Re: [PATCH] shmem: add support for user extended attributes

[Oleksandr Tymoshenko <ovt@google.com>]

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Thanks for working on this.

On Mon, Aug 21, 2023 at 10:52 AM Hugh Dickins <hughd@google.com> wrote:

> On Tue, 15 Aug 2023, Christian Brauner wrote:
> > On Tue, Aug 15, 2023 at 09:46:22AM +0200, Franklin “Snaipe” Mathieu
> wrote:
> > >
> > > So, it's likely that there's some more work to do in that area; I'd
> > > certainly expect the OOM killer to take the overall memory footprint
> > > of mount namespaces into account when selecting which processes to
> > > kill. It's also possible my experiment was flawed and not
> > > representative of a real-life scenario, as I clearly have interacted
> > > with misbehaving containers before, which got killed when they wrote
> > > too much to tmpfs. But then again, my experiment also didn't take
> > > memory cgroups into account.
> >
> > So mount namespaces are orthogonal to that and they would be the wrong
> > layer to handle this.
> >
> > Note that an unprivileged user (regular or via containers) on the system
> > can just exhaust all memory in various ways. Ultimately the container or
> > user would likely be taken down by in-kernel OOM or systemd-oomd or
> > similar tools under memory pressure.
> >
> > Of course, all that means is that untrusted workloads need to have
> > cgroup memory limits. That also limits tmpfs instances and prevents
> > unprivileged user from using all memory.
> >
> > If you don't set a memory limit then yes, the container might be able to
> > exhaust all memory but that's a bug in the container runtime. Also, at
> > some point the OOM killer or related userspace tools will select the
> > container init process for termination at which point all the namespaces
> > and mounts go away. That's probably what you experience as misbehaving
> > containers. The real bug there is probably that they're allowed to run
> > without memory limits in the first place.
>
> Thanks, this was a good reminder that I very much needed to look back at
> the memory cgroup limiting of xattrs on tmpfs - I'd had the patch in the
> original series, then was alarmed to find shmem_alloc_inode() using
> GFP_KERNEL, so there seemed no point in accounting the xattrs if the
> inodes were not being accounted: so dropped it temporarily.  I had
> forgotten that SLAB_ACCOUNT on the kmem_cache ensures that accounting.
>
> "tmpfs,xattr: GFP_KERNEL_ACCOUNT for simple xattrs" just sent to fix it:
>
> https://lore.kernel.org/linux-fsdevel/f6953e5a-4183-8314-38f2-40be60998615@google.com/
>
> Thanks,
> Hugh

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