On Sat, 2010-11-27 at 21:19 -0600, C Anthony Risinger wrote:
> i have read just recently and in the past that btrfs supports COW for
> _any_ file/directory (this is reflinks, yes?), and today i
> accidentally noticed that i can mount a directory as well (if it's in
> the top level volume at least).
>
> eg. if i have a "regular" directory (not a subvolume) in the top-level:
>
> /__boot
>
> i can mount it with:
>
> mount -o subvol=__boot /dev/sda /mnt
The 'subvol' option actually works using the same mechanism as a bind
mount. The fact that it works using a directory is purely a coincidence
- I would not expect it to be officially supported, and you shouldn't
rely on it.
> i am working on an update to my initramfs hook that will utilize
> extlinux, and this property to provide seamless kernel level system
> rollbacks, and i want to make sure it's safe to do this.
To handle system rollbacks, you really should be using subvolumes and
snapshots, not regular directories.
> also, is there a way to target an arbitrary directory? does "subvol"
> support paths yet, or maybe via "subvolid" somehow? essentially i
I don't think that it would be very hard to make subvol= support a path
instead of only one level deep. Actually, I think I could make a patch
for that myself... I've included it here. Mildly tested, but I'm not
really a kernel programmer and might have missed something -
particularly with regards to the locking.
> just want to mount a directory inside a snapshot at /boot, so when
> users upgrade there kernels, the images are visible to extlinux (which
> cannot yet peek inside or use subvolumes, so it has to be a regular
> directory in the top-level volume)
Ah, this is the first I've heard that extlinux doesn't support reading
files in subvolumes. That's an unfortunate limitation :/
------------------------8<----------8<---------------------
From: Calvin Walton <calvin.walton@xxxxxxxxx>
Date: Sat, 27 Nov 2010 23:17:55 -0500
Subject: [PATCH] Btrfs: Allow mounting sub-sub(-sub...)-volumes using subvol=a/b/c
Currently you can only mount subvolumes which are direct children of the
'default' root. I.e. with
ID 258 top level 5 path a
ID 259 top level 5 path a/b
you could mount with "-o subvol=a" but not "-o subvol=a/b"
This patch fixes that by recursively following the path to the subvolume.
Signed-off-by: Calvin Walton <calvin.walton@xxxxxxxxx>
---
fs/btrfs/super.c | 41 +++++++++++++++++++++++------------------
1 files changed, 23 insertions(+), 18 deletions(-)
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 8299a25..5e78c86 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -646,26 +646,31 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
/* if they gave us a subvolume name bind mount into that */
if (strcmp(subvol_name, ".")) {
struct dentry *new_root;
- mutex_lock(&root->d_inode->i_mutex);
- new_root = lookup_one_len(subvol_name, root,
- strlen(subvol_name));
- mutex_unlock(&root->d_inode->i_mutex);
-
- if (IS_ERR(new_root)) {
- deactivate_locked_super(s);
- error = PTR_ERR(new_root);
- dput(root);
- goto error_free_subvol_name;
- }
- if (!new_root->d_inode) {
+ char *subvol_name_next = subvol_name;
+ char *subvol_name_part;
+
+ while ((subvol_name_part = strsep(&subvol_name_next, "/"))) {
+ mutex_lock(&root->d_inode->i_mutex);
+ new_root = lookup_one_len(subvol_name, root,
+ strlen(subvol_name));
+ mutex_unlock(&root->d_inode->i_mutex);
+
+ if (IS_ERR(new_root)) {
+ deactivate_locked_super(s);
+ error = PTR_ERR(new_root);
+ dput(root);
+ goto error_free_subvol_name;
+ }
+ if (!new_root->d_inode) {
+ dput(root);
+ dput(new_root);
+ deactivate_locked_super(s);
+ error = -ENXIO;
+ goto error_free_subvol_name;
+ }
dput(root);
- dput(new_root);
- deactivate_locked_super(s);
- error = -ENXIO;
- goto error_free_subvol_name;
+ root = new_root;
}
- dput(root);
- root = new_root;
}
kfree(subvol_name);
--
1.7.3.2
--
Calvin Walton <calvin.walton@xxxxxxxxx>
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