fs: buffer: use raw page_memcg() on locked page

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.

Link: https://lkml.kernel.org/r/20210209190126.97842-1-hannes@cmpxchg.orgSigned-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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.

include/linux/memcontrol.h

@@ -680,8 +680,6 @@ 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,
@@ -1191,11 +1189,6 @@ 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)
 {
 }

mm/memcontrol.c

@@ -1047,29 +1047,6 @@ 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())