linux-kernel - Re: [RFC PATCH 2/4] Mount changes to support union mount.

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Message-ID: <20070621164020.GB9457@filer.fsl.cs.sunysb.edu>
Date:	Thu, 21 Jun 2007 12:40:20 -0400
From:	Josef Sipek <jsipek@....cs.sunysb.edu>
To:	Bharata B Rao <bharata.rao@...il.com>
Cc:	Jan Blunck <jblunck@...e.de>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, bharata@...ux.vnet.ibm.com
Subject: Re: [RFC PATCH 2/4] Mount changes to support union mount.

On Wed, Jun 20, 2007 at 02:23:30PM +0530, Bharata B Rao wrote:
> (replying from a different ID as you didn't copy me on reply)
> 
> On 6/20/07, Jan Blunck <jblunck@...e.de> wrote:
> >On Wed, 20 Jun 2007 11:22:41 +0530, Bharata B Rao wrote:
> >
> >> +/*
> >> + * When propagating mount events to peer group, this is called under
> >> + * vfsmount_lock. Hence using GFP_ATOMIC for kmalloc here.
> >> + * TODO: Can we use a separate kmem cache for union_mount ?
> >> + */
> >> +struct union_mount *alloc_union_mount(struct vfsmount *src_mnt,
> >> +     struct dentry *src_dentry, struct vfsmount *dst_mnt,
> >> +     struct dentry *dst_dentry)
> >> +{
> >> +     struct union_mount *u;
> >> +     u = kmalloc(sizeof(struct union_mount), GFP_ATOMIC);
> >> +     if (!u)
> >> +             return u;
> >> +     u->dst_mnt = mntget(dst_mnt);
> >> +     u->dst_dentry = dget(dst_dentry);
> >> +     u->src_mnt = src_mnt;
> >> +     u->src_dentry = dget(src_dentry);
> >> +     INIT_LIST_HEAD(&u->hash);
> >> +     INIT_LIST_HEAD(&u->list);
> >> +     return u;
> >> +}
> >
> >Hmm, you pin the dentries in memory until umount. This isn't good. Besides
> >that this doesn't work with file systems that do invalidate their
> >dentries. The file system must have a chance to replace the dentry in the
> >union structure.
> 
> Yes, both top level and next level dentries are pinned until umount of
> the upper layer. I was thinking if we could prune these from
> prune_dcache(). What do you think ?
> 
> Ok, I haven't thought about filesystem invalidating the dentries. Yet
> to understand the dentry invalidation, but would filesystem invalidate
> an inuse dentry ?
 
NFS, and quite possibly other network/cluster filesystems can encounter all
sort of odd conditions where the dentry might change, no?

Josef "Jeff" Sipek.

-- 
Hegh QaQ law'
quvHa'ghach QaQ puS
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