On Tue, Jun 26, 2018 at 12:03 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Mon, Jun 25, 2018 at 04:06:58PM -0700, Shakeel Butt wrote:
> > @@ -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
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