vfs: Allow selection of fs root independent of sb

Commit Message

Andrew Price March 21, 2019, 5:26 p.m. UTC

Take a function pointer 'select_root' in vfs_get_block_super() to allow
callers to select the root dentry based on the context.

This is intended for gfs2 to select between the normal root and the
'meta' root, which must be done whether a root already exists for a
superblock or not, i.e. it cannot be decided in fill_super().

This will allow gfs2 to avoid duplicating a function from the vfs just
to special-case the root dentry selection.

Signed-off-by: Andrew Price <[e-mail]>
---

Depends on "vfs: Create fs_context-aware mount_bdev() replacement"

 fs/super.c                 | 11 ++++++++---
 include/linux/fs_context.h |  4 ++--
 2 files changed, 10 insertions(+), 5 deletions(-)

Comments

David Howells March 21, 2019, 5:59 p.m. UTC | #1

That's not what I meant.  There's a kerneldoc comment on vfs_get_block_super()
that you need to modify.  I would in any case roll your patch into mine.

> This is intended for gfs2 to select between the normal root and the
> 'meta' root, which must be done whether a root already exists for a
> superblock or not, i.e. it cannot be decided in fill_super().

Can this be done in gfs2_get_tree, where it sets fc->root?

David

Matthew Wilcox March 21, 2019, 6:36 p.m. UTC | #2

On Thu, Mar 21, 2019 at 05:26:44PM +0000, Andrew Price wrote:
> Take a function pointer 'select_root' in vfs_get_block_super() to allow
> callers to select the root dentry based on the context.

Can't this be a fs_context_operations pointer?

Patch

diff --git a/fs/super.c b/fs/super.c
index 6d8dbf309241..0cdeaf28126f 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1232,11 +1232,12 @@  static int test_bdev_super_fc(struct super_block *s, struct fs_context *fc)
  * @fill_super: Helper to initialise a new superblock
  */
 int vfs_get_block_super(struct fs_context *fc,
-			int (*fill_super)(struct super_block *,
-					  struct fs_context *))
+      int (*fill_super)(struct super_block *, struct fs_context *),
+      struct dentry* (*select_root)(struct fs_context *, struct super_block *))
 {
 	struct block_device *bdev;
 	struct super_block *s;
+	struct dentry *root;
 	int error = 0;
 
 	fc->bdev_mode = FMODE_READ | FMODE_EXCL;
@@ -1302,7 +1303,11 @@  int vfs_get_block_super(struct fs_context *fc,
 	}
 
 	BUG_ON(fc->root);
-	fc->root = dget(s->s_root);
+	if (select_root)
+		root = select_root(fc, s);
+	else
+		root = s->s_root;
+	fc->root = dget(root);
 	return 0;
 
 error_sb:
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index 8233c873af73..c2e9dc5826ce 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -145,8 +145,8 @@  extern int vfs_get_super(struct fs_context *fc,
 					   struct fs_context *fc));
 
 extern int vfs_get_block_super(struct fs_context *fc,
-			       int (*fill_super)(struct super_block *sb,
-						 struct fs_context *fc));
+      int (*fill_super)(struct super_block *, struct fs_context *),
+      struct dentry* (*select_root)(struct fs_context *, struct super_block *));
 
 extern const struct file_operations fscontext_fops;