[Devel] Re: [PATCH 08/10] Define get_sb_ref()

[Cedric Le Goater]

sukadev at linux.vnet.ibm.com wrote:
> Dave Hansen [dave at linux.vnet.ibm.com] wrote:
> | On Fri, 2008-09-26 at 14:21 -0700, sukadev at linux.vnet.ibm.com wrote:
> | > Dave Hansen [dave at linux.vnet.ibm.com] wrote:
> | > | On Fri, 2008-09-12 at 10:53 -0700, sukadev at us.ibm.com wrote:
> | > | > + *     But for single-mount semantics, devpts cannot use get_sb_single(),
> | > | > + *     because get_sb_single()/sget() find and use the super-block from
> | > | > + *     the most recent mount of devpts. But that recent mount may be a
> | > | > + *     'newinstance' mount and get_sb_single() would pick the newinstance
> | > | > + *     super-block instead of the initial super-block.
> | > | 
> | > | Can't you just override the test() function to get what you want here?
> | > 
> | > get_sb_single() does not take a test() parameter and so I would still
> | > need a get_sb_ref() or get_sb_special() interface right ? 
> | > 
> | > This special interface could call sget() with a custom-test function,
> | > to get the super-block.  But in case of devpts, we already have the
> | > super-block. So we don't need to call sget(). We just need get a reference
> | > and remount.
> | 
> | Well, you shouldn't be using get_sb_single() at all any more, right?
> 
> You mean define something like get_sb_multi_mode() and define a  new
> test function ? I can try that.
> 
> | 
> | At this point, you're doing something super-specialized for devpts.  So,
> | why do this in super.c.  Just put it in place of devpts_get_sb()'s
> | current contents.
> 
> grab_super() is static. I have to either externalize it or define the
> new interface in fs/super.c.  I am not sure if mqueue ns or others have
> similar semantics and if so, defining interface in fs/super.c has a better
> chance of being found/generalized when there is another user.

mqueue ns and proc have similar needs but there are those special little
things that make difficult to share a common routine.

Here's the get_sb_single_ns() we would like to share. 

C.
  
From: Cedric Le Goater <clg at fr.ibm.com>

This patch defines the mqueuefs ->get_sb routine as follows:
  . when allocating a new super_block, the mq namespace is stored in that
    super_block s_fs_info attribute.
  . when looking for a super_block, a comparison is done against the
    s_fs_info attribute (it should be equal to the mq namespace that will
    be passed in as a data parameter).

Signed-off-by: Nadia Derbey <Nadia.Derbey at bull.net>
Signed-off-by: Cedric Le Goater <clg at fr.ibm.com>

---
 ipc/mqueue.c |   52 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 51 insertions(+), 1 deletion(-)

Index: linux-2.6.27-rc7/ipc/mqueue.c
===================================================================
--- linux-2.6.27-rc7.orig/ipc/mqueue.c	2008-09-25 15:03:58.000000000 +0200
+++ linux-2.6.27-rc7/ipc/mqueue.c	2008-09-25 16:28:13.000000000 +0200
@@ -201,6 +201,48 @@ static int mqueue_fill_super(struct supe
 	return 0;
 }

+static int compare_sb_single_ns(struct super_block *sb, void *data)
+{
+	return sb->s_fs_info == data;
+}
+
+static int set_sb_single_ns(struct super_block *sb, void *data)
+{
+	struct mq_namespace *mq_ns = (struct mq_namespace *) data;
+	int error;
+
+	sb->s_fs_info = get_mq_ns(mq_ns);
+	error = set_anon_super(sb, NULL);
+	if (error)
+		put_mq_ns(mq_ns);
+	return error;
+}
+
+static int get_sb_single_ns(struct file_system_type *fs_type,
+	int flags, void *data,
+	int (*fill_super)(struct super_block *, void *, int),
+	struct vfsmount *mnt)
+{
+	struct super_block *s;
+	int error;
+
+	s = sget(fs_type, compare_sb_single_ns, set_sb_single_ns, data);
+	if (IS_ERR(s))
+		return PTR_ERR(s);
+	if (!s->s_root) {
+		s->s_flags = flags;
+		error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
+		if (error) {
+			up_write(&s->s_umount);
+			deactivate_super(s);
+			return error;
+		}
+		s->s_flags |= MS_ACTIVE;
+	}
+	do_remount_sb(s, flags, data, 0);
+	return simple_set_mnt(mnt, s);
+}
+
 static int mqueue_get_sb(struct file_system_type *fs_type,
 			 int flags, const char *dev_name,
 			 void *data, struct vfsmount *mnt)
@@ -208,6 +250,14 @@ static int mqueue_get_sb(struct file_sys
 	return get_sb_single(fs_type, flags, data, mqueue_fill_super, mnt);
 }

+static void mqueue_kill_sb(struct super_block *sb)
+{
+	struct mq_namespace *mq_ns = (struct mq_namespace *) sb->s_fs_info;
+
+	kill_litter_super(sb);
+	put_mq_ns(mq_ns);
+}
+
 static void init_once(void *foo)
 {
 	struct mqueue_inode_info *p = (struct mqueue_inode_info *) foo;
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