Re: [Devel] [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

[Stanislav Kinsbursky]

11.12.2012 18:00, Stanislav Kinsbursky ÐÐÑÐÑ:
> 11.12.2012 00:28, J. Bruce Fields ÐÐÑÐÑ:
> > On Thu, Dec 06, 2012 at 06:34:47PM +0300, Stanislav Kinsbursky wrote:
> > > NFSd does lookup. Lookup is done starting from current->fs->root.
> > > NFSd is a kthread, cloned by kthreadd, and thus have global (but luckely
> > > unshared) root.
> > > So we have to swap root to those, which process, started NFSd, has. Because
> > > that process can be in a container with it's own root.
> >
> > This doesn't sound right to me.
> >
> > Which lookups exactly do you see being done relative to
> > current->fs->root ?
>
> Ok, you are right. I was mistaken here.
> This is not a exactly lookup, but d_path() problem in svc_export_request().
> I.e. without root swapping, d_path() will give not local export path (like "/export")
> but something like this "/root/containers_root/export".

We, actually, can do it less in less aggressive way.
I.e. instead root swap and current svc_export_request() implementation:

void svc_export_request(...)
{
	<snip>
        pth = d_path(&exp->ex_path, *bpp, *blen);
	<snip>
}

we can do something like this:

void svc_export_request(...)
{
	struct nfsd_net *nn = ...
	<snip>
	spin_lock(&dcache_lock);
        pth = __d_path(&exp->ex_path, &nn->root, *bpp, *blen);
	spin_unlock(&dcache_lock);
	<snip>
}

> > --b.
> >
> > > Signed-off-by: Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx>
> > > ---
> > >   fs/nfsd/netns.h  |    1 +
> > >   fs/nfsd/nfssvc.c |   33 ++++++++++++++++++++++++++++++++-
> > >   2 files changed, 33 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> > > index abfc97c..5c423c6 100644
> > > --- a/fs/nfsd/netns.h
> > > +++ b/fs/nfsd/netns.h
> > > @@ -101,6 +101,7 @@ struct nfsd_net {
> > >       struct timeval nfssvc_boot;
> > >
> > >       struct svc_serv *nfsd_serv;
> > > +    struct path    root;
> > >   };
> > >
> > >   extern int nfsd_net_id;
> > > diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> > > index cee62ab..177bb60 100644
> > > --- a/fs/nfsd/nfssvc.c
> > > +++ b/fs/nfsd/nfssvc.c
> > > @@ -392,6 +392,7 @@ int nfsd_create_serv(struct net *net)
> > >
> > >       set_max_drc();
> > >       do_gettimeofday(&nn->nfssvc_boot);        /* record boot time */
> > > +    get_fs_root(current->fs, &nn->root);
> > >       return 0;
> > >   }
> > >
> > > @@ -426,8 +427,10 @@ void nfsd_destroy(struct net *net)
> > >       if (destroy)
> > >           svc_shutdown_net(nn->nfsd_serv, net);
> > >       svc_destroy(nn->nfsd_serv);
> > > -    if (destroy)
> > > +    if (destroy) {
> > > +        path_put(&nn->root);
> > >           nn->nfsd_serv = NULL;
> > > +    }
> > >   }
> > >
> > >   int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
> > > @@ -533,6 +536,25 @@ out:
> > >       return error;
> > >   }
> > >
> > > +/*
> > > + * This function is actually slightly modified set_fs_root()<F9>
> > > + */
> > > +static void nfsd_swap_root(struct net *net)
> > > +{
> > > +    struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> > > +    struct fs_struct *fs = current->fs;
> > > +    struct path old_root;
> > > +
> > > +    path_get(&nn->root);
> > > +    spin_lock(&fs->lock);
> > > +    write_seqcount_begin(&fs->seq);
> > > +    old_root = fs->root;
> > > +    fs->root = nn->root;
> > > +    write_seqcount_end(&fs->seq);
> > > +    spin_unlock(&fs->lock);
> > > +    if (old_root.dentry)
> > > +        path_put(&old_root);
> > > +}
> > >
> > >   /*
> > >    * This is the NFS server kernel thread
> > > @@ -559,6 +581,15 @@ nfsd(void *vrqstp)
> > >       current->fs->umask = 0;
> > >
> > >       /*
> > > +     * We have to swap NFSd kthread's fs->root.
> > > +     * Why so? Because NFSd can be started in container, which has it's own
> > > +     * root.
> > > +     * And so what? NFSd lookup files, and lookup start from
> > > +     * current->fs->root.
> > > +     */
> > > +    nfsd_swap_root(net);
> > > +
> > > +    /*
> > >        * thread is spawned with all signals set to SIG_IGN, re-enable
> > >        * the ones that will bring down the thread
> > >        */


--
Best regards,
Stanislav Kinsbursky
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