Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()

From: Christian Brauner <christian.brauner@ubuntu.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Jia He <justin.he@arm.com>, Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Jonathan Corbet <corbet@lwn.net>, Al Viro <viro@ftp.linux.org.uk>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Ira Weiny <ira.weiny@intel.com>,
	Eric Biggers <ebiggers@google.com>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-s390 <linux-s390@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()
Date: Mon, 10 May 2021 09:20:43 +0200	[thread overview]
Message-ID: <20210510072043.lde2n3hbk7lgeddv@wittgenstein> (raw)
In-Reply-To: <YJcbkJxrFAheQ5yO@zeniv-ca.linux.org.uk>

On Sat, May 08, 2021 at 11:15:28PM +0000, Al Viro wrote:
> On Sat, May 08, 2021 at 10:46:23PM +0000, Al Viro wrote:
> > On Sat, May 08, 2021 at 03:17:44PM -0700, Linus Torvalds wrote:
> > > On Sat, May 8, 2021 at 2:06 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > > >
> > > > On Sat, May 08, 2021 at 01:39:45PM -0700, Linus Torvalds wrote:
> > > >
> > > > > +static inline int prepend_entries(struct prepend_buffer *b, const struct path *path, const struct path *root, struct mount *mnt)
> > > >
> > > > If anything, s/path/dentry/, since vfsmnt here will be equal to &mnt->mnt all along.
> > > 
> > > Too subtle for me.
> > > 
> > > And is it? Because mnt is from
> > > 
> > >      mnt = real_mount(path->mnt);
> > > 
> > > earlier, while vfsmount is plain "path->mnt".
> > 
> > static inline struct mount *real_mount(struct vfsmount *mnt)
> > {
> >         return container_of(mnt, struct mount, mnt);
> > }
> 
> Basically, struct vfsmount instances are always embedded into struct mount ones.
> All information about the mount tree is in the latter (and is visible only if
> you manage to include fs/mount.h); here we want to walk towards root, so...
> 
> Rationale: a lot places use struct vfsmount pointers, but they've no need to
> access all that stuff.  So struct vfsmount got trimmed down, with most of the
> things that used to be there migrating into the containing structure.
> 
> [Christian Browner Cc'd]
> BTW, WTF do we have struct mount.user_ns and struct vfsmount.mnt_userns?
> Can they ever be different?  Christian?

Yes, they can.

> 
> 	Sigh...  Namespace flavours always remind me of old joke -
> Highlander II: There Should've Been Only One...

(I'd prefer if mount propagation would die first.)