[PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

[PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

[Jeff Layton]

We had a couple of reports of people that are mounting NFS filesystems,
and then bind mounting certain local files onto dentries in that nfs
mount (sort of like a poor-man's unionmount).

This all works well until the dentry serving as the mountpoint fails
d_revalidate. The dentry will end up being invalidated which makes the
bind mount unreachable via pathwalk.

It doesn't make much sense to me to allow dentries to serve as
mountpoints to end up invalidated, so there's no real point in
attempting to d_revalidate them at all.

Reported-by: Patrick McLean <chutzpah@gentoo.org>
Reported-by: Guang Cheng Li <liguangc@cn.ibm.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 fs/namei.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index caa2805..5b10ad0 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -585,6 +585,9 @@ drop_root_mnt:
 
 static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
 {
+	/* dentries that serve as mountpoints are always considered valid */
+	if (d_mountpoint(dentry))
+		return 1;
 	return dentry->d_op->d_revalidate(dentry, flags);
 }
 
-- 
1.8.3.1

Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

[NeilBrown]

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On Mon,  4 Nov 2013 20:16:26 -0500 Jeff Layton <jlayton@redhat.com> wrote:

> We had a couple of reports of people that are mounting NFS filesystems,
> and then bind mounting certain local files onto dentries in that nfs
> mount (sort of like a poor-man's unionmount).
> 
> This all works well until the dentry serving as the mountpoint fails
> d_revalidate. The dentry will end up being invalidated which makes the
> bind mount unreachable via pathwalk.
> 
> It doesn't make much sense to me to allow dentries to serve as
> mountpoints to end up invalidated, so there's no real point in
> attempting to d_revalidate them at all.
> 
> Reported-by: Patrick McLean <chutzpah@gentoo.org>
> Reported-by: Guang Cheng Li <liguangc@cn.ibm.com>
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  fs/namei.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index caa2805..5b10ad0 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -585,6 +585,9 @@ drop_root_mnt:
>  
>  static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
>  {
> +	/* dentries that serve as mountpoints are always considered valid */
> +	if (d_mountpoint(dentry))
> +		return 1;
>  	return dentry->d_op->d_revalidate(dentry, flags);
>  }
>  

Makes sense to me. But what about the parent or grandparent of the mountpoint?

Presumably we don't want to invalidate them either, and I don't think
d_mountpoint() returns true for those, does it?

NeilBrown

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Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

[Shirish Pargaonkar]

For a similar issue in cifs vfs (samba bugzilla 8950), I was going to try
unset the bit DCACHE_OP_REVALIDATE of d_flags of the
dentry.  Would something like work for the mountpoint dentry?



On Mon, Nov 4, 2013 at 7:16 PM, Jeff Layton <jlayton@redhat.com> wrote:
> We had a couple of reports of people that are mounting NFS filesystems,
> and then bind mounting certain local files onto dentries in that nfs
> mount (sort of like a poor-man's unionmount).
>
> This all works well until the dentry serving as the mountpoint fails
> d_revalidate. The dentry will end up being invalidated which makes the
> bind mount unreachable via pathwalk.
>
> It doesn't make much sense to me to allow dentries to serve as
> mountpoints to end up invalidated, so there's no real point in
> attempting to d_revalidate them at all.
>
> Reported-by: Patrick McLean <chutzpah@gentoo.org>
> Reported-by: Guang Cheng Li <liguangc@cn.ibm.com>
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  fs/namei.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index caa2805..5b10ad0 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -585,6 +585,9 @@ drop_root_mnt:
>
>  static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
>  {
> +       /* dentries that serve as mountpoints are always considered valid */
> +       if (d_mountpoint(dentry))
> +               return 1;
>         return dentry->d_op->d_revalidate(dentry, flags);
>  }
>
> --
> 1.8.3.1
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-nfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

[Christoph Hellwig]

On Mon, Nov 04, 2013 at 09:00:23PM -0600, Shirish Pargaonkar wrote:
> For a similar issue in cifs vfs (samba bugzilla 8950), I was going to try
> unset the bit DCACHE_OP_REVALIDATE of d_flags of the
> dentry.  Would something like work for the mountpoint dentry?

The DCACHE_OP_REVALIDATE are just caches of the available dentry
operations, please don't manipulate those flags on their own.

Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

[Jeff Layton]

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On Tue, 5 Nov 2013 13:39:59 +1100
NeilBrown <neilb@suse.de> wrote:

> On Mon,  4 Nov 2013 20:16:26 -0500 Jeff Layton <jlayton@redhat.com> wrote:
> 
> > We had a couple of reports of people that are mounting NFS filesystems,
> > and then bind mounting certain local files onto dentries in that nfs
> > mount (sort of like a poor-man's unionmount).
> > 
> > This all works well until the dentry serving as the mountpoint fails
> > d_revalidate. The dentry will end up being invalidated which makes the
> > bind mount unreachable via pathwalk.
> > 
> > It doesn't make much sense to me to allow dentries to serve as
> > mountpoints to end up invalidated, so there's no real point in
> > attempting to d_revalidate them at all.
> > 
> > Reported-by: Patrick McLean <chutzpah@gentoo.org>
> > Reported-by: Guang Cheng Li <liguangc@cn.ibm.com>
> > Signed-off-by: Jeff Layton <jlayton@redhat.com>
> > ---
> >  fs/namei.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index caa2805..5b10ad0 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -585,6 +585,9 @@ drop_root_mnt:
> >  
> >  static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
> >  {
> > +	/* dentries that serve as mountpoints are always considered valid */
> > +	if (d_mountpoint(dentry))
> > +		return 1;
> >  	return dentry->d_op->d_revalidate(dentry, flags);
> >  }
> >  
> 
> Makes sense to me. But what about the parent or grandparent of the mountpoint?
> 
> Presumably we don't want to invalidate them either, and I don't think
> d_mountpoint() returns true for those, does it?
> 
> NeilBrown

Good point...

I think we're probably saved there (at least on NFS) by the
check_submounts_and_drop() call in nfs_lookup_revalidate.

Hmmm, that said...I did do most of this testing on a slightly older
kernel that doesn't have that call, and I can't reproduce this problem
on a 3.12-ish kernel. I think this is probably already fixed by Miklos'
patches that added that.

Sorry for the noise!
-- 
Jeff Layton <jlayton@redhat.com>

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Re: [PATCH] [RFC] vfs: don't revalidate dentries that serve as mountpoints

[Jeff Layton]

On Mon, 4 Nov 2013 21:00:23 -0600
Shirish Pargaonkar <shirishpargaonkar@gmail.com> wrote:

> For a similar issue in cifs vfs (samba bugzilla 8950), I was going to try
> unset the bit DCACHE_OP_REVALIDATE of d_flags of the
> dentry.  Would something like work for the mountpoint dentry?
> 
> 

I don't think that's the same issue at all. The problem there is that
you need to instantiate a dentry for the root of the mount on the
client, when you don't have any information about the parents.

In this case, we have dentries that are acting as mountpoints that were
getting invalidated.

> 
> On Mon, Nov 4, 2013 at 7:16 PM, Jeff Layton <jlayton@redhat.com> wrote:
> > We had a couple of reports of people that are mounting NFS filesystems,
> > and then bind mounting certain local files onto dentries in that nfs
> > mount (sort of like a poor-man's unionmount).
> >
> > This all works well until the dentry serving as the mountpoint fails
> > d_revalidate. The dentry will end up being invalidated which makes the
> > bind mount unreachable via pathwalk.
> >
> > It doesn't make much sense to me to allow dentries to serve as
> > mountpoints to end up invalidated, so there's no real point in
> > attempting to d_revalidate them at all.
> >
> > Reported-by: Patrick McLean <chutzpah@gentoo.org>
> > Reported-by: Guang Cheng Li <liguangc@cn.ibm.com>
> > Signed-off-by: Jeff Layton <jlayton@redhat.com>
> > ---
> >  fs/namei.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index caa2805..5b10ad0 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -585,6 +585,9 @@ drop_root_mnt:
> >
> >  static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
> >  {
> > +       /* dentries that serve as mountpoints are always considered valid */
> > +       if (d_mountpoint(dentry))
> > +               return 1;
> >         return dentry->d_op->d_revalidate(dentry, flags);
> >  }
> >
> > --
> > 1.8.3.1
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-nfs" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html


-- 
Jeff Layton <jlayton@redhat.com>