[PATCH v1 09/26] writeback: prevent memory cgroup release in writeback module

Qi Zheng
2025-10-28T14:03:12Z

From: Muchun Song <songmuchun@bytedance.com>

In the near future, a folio will no longer pin its corresponding
memory cgroup. To ensure safety, it will only be appropriate to
hold the rcu read lock or acquire a reference to the memory cgroup
returned by folio_memcg(), thereby preventing it from being released.

In the current patch, the function get_mem_cgroup_css_from_folio()
and the rcu read lock are employed to safeguard against the release
of the memory cgroup.

This serves as a preparatory measure for the reparenting of the
LRU pages.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
fs/fs-writeback.c | 22 +++++++++++-----------
include/linux/memcontrol.h | 9 +++++++--
include/trace/events/writeback.h | 3 +++
mm/memcontrol.c | 14 ++++++++------
4 files changed, 29 insertions(+), 19 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 2b35e80037fee..afd81fb8100cb 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -269,15 +269,13 @@ void __inode_attach_wb(struct inode *inode, struct folio *folio)
if (inode_cgwb_enabled(inode)) {
struct cgroup_subsys_state *memcg_css;

- if (folio) {
- memcg_css = mem_cgroup_css_from_folio(folio);
- 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 (folio)
+ memcg_css = get_mem_cgroup_css_from_folio(folio);
+ 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)
@@ -968,16 +966,16 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct folio *folio
if (!wbc->wb || wbc->no_cgroup_owner)
return;

- css = mem_cgroup_css_from_folio(folio);
+ css = get_mem_cgroup_css_from_folio(folio);
/* 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)
@@ -990,6 +988,8 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct folio *folio
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 9fdbd4970021d..174e52d8e7039 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -895,7 +895,7 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
return match;
}

-struct cgroup_subsys_state *mem_cgroup_css_from_folio(struct folio *folio);
+struct cgroup_subsys_state *get_mem_cgroup_css_from_folio(struct folio *folio);
ino_t page_cgroup_ino(struct page *page);

static inline bool mem_cgroup_online(struct mem_cgroup *memcg)
@@ -1577,9 +1577,14 @@ static inline void mem_cgroup_track_foreign_dirty(struct folio *folio,
if (mem_cgroup_disabled())
return;

+ if (!folio_memcg_charged(folio))
+ return;
+
+ rcu_read_lock();
memcg = folio_memcg(folio);
- if (unlikely(memcg && &memcg->css != wb->memcg_css))
+ if (unlikely(&memcg->css != wb->memcg_css))
mem_cgroup_track_foreign_dirty_slowpath(folio, wb);
+ rcu_read_unlock();
}

void mem_cgroup_flush_foreign(struct bdi_writeback *wb);
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index c08aff044e807..3680828727f23 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -295,7 +295,10 @@ 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);
+
+ rcu_read_lock();
__entry->page_cgroup_ino = cgroup_ino(folio_memcg(folio)->css.cgroup);
+ rcu_read_unlock();
),

TP_printk("bdi %s[%llu]: ino=%lu memcg_id=%u cgroup_ino=%lu page_cgroup_ino=%lu",
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1da3ad77054d3..aa8945c4ee383 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -241,7 +241,7 @@ DEFINE_STATIC_KEY_FALSE(memcg_bpf_enabled_key);
EXPORT_SYMBOL(memcg_bpf_enabled_key);

/**
- * mem_cgroup_css_from_folio - css of the memcg associated with a folio
+ * get_mem_cgroup_css_from_folio - acquire a css of the memcg associated with a folio
* @folio: folio of interest
*
* If memcg is bound to the default hierarchy, css of the memcg associated
@@ -251,14 +251,16 @@ EXPORT_SYMBOL(memcg_bpf_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_folio(struct folio *folio)
+struct cgroup_subsys_state *get_mem_cgroup_css_from_folio(struct folio *folio)
{
- struct mem_cgroup *memcg = folio_memcg(folio);
+ struct mem_cgroup *memcg;

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

- return &memcg->css;
+ memcg = get_mem_cgroup_from_folio(folio);
+
+ return memcg ? &memcg->css : &root_mem_cgroup->css;
}

/**
--
2.20.1