Re: [PATCH 0/6] shmem: Add user and group quota support for tmpfs

[Christian Brauner <brauner@kernel.org>]

On Wed, Apr 05, 2023 at 12:44:27PM +0200, Carlos Maiolino wrote:
> Hi Christian.
> 
> On Wed, Apr 05, 2023 at 10:52:44AM +0200, Christian Brauner wrote:
> > On Mon, Apr 03, 2023 at 10:47:53AM +0200, cem@kernel.org wrote:
> > > From: Carlos Maiolino <cmaiolino@redhat.com>
> > >
> > > Hi folks. this work has been done originally by Lukas, but he left the company,
> > > so I'm taking over his work from where he left it of. This series is virtually
> > > done, and he had updated it with comments from the last version, but, I'm
> > 
> > I've commented on the last version:
> > 
> > https://lore.kernel.org/linux-fsdevel/20221129112133.rrpoywlwdw45k3qa@wittgenstein
> > 
> > trying to point out that tmpfs can be mounted in user namespaces. Which
> > means that the quota uids and gids need to take the idmapping of the
> > user namespace in which the tmpfs instances is mounted in into account;
> > not the one on the host.
> > 
> > See the link above for some details. Before we can merge this it would
> > be very good if we could get tests that verify tmpfs being mounted
> > inside a userns with quotas enabled because I don't think this is
> > covered yet by xfstests. Or you punt on it for now and restricted quotas
> > to tmpfs instances mounted on the host.
> > 
> 
> Thanks for the link, I've read it before, and this is by now a limitation I'd
> like to keep in this series. I can extend it to be namespace aware later on, but
> the current goal of this series is to be able tmpfs mounts on the host to limit
> the amount of memory consumed by users. Being namespace aware is something I

This is fine with me. But please point the restriction out in the
documentation and in the commit message. This is especially important
because the check is hidden in the bowls of dquot_load_quota_sb().

Ideally we'd probably check for fc->user_ns == &init_user_ns directly
when parsing the quota mount options instead of waiting until
fill_super.