current setting of
> > > its parent.
> > >
> > > Note that this is subtly different from setting memory.swap.max to 0, as
> > > it still allows for pages to be stored in the zswap pool (which itself
> > > consumes swap space in its current form).
> > >
> > > This patch should be applied on top of the zswap shrinker series:
> > >
> > > https://lore.kernel.org/lkml/20231106183159.3562879-1-nphamcs@gmail.com/
> > >
> > > as it also disables the zswap shrinker, a major source of zswap
> > > writebacks.
> > >
> > > Suggested-by: Johannes Weiner <hannes@...xchg.org>
> > > Signed-off-by: Nhat Pham <nphamcs@...il.com>
> > > ---
> > >  Documentation/admin-guide/cgroup-v2.rst | 12 ++++++++
> > >  Documentation/admin-guide/mm/zswap.rst  |  6 ++++
> > >  include/linux/memcontrol.h              | 12 ++++++++
> > >  include/linux/zswap.h                   |  6 ++++
> > >  mm/memcontrol.c                         | 38 +++++++++++++++++++++++++
> > >  mm/page_io.c                            |  6 ++++
> > >  mm/shmem.c                              |  3 +-
> > >  mm/zswap.c                              | 14 +++++++++
> > >  8 files changed, 95 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > > index 3f85254f3cef..2b4ac43efdc8 100644
> > > --- a/Documentation/admin-guide/cgroup-v2.rst
> > > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > > @@ -1679,6 +1679,18 @@ PAGE_SIZE multiple when read back.
> > >         limit, it will refuse to take any more stores before existing
> > >         entries fault back in or are written out to disk.
> > >
> > > +  memory.zswap.writeback
> > > +       A read-write single value file. The default value is "1". The
> > > +       initial value of the root cgroup is 1, and when a new cgroup is
> > > +       created, it inherits the current value of its parent.
> > > +
> > > +       When this is set to 0, all swapping attempts to swapping devices
> > > +       are disabled. This included both zswap writebacks, and swapping due
> > > +       to zswap store failure.
> > > +
> > > +       Note that this is subtly different from setting memory.swap.max to
> > > +       0, as it still allows for pages to be written to the zswap pool.
> > > +
> > >    memory.pressure
> > >         A read-only nested-keyed file.
> > >
> > > diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
> > > index 522ae22ccb84..b987e58edb70 100644
> > > --- a/Documentation/admin-guide/mm/zswap.rst
> > > +++ b/Documentation/admin-guide/mm/zswap.rst
> > > @@ -153,6 +153,12 @@ attribute, e. g.::
> > >
> > >  Setting this parameter to 100 will disable the hysteresis.
> > >
> > > +Some users cannot tolerate the swapping that comes with zswap store failures
> > > +and zswap writebacks. Swapping can be disabled entirely (without disabling
> > > +zswap itself) on a cgroup-basis as follows:
> > > +
> > > +       echo 0 > /sys/fs/cgroup/<cgroup-name>/memory.zswap.writeback
> > > +
> > >  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 and reclaim
> > >  the memory for other use cases. By default, the zswap shrinker is disabled.
> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index 83590fd0d6d1..3901ff4dae63 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -219,6 +219,12 @@ struct mem_cgroup {
> > >
> > >  #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
> > >         unsigned long zswap_max;
> > > +
> > > +       /*
> > > +        * Prevent pages from this memcg from being written back from zswap to
> > > +        * swap, and from being swapped out on zswap store failures.
> > > +        */
> > > +       bool zswap_writeback;
> > >  #endif
> > >
> > >         unsigned long soft_limit;
> > > @@ -1931,6 +1937,7 @@ static inline void count_objcg_event(struct obj_cgroup *objcg,
> > >  bool obj_cgroup_may_zswap(struct obj_cgroup *objcg);
> > >  void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size);
> > >  void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size);
> > > +bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg);
> > >  #else
> > >  static inline bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
> > >  {
> > > @@ -1944,6 +1951,11 @@ static inline void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg,
> > >                                              size_t size)
> > >  {
> > >  }
> > > +static inline bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
> > > +{
> > > +       /* if zswap is disabled, do not block pages going to the swapping device */
> > > +       return true;
> > > +}
> > >  #endif
> > >
> > >  #endif /* _LINUX_MEMCONTROL_H */
> > > diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> > > index cbd373ba88d2..b4997e27a74b 100644
> > > --- a/include/linux/zswap.h
> > > +++ b/include/linux/zswap.h
> > > @@ -35,6 +35,7 @@ void zswap_swapoff(int type);
> > >  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
> > >  void zswap_lruvec_state_init(struct lruvec *lruvec);
> > >  void zswap_lruvec_swapin(struct page *page);
> > > +bool is_zswap_enabled(void);
> > >  #else
> > >
> > >  struct zswap_lruvec_state {};
> > > @@ -55,6 +56,11 @@ static inline void zswap_swapoff(int type) {}
> > >  static inline void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg) {}
> > >  static inline void zswap_lruvec_init(struct lruvec *lruvec) {}
> > >  static inline void zswap_lruvec_swapin(struct page *page) {}
> > > +
> > > +static inline bool is_zswap_enabled(void)
> > > +{
> > > +       return false;
> > > +}
> > >  #endif
> > >
> > >  #endif /* _LINUX_ZSWAP_H */
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 786c7edf5836..5ad71ce31c74 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -5522,6 +5522,8 @@ 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 = PAGE_COUNTER_MAX;
> > > +       WRITE_ONCE(memcg->zswap_writeback,
> > > +               !parent || READ_ONCE(parent->zswap_writeback));
> > >  #endif
> > >         page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
> > >         if (parent) {
> > > @@ -8146,6 +8148,12 @@ void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size)
> > >         rcu_read_unlock();
> > >  }
> > >
> > > +bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
> > > +{
> > > +       /* if zswap is disabled, do not block pages going to the swapping device */
> > > +       return !is_zswap_enabled() || !memcg || READ_ONCE(memcg->zswap_writeback);
> > > +}
> > > +
> > >  static u64 zswap_current_read(struct cgroup_subsys_state *css,
> > >                               struct cftype *cft)
> > >  {
> > > @@ -8176,6 +8184,31 @@ static ssize_t zswap_max_write(struct kernfs_open_file *of,
> > >         return nbytes;
> > >  }
> > >
> > > +static int zswap_writeback_show(struct seq_file *m, void *v)
> > > +{
> > > +       struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
> > > +
> > > +       seq_printf(m, "%d\n", READ_ONCE(memcg->zswap_writeback));
> > > +       return 0;
> > > +}
> > > +
> > > +static ssize_t zswap_writeback_write(struct kernfs_open_file *of,
> > > +                               char *buf, size_t nbytes, loff_t off)
> > > +{
> > > +       struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> > > +       int zswap_writeback;
> > > +       ssize_t parse_ret = kstrtoint(strstrip(buf), 0, &zswap_writeback);
> > > +
> > > +       if (parse_ret)
> > > +               return parse_ret;
> > > +
> > > +       if (zswap_writeback != 0 && zswap_writeback != 1)
> > > +               return -EINVAL;
> > > +
> > > +       WRITE_ONCE(memcg->zswap_writeback, zswap_writeback);
> > > +       return nbytes;
> > > +}
> > > +
> > >  static struct cftype zswap_files[] = {
> > >         {
> > >                 .name = "zswap.current",
> > > @@ -8188,6 +8221,11 @@ static struct cftype zswap_files[] = {
> > >                 .seq_show = zswap_max_show,
> > >                 .write = zswap_max_write,
> > >         },
> > > +       {
> > > +               .name = "zswap.writeback",
> > > +               .seq_show = zswap_writeback_show,
> > > +               .write = zswap_writeback_write,
> > > +       },
> > >         { }     /* terminate */
> > >  };
> > >  #endif /* CONFIG_MEMCG_KMEM && CONFIG_ZSWAP */
> > > diff --git a/mm/page_io.c b/mm/page_io.c
> > > index cb559ae324c6..5e606f1aa2f6 100644
> > > --- a/mm/page_io.c
> > > +++ b/mm/page_io.c
> > > @@ -201,6 +201,12 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
> > >                 folio_end_writeback(folio);
> > >                 return 0;
> > >         }
> > > +
> > > +       if (!mem_cgroup_zswap_writeback_enabled(folio_memcg(folio))) {
> > > +               folio_mark_dirty(folio);
> > > +               return AOP_WRITEPAGE_ACTIVATE;
> > > +       }
> > > +
> > >         __swap_writepage(&folio->page, wbc);
> > >         return 0;
> > >  }
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 0d1ce70bce38..ccbaaa5f1c16 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -1514,8 +1514,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> > >
> > >                 mutex_unlock(&shmem_swaplist_mutex);
> > >                 BUG_ON(folio_mapped(folio));
> > > -               swap_writepage(&folio->page, wbc);
> > > -               return 0;
> > > +               return swap_writepage(&folio->page, wbc);
> > >         }
> > >
> > >         mutex_unlock(&shmem_swaplist_mutex);
> > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > index 943090dfe793..caa467e40009 100644
> > > --- a/mm/zswap.c
> > > +++ b/mm/zswap.c
> > > @@ -152,6 +152,11 @@ module_param_named(exclusive_loads, zswap_exclusive_loads_enabled, bool, 0644);
> > >  static bool zswap_shrinker_enabled;
> > >  module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
> > >
> > > +bool is_zswap_enabled(void)
> > > +{
> > > +       return zswap_enabled;
> > > +}
> > > +
> > >  /*********************************
> > >  * data structures
> > >  **********************************/
> > > @@ -589,6 +594,9 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
> > >         struct zswap_pool *pool = shrinker->private_data;
> > >         bool encountered_page_in_swapcache = false;
> > >
> > > +       if (!mem_cgroup_zswap_writeback_enabled(sc->memcg))
> > > +               return SHRINK_STOP;
> > > +
> > >         nr_protected =
> > >                 atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
> > >         lru_size = list_lru_shrink_count(&pool->list_lru, sc);
> > > @@ -619,6 +627,9 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
> > >         struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
> > >         unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> > >
> > > +       if (!mem_cgroup_zswap_writeback_enabled(memcg))
> > > +               return 0;
> > > +
> > >  #ifdef CONFIG_MEMCG_KMEM
> > >         cgroup_rstat_flush(memcg->css.cgroup);
> > >         nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
> > > @@ -934,6 +945,9 @@ static int shrink_memcg(struct mem_cgroup *memcg)
> > >         struct zswap_pool *pool;
> > >         int nid, shrunk = 0;
> > >
> > > +       if (!mem_cgroup_zswap_writeback_enabled(memcg))
> > > +               return -EINVAL;
> > > +
> > >         /*
> > >          * Skip zombies because their LRUs are reparented and we would be
> > >          * reclaiming from the parent instead of the dead memcg.
> > > --
> > > 2.34.1
>

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