Re: [PATCH v7 01/12] btrfs: lift rw mount setup from mount and remount

[David Sterba <dsterba@suse.cz>]

On Wed, Nov 18, 2020 at 03:06:16PM -0800, Boris Burkov wrote:
> Mounting rw and remounting from ro to rw naturally share invariants and
> functionality which result in a correctly setup rw filesystem. Luckily,
> there is even a strong unity in the code which implements them. In
> mount's open_ctree, these operations mostly happen after an early return
> for ro file systems, and in remount, they happen in a section devoted to
> remounting ro->rw, after some remount specific validation passes.
> 
> However, there are unfortunately a few differences. There are small
> deviations in the order of some of the operations, remount does not
> cleanup orphan inodes in root_tree or fs_tree, remount does not create
> the free space tree, and remount does not handle "one-shot" mount
> options like clear_cache and uuid tree rescan.
> 
> Since we want to add building the free space tree to remount, and since
> it is possible to leak orphans on a filesystem mounted as ro then
> remounted rw

The statement is not specific if the orphans are files or roots. But I
don't agree that a leak is possible, or need a proof of the claim above.

The mount-time orphan cleanup will start early, but otherwise orphan
cleanup is checked and started on dentry lookups (btrfs_lookup_dentry).
Deleted but not clened tree roorts are all found and removed, regardless
of rw or ro->rw mount.

So I wonder if you claim there's a leak just by lack of an explicit call
on the remount path.