Re: [PATCH] inotify, memcg: account inotify instances to kmemcg

[Shakeel Butt]

On Sat, Dec 19, 2020 at 1:48 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> On Sat, Dec 19, 2020 at 12:11 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> >
> > Currently the fs sysctl inotify/max_user_instances is used to limit the
> > number of inotify instances on the system. For systems running multiple
> > workloads, the per-user namespace sysctl max_inotify_instances can be
> > used to further partition inotify instances. However there is no easy
> > way to set a sensible system level max limit on inotify instances and
> > further partition it between the workloads. It is much easier to charge
> > the underlying resource (i.e. memory) behind the inotify instances to
> > the memcg of the workload and let their memory limits limit the number
> > of inotify instances they can create.
>
> Not that I have a problem with this patch, but what problem does it try to
> solve?

I am aiming for the simplicity to not set another limit which can
indirectly be limited by memcg limits. I just want to set the memcg
limit on our production environment which runs multiple workloads on a
system and not think about setting a sensible value to
max_user_instances in production. I would prefer to set
max_user_instances to max int and let the memcg limits of the
workloads limit their inotify usage.

> Are you concerned of users depleting system memory by creating
> userns's and allocating 128 * (struct fsnotify_group) at a time?
>
> IMO, that is not what max_user_instances was meant to protect against.
> There are two reasons I can think of to limit user instances:
> 1. Pre-memgc, user allocation of events is limited to
>     <max_user_instances>*<max_queued_events>
> 2. Performance penalty. User can place <max_user_instances>
>     watches on the same "hot" directory, that will cause any access to
>     that directory by any task on the system to pay the penalty of traversing
>     <max_user_instances> marks and attempt to queue <max_user_instances>
>     events. That cost, including <max_user_instances> inotify_merge() loops
>     could be significant
>
> #1 is not a problem anymore, since you already took care of accounting events
> to the user's memcg.
> #2 is not addressed by your patch.

Yes, I am not addressing #2. Our workloads in prod have their own
private filesystems, so this is not an issue we observed.

>
> >
> > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > ---
> >  fs/notify/group.c                | 14 ++++++++++++--
> >  fs/notify/inotify/inotify_user.c |  5 +++--
> >  include/linux/fsnotify_backend.h |  2 ++
> >  3 files changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/notify/group.c b/fs/notify/group.c
> > index a4a4b1c64d32..fab3cfdb4d9e 100644
> > --- a/fs/notify/group.c
> > +++ b/fs/notify/group.c
> > @@ -114,11 +114,12 @@ EXPORT_SYMBOL_GPL(fsnotify_put_group);
> >  /*
> >   * Create a new fsnotify_group and hold a reference for the group returned.
> >   */
> > -struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
> > +struct fsnotify_group *fsnotify_alloc_group_gfp(const struct fsnotify_ops *ops,
> > +                                               gfp_t gfp)
> >  {
> >         struct fsnotify_group *group;
> >
> > -       group = kzalloc(sizeof(struct fsnotify_group), GFP_KERNEL);
> > +       group = kzalloc(sizeof(struct fsnotify_group), gfp);
> >         if (!group)
> >                 return ERR_PTR(-ENOMEM);
> >
> > @@ -139,6 +140,15 @@ struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
> >
> >         return group;
> >  }
> > +EXPORT_SYMBOL_GPL(fsnotify_alloc_group_gfp);
> > +
> > +/*
> > + * Create a new fsnotify_group and hold a reference for the group returned.
> > + */
> > +struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
> > +{
> > +       return fsnotify_alloc_group_gfp(ops, GFP_KERNEL);
> > +}
> >  EXPORT_SYMBOL_GPL(fsnotify_alloc_group);
> >
> >  int fsnotify_fasync(int fd, struct file *file, int on)
> > diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
> > index 59c177011a0f..7cb528c6154c 100644
> > --- a/fs/notify/inotify/inotify_user.c
> > +++ b/fs/notify/inotify/inotify_user.c
> > @@ -632,11 +632,12 @@ static struct fsnotify_group *inotify_new_group(unsigned int max_events)
> >         struct fsnotify_group *group;
> >         struct inotify_event_info *oevent;
> >
> > -       group = fsnotify_alloc_group(&inotify_fsnotify_ops);
> > +       group = fsnotify_alloc_group_gfp(&inotify_fsnotify_ops,
> > +                                        GFP_KERNEL_ACCOUNT);
> >         if (IS_ERR(group))
> >                 return group;
> >
> > -       oevent = kmalloc(sizeof(struct inotify_event_info), GFP_KERNEL);
> > +       oevent = kmalloc(sizeof(struct inotify_event_info), GFP_KERNEL_ACCOUNT);
> >         if (unlikely(!oevent)) {
> >                 fsnotify_destroy_group(group);
> >                 return ERR_PTR(-ENOMEM);
>
> Any reason why you did not include fanotify in this patch?

The motivation was inotify's max_user_instances but we can charge
fsnotify_group for fanotify as well. Though I would prefer that to be
a separate patch. Let me know what you prefer?

>
> Thanks,
> Amir.

Thanks for the review. I really appreciate your time.

Shakeel