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

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Message-ID: <344eb09a0706200153j3b856dcvb983218714d1eda0@mail.gmail.com>
Date:	Wed, 20 Jun 2007 14:23:30 +0530
From:	"Bharata B Rao" <bharata.rao@...il.com>
To:	"Jan Blunck" <jblunck@...e.de>
Cc:	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.

(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 ?

Regards,
Bharata.
-- 
"Men come and go but mountains remain" -- Ruskin Bond.
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