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Subject[PATCH] fs: buffer: use raw page_memcg() on locked page
Date
alloc_page_buffers() currently uses get_mem_cgroup_from_page() for
charging the buffers to the page owner, which does an rcu-protected
page->memcg lookup and acquires a reference. But buffer allocation has
the page lock held throughout, which pins the page to the memcg and
thereby the memcg - neither rcu nor holding an extra reference during
the allocation are necessary. Use a raw page_memcg() instead.

This was the last user of get_mem_cgroup_from_page(), delete it.

Reported-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
fs/buffer.c | 4 ++--
include/linux/memcontrol.h | 7 -------
mm/memcontrol.c | 23 -----------------------
3 files changed, 2 insertions(+), 32 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 96c7604f69b3..dc5d3e22c493 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -847,7 +847,8 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
if (retry)
gfp |= __GFP_NOFAIL;

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

head = NULL;
@@ -868,7 +869,6 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
}
out:
set_active_memcg(old_memcg);
- mem_cgroup_put(memcg);
return head;
/*
* In case anything failed, we just free everything we got.
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index a8c7a0ccc759..a44b2d51aecc 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -687,8 +687,6 @@ struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);

struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm);

-struct mem_cgroup *get_mem_cgroup_from_page(struct page *page);
-
struct lruvec *lock_page_lruvec(struct page *page);
struct lruvec *lock_page_lruvec_irq(struct page *page);
struct lruvec *lock_page_lruvec_irqsave(struct page *page,
@@ -1169,11 +1167,6 @@ static inline struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
return NULL;
}

-static inline struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
-{
- return NULL;
-}
-
static inline void mem_cgroup_put(struct mem_cgroup *memcg)
{
}
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 51778fa9b462..9e455815fb7a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1048,29 +1048,6 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
}
EXPORT_SYMBOL(get_mem_cgroup_from_mm);

-/**
- * get_mem_cgroup_from_page: Obtain a reference on given page's memcg.
- * @page: page from which memcg should be extracted.
- *
- * Obtain a reference on page->memcg and returns it if successful. Otherwise
- * root_mem_cgroup is returned.
- */
-struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
-{
- struct mem_cgroup *memcg = page_memcg(page);
-
- if (mem_cgroup_disabled())
- return NULL;
-
- rcu_read_lock();
- /* Page should not get uncharged and freed memcg under us. */
- if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
- memcg = root_mem_cgroup;
- rcu_read_unlock();
- return memcg;
-}
-EXPORT_SYMBOL(get_mem_cgroup_from_page);
-
static __always_inline struct mem_cgroup *active_memcg(void)
{
if (in_interrupt())
--
2.30.0
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