Re: [PATCH v2 3/5] fs: fix __lookup_mnt() documentation

[Seth Forshee <sforshee@kernel.org>]

On Tue, Mar 28, 2023 at 06:13:08PM +0200, Christian Brauner wrote:
> The comment on top of __lookup_mnt() states that it finds the first
> mount implying that there could be multiple mounts mounted at the same
> dentry with the same parent.
> 
> This was true on old kernels where __lookup_mnt() could encounter a
> stack of child mounts such that each child had the same parent mount and
> was mounted at the same dentry. These were called "shadow mounts" and
> were created during mount propagation. So back then if a mount @m in the
> destination propagation tree already had a child mount @p mounted at
> @mp then any mount @n we propagated to @m at the same @mp would be
> appended after the preexisting mount @p in @mount_hashtable.
> 
> This hasn't been the case for quite a while now and I don't see an
> obvious way how such mount stacks could be created in another way. And
> if that's possible it would invalidate assumptions made in other parts
> of the code.
> 
> So for a long time on all relevant kernels the child-parent relationship
> is unique per dentry. So given a child mount @c mounted at its parent
> mount @p on dentry @mp means that @c is the only child mounted on
> @p at @mp. Should a mount @m be propagated to @p on @mp then @m will be
> mounted on @p at @mp and the preexisting child @c will be remounted on
> top of @m at @m->mnt_root.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>

I've been confused by the comment on __lookup_mnt() before, so this is a
helpful update.

Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>