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@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 >
