linux-kernel - Re: [PATCH RFC 1/3] fs: introduce helper d_path

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Date:   Mon, 10 May 2021 09:20:43 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jia He <justin.he@....com>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@....linux.org.uk>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()

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@...iv.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.)