Re: [PATCH v5] zswap: memcontrol: implement zswap writeback disabling
From: Nhat Pham
Date: Sat Nov 18 2023 - 14:24:00 EST
On Thu, Nov 16, 2023 at 7:39 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
>
> 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
Hmm how about this - in the future, we support the following
options:
1. zswap.writeback == 1: no limitation to zswap writeback.
All backing swap devices (sorted by priorities?) are fair game.
2. zswap.writeback == 0: disable all forms of zswap writeback.
3. zswap.writeback == <tiers description>: attempt to write to each
tier, one at a time.
The first two are basically what we have for this patch.
The last one will be added in a future patch.
This is from the userspace perspective. Internally, we can modify
memcg->writeback to be a pointer or a struct instead of this bool.
(as you suggested).
This way, the API remains intact and backward compatible
(and FWIW, I think there are still a lot of values in having simple
options for the users who have simple memory hierarchies).
>
> On Thu, Nov 16, 2023 at 1:01 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> > On Thu, Nov 16, 2023 at 12:53 PM Chris Li <chrisl@xxxxxxxxxx> 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@xxxxxxxxxxxxxx/
> >
> > 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@xxxxxxxxx> 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@xxxxxxxxx/
> > > >
> > > > as it also disables the zswap shrinker, a major source of zswap
> > > > writebacks.
> > > >
> > > > Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> > > > ---
> > > > 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
> >