From: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
The problem is with parallel mounting multiple subvolumes rw when the
root filesystem is marked as read-only such as a boot sequence
using systemd. Not all subvolumes will be mounted because some of
them will return -EBUSY.
Here is a sample execution time.
Root filesystem is mounted read-only so s->s_flags are set to MS_RDONLY
flags is the parameter passed and s_flags is the one recorded in sb.
btrfs_mount is called via vfs_kern_mount().
Mount Thread 1 Mount Thread 2
btrfs_mount(flags & ~MS_RDONLY)
check (flags ^ s_flags) & MS_RDONLY
returns -EBUSY btrfs_mount(flags & ~MS_RDONLY)
check (flags ^ s_flags) & MS_RDONLY
returns -EBUSY
btrfs_mount(flags | MS_RDONLY)
btrfs_remount(flags & ~MS_RDONLY)
s->s_flags &= ~MS_RDONLY
btrfs_mount(flags | MS_RDONLY)
check (flags ^ s_flags) & MS_RDONLY)
returns -EBUSY
mount FAILS
The -EBUSY was originally introduced in:
4b82d6e ("Btrfs: Add mount into directory support")
as a copy of (then) get_sb_dev().
Later commit 0723a04 ("btrfs: allow mounting btrfs subvolumes
with different ro/rw options") added the option of allowing
subvolumes in rw/ro modes.
This fix is instead of toggling the flags in remount, we set
s_flags &= MS_RDONLY when we see a conflict in s_flags and passed parameter
flags and let mount continue as it is. This will allow the first mount attempt
to succeed, and we can get rid of the re-kern_mount() and remount sequence
altogether.
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>
---
fs/btrfs/super.c | 36 ++++++++++--------------------------
1 file changed, 10 insertions(+), 26 deletions(-)
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index e9ae93e..978b4a6 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -67,8 +67,6 @@
static const struct super_operations btrfs_super_ops;
static struct file_system_type btrfs_fs_type;
-static int btrfs_remount(struct super_block *sb, int *flags, char *data);
-
const char *btrfs_decode_error(int errno)
{
char *errstr = "unknown";
@@ -1379,28 +1377,6 @@ static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
}
mnt = vfs_kern_mount(&btrfs_fs_type, flags, device_name, newargs);
- if (PTR_ERR_OR_ZERO(mnt) == -EBUSY) {
- if (flags & MS_RDONLY) {
- mnt = vfs_kern_mount(&btrfs_fs_type, flags & ~MS_RDONLY,
- device_name, newargs);
- } else {
- mnt = vfs_kern_mount(&btrfs_fs_type, flags | MS_RDONLY,
- device_name, newargs);
- if (IS_ERR(mnt)) {
- root = ERR_CAST(mnt);
- mnt = NULL;
- goto out;
- }
-
- down_write(&mnt->mnt_sb->s_umount);
- ret = btrfs_remount(mnt->mnt_sb, &flags, NULL);
- up_write(&mnt->mnt_sb->s_umount);
- if (ret < 0) {
- root = ERR_PTR(ret);
- goto out;
- }
- }
- }
if (IS_ERR(mnt)) {
root = ERR_CAST(mnt);
mnt = NULL;
@@ -1606,8 +1582,16 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
if (s->s_root) {
btrfs_close_devices(fs_devices);
free_fs_info(fs_info);
- if ((flags ^ s->s_flags) & MS_RDONLY)
- error = -EBUSY;
+ /* If s_flags is MS_RDONLY and flags is rw (~MS_RDONLY)
+ * we need to reset s_flags so that sb can be writable
+ * since we can be called from mount_subvol().
+ * The vfsmount manages to preserve the ro/rw flags
+ * of the root/orignal mount.
+ * In case of vice-versa, s_flags is already does not
+ * have MS_RDONLY set, so don't bother.
+ */
+ if ((s->s_flags & MS_RDONLY) && !(flags & MS_RDONLY))
+ s->s_flags &= ~MS_RDONLY;
} else {
snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
btrfs_sb(s)->bdev_holder = fs_type;
--
2.10.2
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