Re: [PATCH v2 1/7] mm: memcontrol: charge swap to cgroup2

Michal Hocko
2016-01-13T16:45:13Z

[Sorry to come back late - I was too busy after holidays]

On Thu 17-12-15 15:29:54, Vladimir Davydov wrote:
> In the legacy hierarchy we charge memsw, which is dubious, because:
>
> - memsw.limit must be >= memory.limit, so it is impossible to limit
> swap usage less than memory usage. Taking into account the fact that
> the primary limiting mechanism in the unified hierarchy is
> memory.high while memory.limit is either left unset or set to a very
> large value, moving memsw.limit knob to the unified hierarchy would
> effectively make it impossible to limit swap usage according to the
> user preference.
>
> - memsw.usage != memory.usage + swap.usage, because a page occupying
> both swap entry and a swap cache page is charged only once to memsw
> counter. As a result, it is possible to effectively eat up to
> memory.limit of memory pages *and* memsw.limit of swap entries, which
> looks unexpected.
>
> That said, we should provide a different swap limiting mechanism for
> cgroup2.
>
> This patch adds mem_cgroup->swap counter, which charges the actual
> number of swap entries used by a cgroup. It is only charged in the
> unified hierarchy, while the legacy hierarchy memsw logic is left
> intact.
>
> The swap usage can be monitored using new memory.swap.current file and
> limited using memory.swap.max.
>
> Note, to charge swap resource properly in the unified hierarchy, we have
> to make swap_entry_free uncharge swap only when ->usage reaches zero,
> not just ->count, i.e. when all references to a swap entry, including
> the one taken by swap cache, are gone. This is necessary, because
> otherwise swap-in could result in uncharging swap even if the page is
> still in swap cache and hence still occupies a swap entry. At the same
> time, this shouldn't break memsw counter logic, where a page is never
> charged twice for using both memory and swap, because in case of legacy
> hierarchy we uncharge swap on commit (see mem_cgroup_commit_charge).
>
> Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>

OK, let's go with this in the current form but I am still worried that
configuring the swap limit properly is really non-trivial, especially
when hard limits are not in place. But let's wait with further
heuristics after users start complaining.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> Changes in v2:
> - Rename mem_cgroup_charge_swap -> mem_cgroup_try_charge_swap to
> conform to other memcg charging methods.
> - Remove unnecessary READ_ONCE from mem_cgroup_get_limit.
> - Uncharge swap only when the last reference is gone, including the one
> taken by swap cache. In other words, make swap_entry_free call
> mem_cgroup_uncharge_swap only when ->usage reaches zero, not just
> ->count.
> - Note: I left legacy and default swap charge paths separated for now,
> because unifying them (i.e. moving swap_cgroup_record to
> mem_cgroup_try_charge_swap) would result in changing behavior of
> MEM_CGROUP_STAT_SWAP counter on the legacy hierarchy.
>
> include/linux/memcontrol.h | 1 +
> include/linux/swap.h | 6 +++
> mm/memcontrol.c | 118 ++++++++++++++++++++++++++++++++++++++++++---
> mm/shmem.c | 4 ++
> mm/swap_state.c | 5 ++
> mm/swapfile.c | 4 +-
> 6 files changed, 128 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 27123e597eca..6e0126230878 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -163,6 +163,7 @@ struct mem_cgroup {
>
> /* Accounted resources */
> struct page_counter memory;
> + struct page_counter swap;
>
> /* Legacy consumer-oriented counters */
> struct page_counter memsw;
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 457181844b6e..478e7dd038c7 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -368,11 +368,17 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
> #endif
> #ifdef CONFIG_MEMCG_SWAP
> extern void mem_cgroup_swapout(struct page *page, swp_entry_t entry);
> +extern int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry);
> extern void mem_cgroup_uncharge_swap(swp_entry_t entry);
> #else
> static inline void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
> {
> }
> +static inline int mem_cgroup_try_charge_swap(struct page *page,
> + swp_entry_t entry)
> +{
> + return 0;
> +}
> static inline void mem_cgroup_uncharge_swap(swp_entry_t entry)
> {
> }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9d951faebbd4..567b56da2c23 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1222,7 +1222,7 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
> pr_cont(":");
>
> for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
> - if (i == MEM_CGROUP_STAT_SWAP && !do_memsw_account())
> + if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)
> continue;
> pr_cont(" %s:%luKB", mem_cgroup_stat_names[i],
> K(mem_cgroup_read_stat(iter, i)));
> @@ -1261,9 +1261,12 @@ static unsigned long mem_cgroup_get_limit(struct mem_cgroup *memcg)
> limit = memcg->memory.limit;
> if (mem_cgroup_swappiness(memcg)) {
> unsigned long memsw_limit;
> + unsigned long swap_limit;
>
> memsw_limit = memcg->memsw.limit;
> - limit = min(limit + total_swap_pages, memsw_limit);
> + swap_limit = memcg->swap.limit;
> + swap_limit = min(swap_limit, (unsigned long)total_swap_pages);
> + limit = min(limit + swap_limit, memsw_limit);
> }
> return limit;
> }
> @@ -4198,11 +4201,13 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
> if (parent && parent->use_hierarchy) {
> memcg->use_hierarchy = true;
> page_counter_init(&memcg->memory, &parent->memory);
> + page_counter_init(&memcg->swap, &parent->swap);
> page_counter_init(&memcg->memsw, &parent->memsw);
> page_counter_init(&memcg->kmem, &parent->kmem);
> page_counter_init(&memcg->tcpmem, &parent->tcpmem);
> } else {
> page_counter_init(&memcg->memory, NULL);
> + page_counter_init(&memcg->swap, NULL);
> page_counter_init(&memcg->memsw, NULL);
> page_counter_init(&memcg->kmem, NULL);
> page_counter_init(&memcg->tcpmem, NULL);
> @@ -5212,7 +5217,7 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
> if (page->mem_cgroup)
> goto out;
>
> - if (do_memsw_account()) {
> + if (do_swap_account) {
> swp_entry_t ent = { .val = page_private(page), };
> unsigned short id = lookup_swap_cgroup_id(ent);
>
> @@ -5665,26 +5670,66 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
> memcg_check_events(memcg, page);
> }
>
> +/*
> + * mem_cgroup_try_charge_swap - try charging a swap entry
> + * @page: page being added to swap
> + * @entry: swap entry to charge
> + *
> + * Try to charge @entry to the memcg that @page belongs to.
> + *
> + * Returns 0 on success, -ENOMEM on failure.
> + */
> +int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
> +{
> + struct mem_cgroup *memcg;
> + struct page_counter *counter;
> + unsigned short oldid;
> +
> + if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) || !do_swap_account)
> + return 0;
> +
> + memcg = page->mem_cgroup;
> +
> + /* Readahead page, never charged */
> + if (!memcg)
> + return 0;
> +
> + if (!mem_cgroup_is_root(memcg) &&
> + !page_counter_try_charge(&memcg->swap, 1, &counter))
> + return -ENOMEM;
> +
> + oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
> + VM_BUG_ON_PAGE(oldid, page);
> + mem_cgroup_swap_statistics(memcg, true);
> +
> + css_get(&memcg->css);
> + return 0;
> +}
> +
> /**
> * mem_cgroup_uncharge_swap - uncharge a swap entry
> * @entry: swap entry to uncharge
> *
> - * Drop the memsw charge associated with @entry.
> + * Drop the swap charge associated with @entry.
> */
> void mem_cgroup_uncharge_swap(swp_entry_t entry)
> {
> struct mem_cgroup *memcg;
> unsigned short id;
>
> - if (!do_memsw_account())
> + if (!do_swap_account)
> return;
>
> id = swap_cgroup_record(entry, 0);
> rcu_read_lock();
> memcg = mem_cgroup_from_id(id);
> if (memcg) {
> - if (!mem_cgroup_is_root(memcg))
> - page_counter_uncharge(&memcg->memsw, 1);
> + if (!mem_cgroup_is_root(memcg)) {
> + if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> + page_counter_uncharge(&memcg->swap, 1);
> + else
> + page_counter_uncharge(&memcg->memsw, 1);
> + }
> mem_cgroup_swap_statistics(memcg, false);
> css_put(&memcg->css);
> }
> @@ -5708,6 +5753,63 @@ static int __init enable_swap_account(char *s)
> }
> __setup("swapaccount=", enable_swap_account);
>
> +static u64 swap_current_read(struct cgroup_subsys_state *css,
> + struct cftype *cft)
> +{
> + struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> +
> + return (u64)page_counter_read(&memcg->swap) * PAGE_SIZE;
> +}
> +
> +static int swap_max_show(struct seq_file *m, void *v)
> +{
> + struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> + unsigned long max = READ_ONCE(memcg->swap.limit);
> +
> + if (max == PAGE_COUNTER_MAX)
> + seq_puts(m, "max\n");
> + else
> + seq_printf(m, "%llu\n", (u64)max * PAGE_SIZE);
> +
> + return 0;
> +}
> +
> +static ssize_t swap_max_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));
> + unsigned long max;
> + int err;
> +
> + buf = strstrip(buf);
> + err = page_counter_memparse(buf, "max", &max);
> + if (err)
> + return err;
> +
> + mutex_lock(&memcg_limit_mutex);
> + err = page_counter_limit(&memcg->swap, max);
> + mutex_unlock(&memcg_limit_mutex);
> + if (err)
> + return err;
> +
> + return nbytes;
> +}
> +
> +static struct cftype swap_files[] = {
> + {
> + .name = "swap.current",
> + .flags = CFTYPE_NOT_ON_ROOT,
> + .read_u64 = swap_current_read,
> + },
> + {
> + .name = "swap.max",
> + .flags = CFTYPE_NOT_ON_ROOT,
> + .seq_show = swap_max_show,
> + .write = swap_max_write,
> + },
> + { } /* terminate */
> +};
> +
> static struct cftype memsw_cgroup_files[] = {
> {
> .name = "memsw.usage_in_bytes",
> @@ -5739,6 +5841,8 @@ static int __init mem_cgroup_swap_init(void)
> {
> if (!mem_cgroup_disabled() && really_do_swap_account) {
> do_swap_account = 1;
> + WARN_ON(cgroup_add_dfl_cftypes(&memory_cgrp_subsys,
> + swap_files));
> WARN_ON(cgroup_add_legacy_cftypes(&memory_cgrp_subsys,
> memsw_cgroup_files));
> }
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 9b051115a100..95e89b26dac3 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -912,6 +912,9 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> if (!swap.val)
> goto redirty;
>
> + if (mem_cgroup_try_charge_swap(page, swap))
> + goto free_swap;
> +
> /*
> * Add inode to shmem_unuse()'s list of swapped-out inodes,
> * if it's not already there. Do it now before the page is
> @@ -940,6 +943,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> }
>
> mutex_unlock(&shmem_swaplist_mutex);
> +free_swap:
> swapcache_free(swap);
> redirty:
> set_page_dirty(page);
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index d783872d746c..934b33879af1 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -170,6 +170,11 @@ int add_to_swap(struct page *page, struct list_head *list)
> if (!entry.val)
> return 0;
>
> + if (mem_cgroup_try_charge_swap(page, entry)) {
> + swapcache_free(entry);
> + return 0;
> + }
> +
> if (unlikely(PageTransHuge(page)))
> if (unlikely(split_huge_page_to_list(page, list))) {
> swapcache_free(entry);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 7073faecb38f..efa279221302 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -788,14 +788,12 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
> count--;
> }
>
> - if (!count)
> - mem_cgroup_uncharge_swap(entry);
> -
> usage = count | has_cache;
> p->swap_map[offset] = usage;
>
> /* free if no reference */
> if (!usage) {
> + mem_cgroup_uncharge_swap(entry);
> dec_cluster_info_page(p, p->cluster_info, offset);
> if (offset < p->lowest_bit)
> p->lowest_bit = offset;
> --
> 2.1.4
>
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--
Michal Hocko
SUSE Labs