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

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11.12.2012 18:12, Stanislav Kinsbursky пишет:
11.12.2012 18:00, Stanislav Kinsbursky пишет:
11.12.2012 00:28, J. Bruce Fields пишет:
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>
}


No, this won't work. Sorry for noise.

--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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