[PATCH 1/1] btrfs: introduce feature to forget a btrfs device

[Anand Jain <anand.jain@oracle.com>]

Support for a new command 'btrfs dev forget [dev]' is proposed here,
to undo the effects of 'btrfs dev scan [dev]'. For this purpose,
this patch proposes to use ioctl #5 as it was empty.
	IOW(BTRFS_IOCTL_MAGIC, 5, ..)
This patch adds new ioctl BTRFS_IOC_FORGET_DEV which can be sent from
the /dev/btrfs-control to forget one or all devices, (devices which are
not mounted) from the btrfs kernel.

The argument it takes is struct btrfs_ioctl_vol_args_v2, and ::name can be
set to specify the device path. And all unmounted devices can be removed
from the kernel using the BTRFS_DEVICE_SPEC_ALL_DEV flag. Remove all
devices functionality would override remove one device when both are
specified in an IOCTL call.
Again, the devices are removed only if the relevant fsid aren't mounted.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/super.c           | 27 +++++++++++++++++++++++----
 fs/btrfs/volumes.c         |  9 +++++++++
 fs/btrfs/volumes.h         |  1 +
 include/uapi/linux/btrfs.h |  6 +++++-
 4 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 559fc53ff59e..6a9a5ce8af3b 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2219,21 +2219,37 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
 				unsigned long arg)
 {
 	struct btrfs_ioctl_vol_args *vol;
+	struct btrfs_ioctl_vol_args_v2 *vol2;
 	struct btrfs_fs_devices *fs_devices;
 	int ret = -ENOTTY;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	vol = memdup_user((void __user *)arg, sizeof(*vol));
-	if (IS_ERR(vol))
-		return PTR_ERR(vol);
+	if (cmd == BTRFS_IOC_FORGET_DEV) {
+		vol2 = memdup_user((void __user *)arg, sizeof(*vol2));
+		if (IS_ERR(vol2))
+			return PTR_ERR(vol2);
+
+		if (vol2->flags & ~BTRFS_VOL_ARG_V2_FLAGS_SUPPORTED)
+			return -EOPNOTSUPP;
+	} else {
+		vol = memdup_user((void __user *)arg, sizeof(*vol));
+		if (IS_ERR(vol))
+			return PTR_ERR(vol);
+	}
 
 	switch (cmd) {
 	case BTRFS_IOC_SCAN_DEV:
 		ret = btrfs_scan_one_device(vol->name, FMODE_READ,
 					    &btrfs_fs_type, &fs_devices);
 		break;
+	case BTRFS_IOC_FORGET_DEV:
+		if (vol2->flags & BTRFS_DEVICE_SPEC_ALL_DEV)
+			ret = btrfs_forget_devices(NULL);
+		else
+			ret = btrfs_forget_devices(vol2->name);
+		break;
 	case BTRFS_IOC_DEVICES_READY:
 		ret = btrfs_scan_one_device(vol->name, FMODE_READ,
 					    &btrfs_fs_type, &fs_devices);
@@ -2246,7 +2262,10 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
 		break;
 	}
 
-	kfree(vol);
+	if (cmd == BTRFS_IOC_FORGET_DEV)
+		kfree(vol2);
+	else
+		kfree(vol);
 	return ret;
 }
 
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index e947e47f8fff..b0c9948baf9a 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1171,6 +1171,15 @@ static int btrfs_read_disk_super(struct block_device *bdev, u64 bytenr,
 	return 0;
 }
 
+int btrfs_forget_devices(const char *path)
+{
+	mutex_lock(&uuid_mutex);
+	btrfs_free_stale_devices(path, NULL);
+	mutex_unlock(&uuid_mutex);
+
+	return 0;
+}
+
 /*
  * Look for a btrfs signature on a device. This may be called out of the mount path
  * and we are not allowed to call set_blocksize during the scan. The superblock
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 15216fed918b..b954ca3b79a9 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -422,6 +422,7 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices,
 		       fmode_t flags, void *holder);
 int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
 			  struct btrfs_fs_devices **fs_devices_ret);
+int btrfs_forget_devices(const char *path);
 int btrfs_close_devices(struct btrfs_fs_devices *fs_devices);
 void btrfs_close_extra_devices(struct btrfs_fs_devices *fs_devices, int step);
 void btrfs_assign_next_active_device(struct btrfs_fs_info *fs_info,
diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index c8d99b9ca550..4e8ec1391872 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -41,12 +41,14 @@ struct btrfs_ioctl_vol_args {
 #define BTRFS_SUBVOL_QGROUP_INHERIT	(1ULL << 2)
 
 #define BTRFS_DEVICE_SPEC_BY_ID		(1ULL << 3)
+#define BTRFS_DEVICE_SPEC_ALL_DEV	(1ULL << 4)
 
 #define BTRFS_VOL_ARG_V2_FLAGS_SUPPORTED		\
 			(BTRFS_SUBVOL_CREATE_ASYNC |	\
 			BTRFS_SUBVOL_RDONLY |		\
 			BTRFS_SUBVOL_QGROUP_INHERIT |	\
-			BTRFS_DEVICE_SPEC_BY_ID)
+			BTRFS_DEVICE_SPEC_BY_ID |	\
+			BTRFS_DEVICE_SPEC_ALL_DEV)
 
 #define BTRFS_FSID_SIZE 16
 #define BTRFS_UUID_SIZE 16
@@ -745,6 +747,8 @@ enum btrfs_err_code {
 				   struct btrfs_ioctl_vol_args)
 #define BTRFS_IOC_SCAN_DEV _IOW(BTRFS_IOCTL_MAGIC, 4, \
 				   struct btrfs_ioctl_vol_args)
+#define BTRFS_IOC_FORGET_DEV _IOW(BTRFS_IOCTL_MAGIC, 5, \
+				   struct btrfs_ioctl_vol_args)
 /* trans start and trans end are dangerous, and only for
  * use by applications that know how to avoid the
  * resulting deadlocks
-- 
2.7.0