btrfs: fix unmountable seed device after fstrim

The following test case reproduces an issue of wrongly freeing in-use
blocks on the readonly seed device when fstrim is called on the rw sprout
device. As shown below.

Create a seed device and add a sprout device to it:

  $ mkfs.btrfs -fq -dsingle -msingle /dev/loop0
  $ btrfstune -S 1 /dev/loop0
  $ mount /dev/loop0 /btrfs
  $ btrfs dev add -f /dev/loop1 /btrfs
  BTRFS info (device loop0): relocating block group 290455552 flags system
  BTRFS info (device loop0): relocating block group 1048576 flags system
  BTRFS info (device loop0): disk added /dev/loop1
  $ umount /btrfs

Mount the sprout device and run fstrim:

  $ mount /dev/loop1 /btrfs
  $ fstrim /btrfs
  $ umount /btrfs

Now try to mount the seed device, and it fails:

  $ mount /dev/loop0 /btrfs
  mount: /btrfs: wrong fs type, bad option, bad superblock on /dev/loop0, missing codepage or helper program, or other error.

Block 5292032 is missing on the readonly seed device:

 $ dmesg -kt | tail
 <snip>
 BTRFS error (device loop0): bad tree block start, want 5292032 have 0
 BTRFS warning (device loop0): couldn't read-tree root
 BTRFS error (device loop0): open_ctree failed

From the dump-tree of the seed device (taken before the fstrim). Block
5292032 belonged to the block group starting at 5242880:

  $ btrfs inspect dump-tree -e /dev/loop0 | grep -A1 BLOCK_GROUP
  <snip>
  item 3 key (5242880 BLOCK_GROUP_ITEM 8388608) itemoff 16169 itemsize 24
  	block group used 114688 chunk_objectid 256 flags METADATA
  <snip>

From the dump-tree of the sprout device (taken before the fstrim).
fstrim used block-group 5242880 to find the related free space to free:

  $ btrfs inspect dump-tree -e /dev/loop1 | grep -A1 BLOCK_GROUP
  <snip>
  item 1 key (5242880 BLOCK_GROUP_ITEM 8388608) itemoff 16226 itemsize 24
  	block group used 32768 chunk_objectid 256 flags METADATA
  <snip>

BPF kernel tracing the fstrim command finds the missing block 5292032
within the range of the discarded blocks as below:

  kprobe:btrfs_discard_extent {
  	printf("freeing start %llu end %llu num_bytes %llu:\n",
  		arg1, arg1+arg2, arg2);
  }

  freeing start 5259264 end 5406720 num_bytes 147456
  <snip>

Fix this by avoiding the discard command to the readonly seed device.
Reported-by: NChris Murphy <lists@colorremedies.com>
CC: stable@vger.kernel.org # 4.4+
Reviewed-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -1340,12 +1340,16 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
 		stripe = bbio->stripes;
 		for (i = 0; i < bbio->num_stripes; i++, stripe++) {
 			u64 bytes;
+			struct btrfs_device *device = stripe->dev;
 
-			if (!stripe->dev->bdev) {
+			if (!device->bdev) {
 				ASSERT(btrfs_test_opt(fs_info, DEGRADED));
 				continue;
 			}
 
+			if (!test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state))
+				continue;
+
 			ret = do_discard_extent(stripe, &bytes);
 			if (!ret) {
 				discarded_bytes += bytes;