[PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution

[Al Viro]

On Wed, Sep 04, 2019 at 03:38:20PM -0700, Linus Torvalds wrote:
> On Wed, Sep 4, 2019 at 3:31 PM David Howells <dhowells at redhat.com> wrote:
> >
> > It ought to be reasonably easy to make them per-sb at least, I think.  We
> > don't allow cross-super rename, right?
> 
> Right now the sequence count handling very much depends on it being a
> global entity on the reader side, at least.
> 
> And while the rename sequence count could (and probably should) be
> per-sb, the same is very much not true of the mount one.

Huh?  That will cost us having to have a per-superblock dentry
hash table; recall that lockless lockup can give false negatives
if something gets moved from chain to chain, and rename_lock is
first and foremost used to catch those and retry.  If we split
it on per-superblock basis, we can't have dentries from different
superblocks in the same chain anymore...