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Date:   Fri, 17 Sep 2021 11:38:38 +0200
From:   Jan Kara <>
To:     "Darrick J. Wong" <>
Cc:     xfs <>,
        linux-ext4 <>,
        linux-btrfs <>,
        linux-fsdevel <>,
        Amir Goldstein <>
Subject: Re: Shameless plug for the FS Track at LPC next week!

On Fri 17-09-21 10:36:08, Jan Kara wrote:
> Let me also post Amir's thoughts on this from a private thread:

And now I'm actually replying to Amir :-p

> On Fri 17-09-21 10:30:43, Jan Kara wrote:
> > We did a small update to the schedule:
> > 
> > > Christian Brauner will run the second session, discussing what idmapped
> > > filesystem mounts are for and the current status of supporting more
> > > filesystems.
> > 
> > We have extended this session as we'd like to discuss and get some feedback
> > from users about project quotas and project ids:
> > 
> > Project quotas were originally mostly a collaborative feature and later got
> > used by some container runtimes to implement limitation of used space on a
> > filesystem shared by multiple containers. As a result current semantics of
> > project quotas are somewhat surprising and handling of project ids is not
> > consistent among filesystems. The main two contending points are:
> > 
> > 1) Currently the inode owner can set project id of the inode to any
> > arbitrary number if he is in init_user_ns. It cannot change project id at
> > all in other user namespaces.
> > 
> > 2) Should project IDs be mapped in user namespaces or not? User namespace
> > code does implement the mapping, VFS quota code maps project ids when using
> > them. However e.g. XFS does not map project IDs in its calls setting them
> > in the inode. Among other things this results in some funny errors if you
> > set project ID to (unsigned)-1.
> > 
> > In the session we'd like to get feedback how project quotas / ids get used
> > / could be used so that we can define the common semantics and make the
> > code consistently follow these rules.
> I think that legacy projid semantics might not be a perfect fit for
> container isolation requirements. I added project quota support to docker
> at the time because it was handy and it did the job of limiting and
> querying disk usage of containers with an overlayfs storage driver.
> With btrfs storage driver, subvolumes are used to create that isolation.
> The TREE_ID proposal [1] got me thinking that it is not so hard to
> implement "tree id" as an extention or in addition to project id.
> The semantics of "tree id" would be:
> 1. tree id is a quota entity accounting inodes and blocks
> 2. tree id can be changed only on an empty directory
> 3. tree id can be set to TID only if quota inode usage of TID is 0
> 4. tree id is always inherited from parent
> 5. No rename() or link() across tree id (clone should be possible)
> AFAIK btrfs subvol meets all the requirements of "tree id".
> Implementing tree id in ext4/xfs could be done by adding a new field to
> inode on-disk format and a new quota entity to quota on-disk format and
> quotatools.
> An alternative simpler way is to repurpose project id and project quota:
> * Add filesystem feature projid-is-treeid
> * The feature can be enabled on fresh mkfs or after fsck verifies "tree id"
>    rules are followed for all usage of projid
> * Once the feature is enabled, filesystem enforces the new semantics
>   about setting projid and projid_inherit
> This might be a good option if there is little intersection between
> systems that need to use the old project semantics and systems
> that would rather have the tree id semantics.

Yes, I actually think that having both tree-id and project-id on a
filesystem would be too confusing. And I'm not aware of realistic usecases.
I've heard only of people wanting current semantics (although these we more
of the kind: "sometime in the past people used the feature like this") and
the people complaining current semantics is not useful for them. This was
discussed e.g. in ext4 list [2].

> I think that with the "tree id" semantics, the user_ns/idmapped
> questions become easier to answer.
> Allocating tree id ranges per userns to avoid exhausting the tree id
> namespace is a very similar problem to allocating uids per userns.

It still depends how exactly tree ids get used - if you want to use them to
limit space usage of a container, you still have to forbid changing of tree
ids inside the container, don't you?

> [1]



Jan Kara <>

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