[02/16] btrfs: fix replace of seed device

Commit Message

Anand Jain Sept. 4, 2020, 5:34 p.m. UTC

If you replace a seed device in a sprouted fs, it appears to have
successfully replaced the seed device, but if you look closely, it didn't.

Here is an example.

mkfs.btrfs -fq /dev/sda
btrfstune -S1 /dev/sda
mount /dev/sda /btrfs
btrfs dev add /dev/sdb /btrfs
umount /btrfs; btrfs dev scan --forget
mount -o device=/dev/sda /dev/sdb /btrfs
btrfs rep start -f /dev/sda /dev/sdc /btrfs; echo $?
0

  BTRFS info (device sdb): dev_replace from /dev/sda (devid 1) to /dev/sdc started
  BTRFS info (device sdb): dev_replace from /dev/sda (devid 1) to /dev/sdc finished

btrfs fi show
Label: none  uuid: ab2c88b7-be81-4a7e-9849-c3666e7f9f4f
	Total devices 2 FS bytes used 256.00KiB
	devid    1 size 3.00GiB used 520.00MiB path /dev/sdc
	devid    2 size 3.00GiB used 896.00MiB path /dev/sdb

Label: none  uuid: 10bd3202-0415-43af-96a8-d5409f310a7e
	Total devices 1 FS bytes used 128.00KiB
	devid    1 size 3.00GiB used 536.00MiB path /dev/sda

So as per the replace start command and kernel log replace was successful.

Now let's try to clean mount.

umount /btrfs;  btrfs dev scan --forget

mount -o device=/dev/sdc /dev/sdb /btrfs
mount: /btrfs: wrong fs type, bad option, bad superblock on /dev/sdb, missing codepage or helper program, or other error.

[  636.157517] BTRFS error (device sdc): failed to read chunk tree: -2
[  636.180177] BTRFS error (device sdc): open_ctree failed

That's because per dev items it is still looking for the original seed
device.

btrfs in dump-tree -d /dev/sdb

	item 0 key (DEV_ITEMS DEV_ITEM 1) itemoff 16185 itemsize 98
		devid 1 total_bytes 3221225472 bytes_used 545259520
		io_align 4096 io_width 4096 sector_size 4096 type 0
		generation 6 start_offset 0 dev_group 0
		seek_speed 0 bandwidth 0
		uuid 59368f50-9af2-4b17-91da-8a783cc418d4  <--- seed uuid
		fsid 10bd3202-0415-43af-96a8-d5409f310a7e  <--- seed fsid
	item 1 key (DEV_ITEMS DEV_ITEM 2) itemoff 16087 itemsize 98
		devid 2 total_bytes 3221225472 bytes_used 939524096
		io_align 4096 io_width 4096 sector_size 4096 type 0
		generation 0 start_offset 0 dev_group 0
		seek_speed 0 bandwidth 0
		uuid 56a0a6bc-4630-4998-8daf-3c3030c4256a  <- sprout uuid
		fsid ab2c88b7-be81-4a7e-9849-c3666e7f9f4f <- sprout fsid

But the replaced target has the following uuid+fsid in its superblock
which doesn't match with the expected uuid+fsid in its devitem.

btrfs in dump-super /dev/sdc | egrep '^generation|dev_item.uuid|dev_item.fsid|devid'
generation	20
dev_item.uuid	59368f50-9af2-4b17-91da-8a783cc418d4
dev_item.fsid	ab2c88b7-be81-4a7e-9849-c3666e7f9f4f [match]
dev_item.devid	1

So if you provide the original seed device the mount shall be successful.
Which so long happening in the test case btrfs/163.

btrfs dev scan --forget
mount -o device=/dev/sda /dev/sdb /btrfs

Fix in this patch:
If a seed is not sprouted then there is no replacement of it, because of
its RO filesystem with an RO device. Similarly, in the case of a sprouted
filesystem, the seed device is still RO. So, mark it as you can't replace
a seed device, you can only add a new device and then delete the seed
device. If replace is attempted then returns -EINVAL.

Signed-off-by: Anand Jain <[e-mail]>
---
 fs/btrfs/dev-replace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index 580e60fe07d0..7637b1f02c63 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -230,7 +230,7 @@  static int btrfs_init_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	*device_out = NULL;
-	if (fs_info->fs_devices->seeding) {
+	if (srcdev->fs_devices->seeding) {
 		btrfs_err(fs_info, "the filesystem is a seed filesystem!");
 		return -EINVAL;
 	}