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

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Message-ID: <CAOQ4uxhNw12XSb7dVbUAGh+LdDzpVaF=LozoPTuXOjL8DGXn4Q@mail.gmail.com>
Date:   Sun, 20 Dec 2020 13:31:07 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] inotify, memcg: account inotify instances to kmemcg

On Sun, Dec 20, 2020 at 6:24 AM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Sat, Dec 19, 2020 at 8:25 AM Amir Goldstein <amir73il@...il.com> wrote:
> >
> > On Sat, Dec 19, 2020 at 4:31 PM Shakeel Butt <shakeelb@...gle.com> wrote:
> > >
> > > On Sat, Dec 19, 2020 at 1:48 AM Amir Goldstein <amir73il@...il.com> wrote:
> > > >
> > > > On Sat, Dec 19, 2020 at 12:11 AM Shakeel Butt <shakeelb@...gle.com> 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.
> > >
> >
> > understood.
> > and I guess the multiple workloads cannot run each in their own userns?
> > because then you wouldn't need to change max_user_instances limit.
> >
>
> No workloads can run in their own user namespace but please note that
> max_user_instances is shared between all the user namespaces.

/proc/sys/fs/inotify/max_user_instances is shared between all the user
namespaces, but it only controls the init_user_ns limits.
/proc/sys/user/max_inotify_instances is per user ns and it is the one that
actually controls the inotify limits in non init_user_ns.

That said, I see that it is always initialized to MAX_INT on non init user ns,
which is exactly the setup that you are aiming at:

$ unshare -U
$ cat /proc/sys/user/max_inotify_instances
2147483647
$ cat /proc/sys/fs/inotify/max_user_instances
128

Thanks,
Amir.