Date:	Wed, 13 Jan 2016 17:44:42 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/7] mm: memcontrol: charge swap to cgroup2

[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@...tuozzo.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@...e.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

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