Commit a5b8a5f9 authored by Anand Jain's avatar Anand Jain Committed by David Sterba
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btrfs: support cloned-device mount capability 

Guilherme's previous work [1] aimed at the mounting of cloned devices
using a superblock flag SINGLE_DEV during mkfs.
 [1] https://lore.kernel.org/linux-btrfs/20230831001544.3379273-1-gpiccoli@igalia.com/



Building upon this work, here is in memory only approach. As it mounts
we determine if the same fsid is already mounted if then we generate a
random temp fsid which shall be used the mount, in memory only not
written to the disk. We distinguish devices by devt.

Example:
  $ fallocate -l 300m ./disk1.img
  $ mkfs.btrfs -f ./disk1.img
  $ cp ./disk1.img ./disk2.img
  $ cp ./disk1.img ./disk3.img
  $ mount -o loop ./disk1.img /btrfs
  $ mount -o ./disk2.img /btrfs1
  $ mount -o ./disk3.img /btrfs2

  $ btrfs fi show -m
  Label: none  uuid: 4a212b48-1bec-46a5-938a-783c8c1f0b02
	Total devices 1 FS bytes used 144.00KiB
	devid    1 size 300.00MiB used 88.00MiB path /dev/loop0

  Label: none  uuid: adabf2fe-5515-4ad0-95b4-7b1609218c16
	Total devices 1 FS bytes used 144.00KiB
	devid    1 size 300.00MiB used 88.00MiB path /dev/loop1

  Label: none  uuid: 1d77d0df-7d92-439e-adbd-20b9b86fdedb
	Total devices 1 FS bytes used 144.00KiB
	devid    1 size 300.00MiB used 88.00MiB path /dev/loop2

Co-developed-by: default avatarGuilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: default avatarAnand Jain <anand.jain@oracle.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 69d427f3

fs/btrfs/disk-io.c

+2 −1
Original line number Diff line number Diff line
@@ -2399,7 +2399,8 @@ int btrfs_validate_super(struct btrfs_fs_info *fs_info, 
		ret = -EINVAL;

	}



	if (memcmp(fs_info->fs_devices->fsid, sb->fsid, BTRFS_FSID_SIZE) != 0) {

	if (!fs_info->fs_devices->temp_fsid &&

	    memcmp(fs_info->fs_devices->fsid, sb->fsid, BTRFS_FSID_SIZE) != 0) {

		btrfs_err(fs_info,

		"superblock fsid doesn't match fsid of fs_devices: %pU != %pU",

			  sb->fsid, fs_info->fs_devices->fsid);

fs/btrfs/volumes.c

+57 −3
Original line number Diff line number Diff line
@@ -554,17 +554,63 @@ static int btrfs_free_stale_devices(dev_t devt, struct btrfs_device *skip_device 
}



static struct btrfs_fs_devices *find_fsid_by_device(

					struct btrfs_super_block *disk_super)

					struct btrfs_super_block *disk_super,

					dev_t devt, bool *same_fsid_diff_dev)

{

	struct btrfs_fs_devices *fsid_fs_devices;

	struct btrfs_fs_devices *devt_fs_devices;

	const bool has_metadata_uuid = (btrfs_super_incompat_flags(disk_super) &

					BTRFS_FEATURE_INCOMPAT_METADATA_UUID);

	bool found_by_devt = false;



	/* Find the fs_device by the usual method, if found use it. */

	fsid_fs_devices = find_fsid(disk_super->fsid,

		    has_metadata_uuid ? disk_super->metadata_uuid : NULL);



	/* The temp_fsid feature is supported only with single device filesystem. */

	if (btrfs_super_num_devices(disk_super) != 1)

		return fsid_fs_devices;



	/* Try to find a fs_devices by matching devt. */

	list_for_each_entry(devt_fs_devices, &fs_uuids, fs_list) {

		struct btrfs_device *device;



		list_for_each_entry(device, &devt_fs_devices->devices, dev_list) {

			if (device->devt == devt) {

				found_by_devt = true;

				break;

			}

		}

		if (found_by_devt)

			break;

	}



	if (found_by_devt) {

		/* Existing device. */

		if (fsid_fs_devices == NULL) {

			if (devt_fs_devices->opened == 0) {

				/* Stale device. */

				return NULL;

			} else {

				/* temp_fsid is mounting a subvol. */

				return devt_fs_devices;

			}

		} else {

			/* Regular or temp_fsid device mounting a subvol. */

			return devt_fs_devices;

		}

	} else {

		/* New device. */

		if (fsid_fs_devices == NULL) {

			return NULL;

		} else {

			/* sb::fsid is already used create a new temp_fsid. */

			*same_fsid_diff_dev = true;

			return NULL;

		}

	}



	/* Not reached. */

}



/*
@@ -670,6 +716,7 @@ static noinline struct btrfs_device *device_list_add(const char *path, 
	u64 devid = btrfs_stack_device_id(&disk_super->dev_item);

	dev_t path_devt;

	int error;

	bool same_fsid_diff_dev = false;

	bool has_metadata_uuid = (btrfs_super_incompat_flags(disk_super) &

		BTRFS_FEATURE_INCOMPAT_METADATA_UUID);
@@ -687,7 +734,7 @@ static noinline struct btrfs_device *device_list_add(const char *path, 
		return ERR_PTR(error);

	}



	fs_devices = find_fsid_by_device(disk_super);

	fs_devices = find_fsid_by_device(disk_super, path_devt, &same_fsid_diff_dev);



	if (!fs_devices) {

		fs_devices = alloc_fs_devices(disk_super->fsid);
@@ -698,6 +745,13 @@ static noinline struct btrfs_device *device_list_add(const char *path, 
		if (IS_ERR(fs_devices))

			return ERR_CAST(fs_devices);



		if (same_fsid_diff_dev) {

			generate_random_uuid(fs_devices->fsid);

			fs_devices->temp_fsid = true;

			pr_info("BTRFS: device %s using temp-fsid %pU\n",

				path, fs_devices->fsid);

		}



		mutex_lock(&fs_devices->device_list_mutex);

		list_add(&fs_devices->fs_list, &fs_uuids);

fs/btrfs/volumes.h

+2 −0
Original line number Diff line number Diff line
@@ -364,6 +364,8 @@ struct btrfs_fs_devices { 
	bool discardable;

	/* The filesystem is a seed filesystem. */

	bool seeding;

	/* The mount needs to use a randomly generated fsid. */

	bool temp_fsid;



	struct btrfs_fs_info *fs_info;

	/* sysfs kobjects */