Re: [PATCH 1/2] fs: fsnotify: account fsnotify metadata to kmemcg

On Tue, Jun 26, 2018 at 12:03 PM Johannes Weiner [off-list ref] wrote:

On Mon, Jun 25, 2018 at 04:06:58PM -0700, Shakeel Butt wrote:

quoted

@@ -140,8 +141,9 @@ struct fanotify_event_info *fanotify_alloc_event(struct fsnotify_group *group,
                                               struct inode *inode, u32 mask,
                                               const struct path *path)
 {
-     struct fanotify_event_info *event;
+     struct fanotify_event_info *event = NULL;
      gfp_t gfp = GFP_KERNEL;
+     struct mem_cgroup *old_memcg = NULL;

      /*
       * For queues with unlimited length lost events are not expected and

@@ -151,19 +153,25 @@ struct fanotify_event_info *fanotify_alloc_event(struct fsnotify_group *group,
      if (group->max_events == UINT_MAX)
              gfp |= __GFP_NOFAIL;

+     /* Whoever is interested in the event, pays for the allocation. */
+     if (group->memcg) {
+             gfp |= __GFP_ACCOUNT;
+             old_memcg = memalloc_use_memcg(group->memcg);
+     }

group->memcg is only NULL when memcg is disabled or there is some
offlining race. Can you make memalloc_use_memcg(NULL) mean that it
should charge root_mem_cgroup instead of current->mm->memcg? That way
we can make this site unconditional while retaining the behavior:

        gfp_t gfp = GFP_KERNEL | __GFP_ACCOUNT;

        memalloc_use_memcg(group->memcg);
        kmem_cache_alloc(..., gfp);
out:
        memalloc_unuse_memcg();

(dropping old_memcg and the unuse parameter as per the other mail)

group->memcg is only NULL when memcg is disabled (i.e.
get_mem_cgroup_from_mm() returns root_mem_cgroup for offlined
mm->memcg). Though group->memcg can point to an offlined memcg.

If I understand you correctly this is what we want:

1. If group->memcg is NULL then __GFP_ACCOUNT is a noop i.e. memcg is disabled.
2. If group->memcg is root_mem_cgroup, then __GFP_ACCOUNT again is a
kind of noop (charges to root_mem_cgroups are bypassed).
3. If group->memcg is offlined memcg, then make __GFP_ACCOUNT noop by
returning root_mem_cgroup from get_mem_cgroup_from_current().
4. Else charge group->memcg.

This seems reasonable. After your Ack and Amir's or Jan's answer to
the nesting query, I will resend the next version of this patch
series.

In future if we find any use-cases of memalloc_use_memcg nesting then
we can make it work for nesting.

thanks,
Shakeel