Re: [PATCH 3/4] vfs: check for autofs expiring dentry in follow_automount()

[Ian Kent <raven@themaw.net>]

On Mon, 2020-03-30 at 23:20 +0000, McIntyre, Vincent (CASS, Marsfield)
wrote:
> On Tue, Mar 31, 2020 at 07:07:02AM +0800, Ian Kent wrote:
> > follow_automount() checks if a stat family system call path walk is
> > being done on a positive dentry and and returns -EISDIR to indicate
> > the dentry should be used as is without attempting an automount.
> > 
> > But if autofs is expiring the dentry at the time it should be
> > remounted following the expire.
> > 
> > There are two cases, in the case of a "nobrowse" indirect autofs
> > mount it would have been mounted on lookup anyway. In the case of
> > a "browse" indirect or direct autofs mount re-mounting it will
> > maintain the mount which is what user space would be expected.
> > 
> > This will defer expiration of the mount which might lead to mounts
> > unexpectedly remaining mounted under heavy stat activity but
> > there's
> > no other choice in order to maintain consistency for user space.
> > 
> > Signed-off-by: Ian Kent <raven@themaw.net>
> > ---
> > fs/autofs/root.c |   10 +++++++++-
> > fs/namei.c       |   13 +++++++++++--
> > 2 files changed, 20 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/autofs/root.c b/fs/autofs/root.c
> > index a1c9c32e104f..b3f748e4df08 100644
> > --- a/fs/autofs/root.c
> > +++ b/fs/autofs/root.c
> > @@ -406,9 +406,17 @@ static int autofs_d_manage(const struct path
> > *path, bool rcu_walk)
> > 
> > 	/* Check for (possible) pending expire */
> > 	if (ino->flags & AUTOFS_INF_WANT_EXPIRE) {
> > +		/* dentry possibly going to be picked for expire,
> > +		 * proceed to ref-walk mode.
> > +		 */
> > 		if (rcu_walk)
> > 			return -ECHILD;
> > -		return 0;
> > +
> > +		/* ref-walk mode, return 1 so follow_automount()
> > +		 * can wait on the expire outcome and possibly
> > +		 * attempt a re-mount.
> > +		 */
> > +		return 1;
> > 	}
> > 
> > 	/*
> > diff --git a/fs/namei.c b/fs/namei.c
> > index db6565c99825..869e0d4bb4d9 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -1227,11 +1227,20 @@ static int follow_automount(struct path
> > *path, struct nameidata *nd,
> > 	 * mounted directory.  Also, autofs may mark negative dentries
> > 	 * as being automount points.  These will need the attentions
> > 	 * of the daemon to instantiate them before they can be used.
> > +	 *
> > +	 * Also if ->d_manage() returns 1 the dentry transit needs
> > +	 * to be managing. For autofs, a return of 1 it tells us the
> 
> Unclear. Do you mean "to be managed." ? Or "managing." ?

Right, and I didn't label these v2 either, bit stressed at the
moment I guess.

Ian
> 
> Cheers
> Vince
> 
> > +	 * dentry might be expired, so proceed to ->d_automount().
> > 	 */
> > 	if (!(nd->flags & (LOOKUP_PARENT | LOOKUP_DIRECTORY |
> > 			   LOOKUP_OPEN | LOOKUP_CREATE |
> > LOOKUP_AUTOMOUNT)) &&
> > -	    path->dentry->d_inode)
> > -		return -EISDIR;
> > +	    path->dentry->d_inode) {
> > +		if (path->dentry->d_flags & DCACHE_MANAGE_TRANSIT) {
> > +			if (!path->dentry->d_op->d_manage(path, false))
> > +				return -EISDIR;
> > +		} else
> > +			return -EISDIR;
> > +	}
> > 
> > 	nd->total_link_count++;
> > 	if (nd->total_link_count >= 40)
> > 
> 
> --