mm: memcg: lookup_page_cgroup (almost) never returns NULL (cfa44946) · Commits · 戴 / test

Pages have their corresponding page_cgroup descriptors set up before they are used in userspace, and thus managed by a memory cgroup. The only time where lookup_page_cgroup() can return NULL is in the CONFIG_DEBUG_VM-only page sanity checking code that executes while feeding pages into the page allocator for the first time. Remove the NULL checks against lookup_page_cgroup() results from all callsites where we know that corresponding page_cgroup descriptors must be allocated, and add a comment to the callsite that actually does have to check the return value. [hughd@google.com: stop oops in mem_cgroup_update_page_stat()] Signed-off-by:

Johannes Weiner <jweiner@redhat.com> Acked-by:

KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Acked-by:

Michal Hocko <mhocko@suse.cz> Cc: Balbir Singh <bsingharora@gmail.com> Cc: David Rientjes <rientjes@google.com> Signed-off-by:

Hugh Dickins <hughd@google.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

+7 −4

Original line number	Diff line number	Diff line
		@@ -1960,7 +1960,7 @@ void mem_cgroup_update_page_stat(struct page *page,
		bool need_unlock = false;
		unsigned long uninitialized_var(flags);

		if (unlikely(!pc))
		if (mem_cgroup_disabled())
		return;

		rcu_read_lock();
		@@ -2735,8 +2735,6 @@ static int mem_cgroup_charge_common(struct page page, struct mm_struct mm,
		}

		pc = lookup_page_cgroup(page);
		BUG_ON(!pc); /* XXX: remove this and move pc lookup into commit */

		ret = __mem_cgroup_try_charge(mm, gfp_mask, nr_pages, &memcg, oom);
		if (ret \|\| !memcg)
		return ret;
		@@ -3008,7 +3006,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
		* Check if our page_cgroup is valid
		*/
		pc = lookup_page_cgroup(page);
		if (unlikely(!pc \|\| !PageCgroupUsed(pc)))
		if (unlikely(!PageCgroupUsed(pc)))
		return NULL;

		lock_page_cgroup(pc);
		@@ -3436,6 +3434,11 @@ static struct page_cgroup lookup_page_cgroup_used(struct page page)
		struct page_cgroup *pc;

		pc = lookup_page_cgroup(page);
		/*
		* Can be NULL while feeding pages into the page allocator for
		* the first time, i.e. during boot or memory hotplug;
		* or when mem_cgroup_disabled().
		*/
		if (likely(pc) && PageCgroupUsed(pc))
		return pc;
		return NULL;

Admin message