On Thu, Nov 2, 2023 at 1:02 PM 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.
>
> 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).
>
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
> Documentation/admin-guide/cgroup-v2.rst | 11 +++++++
> Documentation/admin-guide/mm/zswap.rst | 6 ++++
> include/linux/memcontrol.h | 17 +++++++++++
> mm/memcontrol.c | 38 +++++++++++++++++++++++++
> mm/page_io.c | 6 ++++
> mm/shmem.c | 3 +-
> mm/zswap.c | 9 ++++++
> 7 files changed, 88 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 606b2e0eac4b..18c4171392ea 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1672,6 +1672,17 @@ 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 which exists on non-root
> + cgroups. The default value is "1".
> +
> + 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 95f6c9e60ed1..e3a3a06727dc 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;
> @@ -1615,6 +1621,12 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
> {
> return 0;
> }
> +
> +static inline bool mem_cgroup_swap_disk_enabled(struct mem_cgroup *memcg)
> +{
> + return false;
> +}
> +
This seems to be a leftover from a prior version.
> #endif /* CONFIG_MEMCG */
>
> static inline void __inc_lruvec_kmem_state(void *p, enum node_stat_item idx)
> @@ -1931,6 +1943,7 @@ 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 +1957,10 @@ 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)
> +{
> + return false;
> +}
> #endif
>
> #endif /* _LINUX_MEMCONTROL_H */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e43b5aba8efc..b68c613c23a9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5545,6 +5545,7 @@ 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, true);
> #endif
> page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
> if (parent) {
> @@ -8177,6 +8178,12 @@ 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)
> +{
> + return cgroup_subsys_on_dfl(memory_cgrp_subsys) && memcg
> + && READ_ONCE(memcg->zswap_writeback);
> +}
> +
> static u64 zswap_current_read(struct cgroup_subsys_state *css,
> struct cftype *cft)
> {
> @@ -8209,6 +8216,31 @@ 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",
> @@ -8221,6 +8253,12 @@ static struct cftype zswap_files[] = {
> .seq_show = zswap_max_show,
> .write = zswap_max_write,
> },
> + {
> + .name = "zswap.writeback",
> + .flags = CFTYPE_NOT_ON_ROOT,
> + .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;
> + }
> +
I am not a fan of this, because it will disable using disk swap if
"zswap_writeback" is disabled, even if zswap is disabled or the page
was never in zswap. The term zswap_writeback makes no sense here tbh.
I am still hoping someone else will suggest better semantics, because
honestly I can't think of anything. Perhaps something like
memory.swap.zswap_only or memory.swap.types which accepts a string
(e.g. "zswap"/"all",..).
Don't take my suggestions strongly because I am not very fond of them.
Can anyone else come back with better naming/semantics for "use zswap
but nothing else when swapping"?
> __swap_writepage(&folio->page, wbc);
> return 0;
> }
> diff --git a/mm/shmem.c b/mm/shmem.c
> index cab053831fea..e5044678de8b 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 260e01180ee0..42a478d1a21f 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -590,6 +590,9 @@ 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);
> @@ -620,6 +623,9 @@ 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;
> @@ -935,6 +941,9 @@ 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
