[PATCH v6 07/11] mm: memcontrol: make all the callers of {folio,page}_memcg() safe

From: Muchun Song
Date: Tue Jun 21 2022 - 08:59:05 EST

When we use objcg APIs to charge the LRU pages, the page will not hold
a reference to the memcg associated with the page. So the caller of the
{folio,page}_memcg() should hold an rcu read lock or obtain a reference
to the memcg associated with the page to protect memcg from being
released. So introduce get_mem_cgroup_from_{page,folio}() to obtain a
reference to the memory cgroup associated with the page.

In this patch, make all the callers hold an rcu read lock or obtain a
reference to the memcg to protect memcg from being released when the LRU
pages reparented.

We do not need to adjust the callers of {folio,page}_memcg() during
the whole process of mem_cgroup_move_task(). Because the cgroup migration
and memory cgroup offlining are serialized by @cgroup_mutex. In this
routine, the LRU pages cannot be reparented to its parent memory cgroup.
So {folio,page}_memcg() is stable and cannot be released.

This is a preparation for reparenting the LRU pages.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
---
fs/buffer.c | 4 +--
fs/fs-writeback.c | 23 +++++++-------
include/linux/memcontrol.h | 66 +++++++++++++++++++++++++++++++++-----
include/trace/events/writeback.h | 5 +++
mm/memcontrol.c | 68 +++++++++++++++++++++++++++++-----------
mm/migrate.c | 4 +++
mm/page_io.c | 5 +--
7 files changed, 135 insertions(+), 40 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 898c7f301b1b..04ec53f327e4 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -819,8 +819,7 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
if (retry)
gfp |= __GFP_NOFAIL;

- /* The page lock pins the memcg */
- memcg = page_memcg(page);
+ memcg = get_mem_cgroup_from_page(page);
old_memcg = set_active_memcg(memcg);

head = NULL;
@@ -840,6 +839,7 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
set_bh_page(bh, page, offset);
}
out:
+ mem_cgroup_put(memcg);
set_active_memcg(old_memcg);
return head;
/*
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 05221366a16d..1cbac56c810b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -244,15 +244,13 @@ void __inode_attach_wb(struct inode *inode, struct page *page)
if (inode_cgwb_enabled(inode)) {
struct cgroup_subsys_state *memcg_css;

- if (page) {
- memcg_css = mem_cgroup_css_from_page(page);
- wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
- } else {
- /* must pin memcg_css, see wb_get_create() */
+ /* must pin memcg_css, see wb_get_create() */
+ if (page)
+ memcg_css = get_mem_cgroup_css_from_page(page);
+ else
memcg_css = task_get_css(current, memory_cgrp_id);
- wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
- css_put(memcg_css);
- }
+ wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
+ css_put(memcg_css);
}

if (!wb)
@@ -869,16 +867,16 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
if (!wbc->wb || wbc->no_cgroup_owner)
return;

- css = mem_cgroup_css_from_page(page);
+ css = get_mem_cgroup_css_from_page(page);
/* dead cgroups shouldn't contribute to inode ownership arbitration */
if (!(css->flags & CSS_ONLINE))
- return;
+ goto out;

id = css->id;

if (id == wbc->wb_id) {
wbc->wb_bytes += bytes;
- return;
+ goto out;
}

if (id == wbc->wb_lcand_id)
@@ -891,6 +889,9 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
wbc->wb_tcand_bytes += bytes;
else
wbc->wb_tcand_bytes -= min(bytes, wbc->wb_tcand_bytes);
+
+out:
+ css_put(css);
}
EXPORT_SYMBOL_GPL(wbc_account_cgroup_owner);

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 026b62b206b1..a8bd4bb39502 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -379,7 +379,7 @@ static inline bool folio_memcg_kmem(struct folio *folio);
* a valid memcg, but can be atomically swapped to the parent memcg.
*
* The caller must ensure that the returned memcg won't be released:
- * e.g. acquire the rcu_read_lock or css_set_lock.
+ * e.g. acquire the rcu_read_lock or objcg_lock or cgroup_mutex.
*/
static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
{
@@ -445,8 +445,8 @@ static inline struct obj_cgroup *__folio_objcg(struct folio *folio)
* - lock_page_memcg()
* - exclusive reference
*
- * For a kmem folio a caller should hold an rcu read lock to protect memcg
- * associated with a kmem folio from being released.
+ * Note: The caller should hold an rcu read lock to protect memcg associated
+ * with a folio from being released.
*/
static inline struct mem_cgroup *folio_memcg(struct folio *folio)
{
@@ -455,12 +455,48 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
return __folio_memcg(folio);
}

+/*
+ * page_memcg - Get the memory cgroup associated with a page.
+ * @page: Pointer to the page.
+ *
+ * See the cooments in folio_memcg().
+ */
static inline struct mem_cgroup *page_memcg(struct page *page)
{
return folio_memcg(page_folio(page));
}

-/**
+/*
+ * get_mem_cgroup_from_folio - Obtain a reference on the memory cgroup
+ * associated with a folio.
+ * @folio: Pointer to the folio.
+ *
+ * Returns a pointer to the memory cgroup (and obtain a reference on it)
+ * associated with the folio, or NULL. This function assumes that the
+ * folio is known to have a proper memory cgroup pointer. It's not safe
+ * to call this function against some type of pages, e.g. slab pages or
+ * ex-slab pages.
+ */
+static inline struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
+{
+ struct mem_cgroup *memcg;
+
+ rcu_read_lock();
+retry:
+ memcg = folio_memcg(folio);
+ if (unlikely(memcg && !css_tryget(&memcg->css)))
+ goto retry;
+ rcu_read_unlock();
+
+ return memcg;
+}
+
+static inline struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
+{
+ return get_mem_cgroup_from_folio(page_folio(page));
+}
+
+/*
* folio_memcg_rcu - Locklessly get the memory cgroup associated with a folio.
* @folio: Pointer to the folio.
*
@@ -888,7 +924,7 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
return match;
}

-struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page);
+struct cgroup_subsys_state *get_mem_cgroup_css_from_page(struct page *page);
ino_t page_cgroup_ino(struct page *page);

static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
@@ -1058,19 +1094,25 @@ static inline void count_memcg_events(struct mem_cgroup *memcg,
static inline void count_memcg_page_event(struct page *page,
enum vm_event_item idx)
{
- struct mem_cgroup *memcg = page_memcg(page);
+ struct mem_cgroup *memcg;

+ rcu_read_lock();
+ memcg = page_memcg(page);
if (memcg)
count_memcg_events(memcg, idx, 1);
+ rcu_read_unlock();
}

static inline void count_memcg_folio_events(struct folio *folio,
enum vm_event_item idx, unsigned long nr)
{
- struct mem_cgroup *memcg = folio_memcg(folio);
+ struct mem_cgroup *memcg;

+ rcu_read_lock();
+ memcg = folio_memcg(folio);
if (memcg)
count_memcg_events(memcg, idx, nr);
+ rcu_read_unlock();
}

static inline void count_memcg_event_mm(struct mm_struct *mm,
@@ -1149,6 +1191,16 @@ static inline struct mem_cgroup *page_memcg(struct page *page)
return NULL;
}

+static inline struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
+{
+ return NULL;
+}
+
+static inline struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
+{
+ return NULL;
+}
+
static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
{
WARN_ON_ONCE(!rcu_read_lock_held());
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 86b2a82da546..cdb822339f13 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -258,6 +258,11 @@ TRACE_EVENT(track_foreign_dirty,
__entry->ino = inode ? inode->i_ino : 0;
__entry->memcg_id = wb->memcg_css->id;
__entry->cgroup_ino = __trace_wb_assign_cgroup(wb);
+ /*
+ * TP_fast_assign() is under preemption disabled which can
+ * serve as an RCU read-side critical section so that the
+ * memcg returned by folio_memcg() cannot be freed.
+ */
__entry->page_cgroup_ino = cgroup_ino(folio_memcg(folio)->css.cgroup);
),

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6f171480b2f2..346a954e190e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -369,7 +369,7 @@ EXPORT_SYMBOL(memcg_kmem_enabled_key);
#endif

/**
- * mem_cgroup_css_from_page - css of the memcg associated with a page
+ * get_mem_cgroup_css_from_page - get css of the memcg associated with a page
* @page: page of interest
*
* If memcg is bound to the default hierarchy, css of the memcg associated
@@ -379,13 +379,15 @@ EXPORT_SYMBOL(memcg_kmem_enabled_key);
* If memcg is bound to a traditional hierarchy, the css of root_mem_cgroup
* is returned.
*/
-struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
+struct cgroup_subsys_state *get_mem_cgroup_css_from_page(struct page *page)
{
struct mem_cgroup *memcg;

- memcg = page_memcg(page);
+ if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+ return &root_mem_cgroup->css;

- if (!memcg || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
+ memcg = get_mem_cgroup_from_page(page);
+ if (!memcg)
memcg = root_mem_cgroup;

return &memcg->css;
@@ -768,13 +770,13 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
void __mod_lruvec_page_state(struct page *page, enum node_stat_item idx,
int val)
{
- struct page *head = compound_head(page); /* rmap on tail pages */
+ struct folio *folio = page_folio(page); /* rmap on tail pages */
struct mem_cgroup *memcg;
pg_data_t *pgdat = page_pgdat(page);
struct lruvec *lruvec;

rcu_read_lock();
- memcg = page_memcg(head);
+ memcg = folio_memcg(folio);
/* Untracked pages have no memcg, no lruvec. Update only the node */
if (!memcg) {
rcu_read_unlock();
@@ -2056,7 +2058,9 @@ void folio_memcg_lock(struct folio *folio)
* The RCU lock is held throughout the transaction. The fast
* path can get away without acquiring the memcg->move_lock
* because page moving starts with an RCU grace period.
- */
+ *
+ * The RCU lock also protects the memcg from being freed.
+ */
rcu_read_lock();

if (mem_cgroup_disabled())
@@ -3353,7 +3357,7 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
void split_page_memcg(struct page *head, unsigned int nr)
{
struct folio *folio = page_folio(head);
- struct mem_cgroup *memcg = folio_memcg(folio);
+ struct mem_cgroup *memcg = get_mem_cgroup_from_folio(folio);
int i;

if (mem_cgroup_disabled() || !memcg)
@@ -3366,6 +3370,8 @@ void split_page_memcg(struct page *head, unsigned int nr)
obj_cgroup_get_many(__folio_objcg(folio), nr - 1);
else
css_get_many(&memcg->css, nr - 1);
+
+ css_put(&memcg->css);
}

#ifdef CONFIG_MEMCG_SWAP
@@ -4558,7 +4564,7 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
struct bdi_writeback *wb)
{
- struct mem_cgroup *memcg = folio_memcg(folio);
+ struct mem_cgroup *memcg = get_mem_cgroup_from_folio(folio);
struct memcg_cgwb_frn *frn;
u64 now = get_jiffies_64();
u64 oldest_at = now;
@@ -4605,6 +4611,7 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
frn->memcg_id = wb->memcg_css->id;
frn->at = now;
}
+ css_put(&memcg->css);
}

/* issue foreign writeback flushes for recorded foreign dirtying events */
@@ -6167,6 +6174,14 @@ static void mem_cgroup_move_charge(void)
atomic_dec(&mc.from->moving_account);
}

+/*
+ * The cgroup migration and memory cgroup offlining are serialized by
+ * @cgroup_mutex. If we reach here, it means that the LRU pages cannot
+ * be reparented to its parent memory cgroup. So during the whole process
+ * of mem_cgroup_move_task(), page_memcg(page) is stable. So we do not
+ * need to worry about the memcg (returned from page_memcg()) being
+ * released even if we do not hold an rcu read lock.
+ */
static void mem_cgroup_move_task(void)
{
if (mc.to) {
@@ -7025,7 +7040,7 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
if (folio_memcg(new))
return;

- memcg = folio_memcg(old);
+ memcg = get_mem_cgroup_from_folio(old);
VM_WARN_ON_ONCE_FOLIO(!memcg, old);
if (!memcg)
return;
@@ -7044,6 +7059,8 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
mem_cgroup_charge_statistics(memcg, nr_pages);
memcg_check_events(memcg, folio_nid(new));
local_irq_restore(flags);
+
+ css_put(&memcg->css);
}

DEFINE_STATIC_KEY_FALSE(memcg_sockets_enabled_key);
@@ -7228,6 +7245,10 @@ void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry)
if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
return;

+ /*
+ * Interrupts should be disabled by the caller (see the comments below),
+ * which can serve as RCU read-side critical sections.
+ */
memcg = folio_memcg(folio);

VM_WARN_ON_ONCE_FOLIO(!memcg, folio);
@@ -7289,19 +7310,21 @@ int __mem_cgroup_try_charge_swap(struct folio *folio, swp_entry_t entry)
struct page_counter *counter;
struct mem_cgroup *memcg;
unsigned short oldid;
+ int ret = 0;

if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
return 0;

+ rcu_read_lock();
memcg = folio_memcg(folio);

VM_WARN_ON_ONCE_FOLIO(!memcg, folio);
if (!memcg)
- return 0;
+ goto out;

if (!entry.val) {
memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
- return 0;
+ goto out;
}

memcg = mem_cgroup_id_get_online(memcg);
@@ -7311,7 +7334,8 @@ int __mem_cgroup_try_charge_swap(struct folio *folio, swp_entry_t entry)
memcg_memory_event(memcg, MEMCG_SWAP_MAX);
memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
mem_cgroup_id_put(memcg);
- return -ENOMEM;
+ ret = -ENOMEM;
+ goto out;
}

/* Get references for the tail pages, too */
@@ -7320,8 +7344,10 @@ int __mem_cgroup_try_charge_swap(struct folio *folio, swp_entry_t entry)
oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg), nr_pages);
VM_BUG_ON_FOLIO(oldid, folio);
mod_memcg_state(memcg, MEMCG_SWAP, nr_pages);
+out:
+ rcu_read_unlock();

- return 0;
+ return ret;
}

/**
@@ -7366,6 +7392,7 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
bool mem_cgroup_swap_full(struct page *page)
{
struct mem_cgroup *memcg;
+ bool ret = false;

VM_BUG_ON_PAGE(!PageLocked(page), page);

@@ -7374,19 +7401,24 @@ bool mem_cgroup_swap_full(struct page *page)
if (cgroup_memory_noswap || !cgroup_subsys_on_dfl(memory_cgrp_subsys))
return false;

+ rcu_read_lock();
memcg = page_memcg(page);
if (!memcg)
- return false;
+ goto out;

for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) {
unsigned long usage = page_counter_read(&memcg->swap);

if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
- usage * 2 >= READ_ONCE(memcg->swap.max))
- return true;
+ usage * 2 >= READ_ONCE(memcg->swap.max)) {
+ ret = true;
+ goto out;
+ }
}
+out:
+ rcu_read_unlock();

- return false;
+ return ret;
}

static int __init setup_swap_account(char *s)
diff --git a/mm/migrate.c b/mm/migrate.c
index 1ece23d80bc4..2e49b96fa339 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -451,6 +451,10 @@ int folio_migrate_mapping(struct address_space *mapping,
struct lruvec *old_lruvec, *new_lruvec;
struct mem_cgroup *memcg;

+ /*
+ * Irq is disabled, which can serve as RCU read-side critical
+ * sections.
+ */
memcg = folio_memcg(folio);
old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat);
new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat);
diff --git a/mm/page_io.c b/mm/page_io.c
index 68318134dc92..f75ebbc95ee6 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -222,13 +222,14 @@ static void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
struct cgroup_subsys_state *css;
struct mem_cgroup *memcg;

+ rcu_read_lock();
memcg = page_memcg(page);
if (!memcg)
- return;
+ goto out;

- rcu_read_lock();
css = cgroup_e_css(memcg->css.cgroup, &io_cgrp_subsys);
bio_associate_blkg_from_css(bio, css);
+out:
rcu_read_unlock();
}
#else
--
2.11.0