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

Vladimir Davydov
2015-12-17T12:30:24Z

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>
---
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