From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape
 on ".." resolution
Date: Thu, 5 Sep 2019 00:29:11 +0100
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Howells <dhowells@redhat.com>, Aleksa Sarai <cyphar@cyphar.com>, Jeff Layton <jlayton@kernel.org>, "J. Bruce Fields" <bfields@fieldses.org>, Arnd Bergmann <arnd@arndb.de>, Shuah Khan <shuah@kernel.org>, Shuah Khan <skhan@linuxfoundation.org>, Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Christian Brauner <christian@brauner.io>, Jann Horn <jannh@google.com>, Kees Cook <keescook@chromium.org>, Eric Biederman <ebiederm@xmission.com>, Andy Lutomirski <luto@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Alexei Starovoitov <ast@kernel.org>, Tycho Andersen <tycho@tycho.ws>, David Drysdale <drysdale@google.com>, Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>, Rasmus Villemoes <linux@r>
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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@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...