[RFC PATCH v4 07/12] mm: memcontrol: make all the callers of page_memcg() safe

From: Muchun Song
Date: Thu May 27 2021 - 05:34:51 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
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() 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 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 page_memcg(page) is stable and cannot be released.

This is a preparation for reparenting the LRU pages.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
fs/buffer.c | 3 ++-
fs/fs-writeback.c | 23 +++++++++++----------
include/linux/memcontrol.h | 39 ++++++++++++++++++++++++++++++++---
mm/memcontrol.c | 51 ++++++++++++++++++++++++++++++++++++----------
mm/migrate.c | 4 ++++
mm/page_io.c | 5 +++--
6 files changed, 97 insertions(+), 28 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 673cfbef9eec..a542a47f6e27 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -848,7 +848,7 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
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;
@@ -868,6 +868,7 @@ 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 7c46d1588a19..52b7d4563528 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -255,15 +255,13 @@ 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)
@@ -736,16 +734,16 @@ 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)
@@ -758,6 +756,9 @@ 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 961ca3126d7f..99f745e23607 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -380,7 +380,7 @@ static inline bool PageMemcgKmem(struct page *page);
* 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 css_set_lock or cgroup_mutex.
*/
static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
{
@@ -458,6 +458,31 @@ static inline struct mem_cgroup *page_memcg(struct page *page)
}

/*
+ * get_mem_cgroup_from_page - Obtain a reference on the memory cgroup associated
+ * with a page
+ * @page: a pointer to the page struct
+ *
+ * Returns a pointer to the memory cgroup (and obtain a reference on it)
+ * associated with the page, or NULL. This function assumes that the page
+ * 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_page(struct page *page)
+{
+ struct mem_cgroup *memcg;
+
+ rcu_read_lock();
+retry:
+ memcg = page_memcg(page);
+ if (unlikely(memcg && !css_tryget(&memcg->css)))
+ goto retry;
+ rcu_read_unlock();
+
+ return memcg;
+}
+
+/*
* page_memcg_rcu - locklessly get the memory cgroup associated with a page
* @page: a pointer to the page struct
*
@@ -870,7 +895,7 @@ 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)
@@ -1030,10 +1055,13 @@ 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_event_mm(struct mm_struct *mm,
@@ -1107,6 +1135,11 @@ static inline struct mem_cgroup *page_memcg(struct page *page)
return NULL;
}

+static inline struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
+{
+ return NULL;
+}
+
static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
{
WARN_ON_ONCE(!rcu_read_lock_held());
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 06c6bcaa265a..d294504aea0c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -410,7 +410,7 @@ 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
@@ -420,13 +420,15 @@ 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;
@@ -2015,7 +2017,9 @@ void lock_page_memcg(struct page *page)
* 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())
@@ -4591,7 +4595,7 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
struct bdi_writeback *wb)
{
- struct mem_cgroup *memcg = page_memcg(page);
+ struct mem_cgroup *memcg;
struct memcg_cgwb_frn *frn;
u64 now = get_jiffies_64();
u64 oldest_at = now;
@@ -4600,6 +4604,7 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,

trace_track_foreign_dirty(page, wb);

+ memcg = get_mem_cgroup_from_page(page);
/*
* Pick the slot to use. If there is already a slot for @wb, keep
* using it. If not replace the oldest one which isn't being
@@ -4638,6 +4643,7 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
frn->memcg_id = wb->memcg_css->id;
frn->at = now;
}
+ css_put(&memcg->css);
}

/* issue foreign writeback flushes for recorded foreign dirtying events */
@@ -6168,6 +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) {
@@ -6995,7 +7009,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
if (page_memcg(newpage))
return;

- memcg = page_memcg(oldpage);
+ memcg = get_mem_cgroup_from_page(oldpage);
VM_WARN_ON_ONCE_PAGE(!memcg, oldpage);
if (!memcg)
return;
@@ -7016,6 +7030,8 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
mem_cgroup_charge_statistics(memcg, newpage, nr_pages);
memcg_check_events(memcg, newpage);
local_irq_restore(flags);
+
+ css_put(&memcg->css);
}

DEFINE_STATIC_KEY_FALSE(memcg_sockets_enabled_key);
@@ -7204,6 +7220,10 @@ void mem_cgroup_swapout(struct page *page, 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 = page_memcg(page);

VM_WARN_ON_ONCE_PAGE(!memcg, page);
@@ -7271,15 +7291,16 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
return 0;

+ rcu_read_lock();
memcg = page_memcg(page);

VM_WARN_ON_ONCE_PAGE(!memcg, page);
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);
@@ -7289,6 +7310,7 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
memcg_memory_event(memcg, MEMCG_SWAP_MAX);
memcg_memory_event(memcg, MEMCG_SWAP_FAIL);
mem_cgroup_id_put(memcg);
+ rcu_read_unlock();
return -ENOMEM;
}

@@ -7298,6 +7320,8 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg), nr_pages);
VM_BUG_ON_PAGE(oldid, page);
mod_memcg_state(memcg, MEMCG_SWAP, nr_pages);
+out:
+ rcu_read_unlock();

return 0;
}
@@ -7352,17 +7376,22 @@ 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))
+ usage * 2 >= READ_ONCE(memcg->swap.max)) {
+ rcu_read_unlock();
return true;
+ }
}
+out:
+ rcu_read_unlock();

return false;
}
diff --git a/mm/migrate.c b/mm/migrate.c
index 76cb9b422dc2..c4594b20dcec 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -465,6 +465,10 @@ int migrate_page_move_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 = page_memcg(page);
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 c493ce9ebcf5..81744777ab76 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -269,13 +269,14 @@ 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