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@xxxxxxxxxx> I've been confused by the comment on __lookup_mnt() before, so this is a helpful update. Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@xxxxxxxxxx>
