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

[Michal Hocko]

On Thu 28-06-18 12:21:26, Shakeel Butt wrote:
> On Thu, Jun 28, 2018 at 12:03 PM Jan Kara <jack@xxxxxxx> wrote:
> >
> > On Wed 27-06-18 12:12:49, Shakeel Butt wrote:
> > > A lot of memory can be consumed by the events generated for the huge or
> > > unlimited queues if there is either no or slow listener.  This can cause
> > > system level memory pressure or OOMs.  So, it's better to account the
> > > fsnotify kmem caches to the memcg of the listener.
> > >
> > > However the listener can be in a different memcg than the memcg of the
> > > producer and these allocations happen in the context of the event
> > > producer. This patch introduces remote memcg charging API which the
> > > producer can use to charge the allocations to the memcg of the listener.
> > >
> > > There are seven fsnotify kmem caches and among them allocations from
> > > dnotify_struct_cache, dnotify_mark_cache, fanotify_mark_cache and
> > > inotify_inode_mark_cachep happens in the context of syscall from the
> > > listener.  So, SLAB_ACCOUNT is enough for these caches.
> > >
> > > The objects from fsnotify_mark_connector_cachep are not accounted as they
> > > are small compared to the notification mark or events and it is unclear
> > > whom to account connector to since it is shared by all events attached to
> > > the inode.
> > >
> > > The allocations from the event caches happen in the context of the event
> > > producer.  For such caches we will need to remote charge the allocations
> > > to the listener's memcg.  Thus we save the memcg reference in the
> > > fsnotify_group structure of the listener.
> > >
> > > This patch has also moved the members of fsnotify_group to keep the size
> > > same, at least for 64 bit build, even with additional member by filling
> > > the holes.
> >
> > ...
> >
> > >  static int __init fanotify_user_setup(void)
> > >  {
> > > -     fanotify_mark_cache = KMEM_CACHE(fsnotify_mark, SLAB_PANIC);
> > > +     fanotify_mark_cache = KMEM_CACHE(fsnotify_mark,
> > > +                                      SLAB_PANIC|SLAB_ACCOUNT);
> > >       fanotify_event_cachep = KMEM_CACHE(fanotify_event_info, SLAB_PANIC);
> > >       if (IS_ENABLED(CONFIG_FANOTIFY_ACCESS_PERMISSIONS)) {
> > >               fanotify_perm_event_cachep =
> >
> > Why don't you setup also fanotify_event_cachep and
> > fanotify_perm_event_cachep caches with SLAB_ACCOUNT and instead specify
> > __GFP_ACCOUNT manually? Otherwise the patch looks good to me.
> >
> 
> Hi Jan, IMHO having a visible __GFP_ACCOUNT along with
> memalloc_use_memcg() makes the code more explicit and readable that we
> want to targeted/remote memcg charging.

Agreed. If you had an implicit SLAB_ACCOUNT then you could get
inconsistencies when some allocations would get charged to the current
task while others would not.
-- 
Michal Hocko
SUSE Labs