memcg: css_tryget_online cleanups

Currently multiple locations in memcg code, css_tryget_online() is being used. However it doesn't matter whether the cgroup is online for the callers. Online used to matter when we had reparenting on offlining and we needed a way to prevent new ones from showing up.

The failure case for couple of these css_tryget_online usage is to fallback to root_mem_cgroup which kind of make bypassing the memcg limits possible for some workloads. For example creating an inotify group in a subcontainer and then deleting that container after moving the process to a different container will make all the event objects allocated for that group to the root_mem_cgroup. So, using css_tryget_online() is dangerous for such cases.

Two locations still use the online version. The swapin of offlined memcg's pages and the memcg kmem cache creation. The kmem cache indeed needs the online version as the kernel does the reparenting of memcg kmem caches. For the swapin case, it has been left for later as the fallback is not really that concerning.

Signed-off-by Shakeel Butt <[email protected]>

---

Changes since v1:
- replaced WARN_ON with WARN_ON_ONCE

 mm/memcontrol.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

mm/memcontrol.c +9 -5

index 62b574d0cd3c..75d8883bf975 100644
@@ -656,7 +656,7 @@ __mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
 	 */
 	__mem_cgroup_remove_exceeded(mz, mctz);
 	if (!soft_limit_excess(mz->memcg) ||
-	    !css_tryget_online(&mz->memcg->css))
+	    !css_tryget(&mz->memcg->css))
 		goto retry;
 done:
 	return mz;

@@ -961,7 +961,8 @@ struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
 		return NULL;
 
 	rcu_read_lock();
-	if (!memcg || !css_tryget_online(&memcg->css))
+	/* 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;

@@ -974,10 +975,13 @@ EXPORT_SYMBOL(get_mem_cgroup_from_page);
 static __always_inline struct mem_cgroup *get_mem_cgroup_from_current(void)
 {
 	if (unlikely(current->active_memcg)) {
-		struct mem_cgroup *memcg = root_mem_cgroup;
+		struct mem_cgroup *memcg;
 
 		rcu_read_lock();
-		if (css_tryget_online(&current->active_memcg->css))
+		/* current->active_memcg must hold a ref. */
+		if (WARN_ON_ONCE(!css_tryget(&current->active_memcg->css)))
+			memcg = root_mem_cgroup;
+		else
 			memcg = current->active_memcg;
 		rcu_read_unlock();
 		return memcg;

@@ -6732,7 +6736,7 @@ void mem_cgroup_sk_alloc(struct sock *sk)
 		goto out;
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && !memcg->tcpmem_active)
 		goto out;
-	if (css_tryget_online(&memcg->css))
+	if (css_tryget(&memcg->css))
 		sk->sk_memcg = memcg;
 out:
 	rcu_read_unlock();

Signature · 1 line

2.25.0.265.gbab2e86ba0-goog

Quoted 13 lines · Shakeel Butt

Currently multiple locations in memcg code, css_tryget_online() is being used. However it doesn't matter whether the cgroup is online for the callers. Online used to matter when we had reparenting on offlining and we needed a way to prevent new ones from showing up.

The failure case for couple of these css_tryget_online usage is to fallback to root_mem_cgroup which kind of make bypassing the memcg limits possible for some workloads. For example creating an inotify group in a subcontainer and then deleting that container after moving the process to a different container will make all the event objects allocated for that group to the root_mem_cgroup. So, using css_tryget_online() is dangerous for such cases.

Quoted 5 lines

Two locations still use the online version. The swapin of offlined memcg's pages and the memcg kmem cache creation. The kmem cache indeed needs the online version as the kernel does the reparenting of memcg kmem caches. For the swapin case, it has been left for later as the fallback is not really that concerning.

Could you be more specific about the swap in case please?

Quoted 2 lines

Signed-off-by Shakeel Butt <[email protected]>

Other than that nothing really jumped at me although I have to confess that I am far from deeply familiar with the sk_buff charging path.

Acked-by Michal Hocko <mhocko-IBi9RG/[email protected]>

On Tue, Mar 3, 2020 at 1:32 AM Michal Hocko <[email protected]> wrote:

Quoted 2 lines

Could you be more specific about the swap in case please?

With swap accounting enabled, if the memcg of the swapped out page is not online then the memcg extracted from the given 'mm' will be charged and if 'mm' is NULL then root memcg will be charged. However I could not find a code path where the given 'mm' will be NULL for swap-in case.