[PATCH] btrfs: fix unmountable seed device after fstrim

From: Anand Jain <anand.jain@oracle.com>
To: linux-btrfs@vger.kernel.org
Cc: Anand Jain <anand.jain@oracle.com>,
	fdmanana@suse.com, Chris Murphy <lists@colorremedies.com>
Subject: [PATCH] btrfs: fix unmountable seed device after fstrim
Date: Fri, 30 Apr 2021 19:59:51 +0800	[thread overview]
Message-ID: <d6fcae756c5ce47da3527e5db4760d676420d950.1619783910.git.anand.jain@oracle.com> (raw)
In-Reply-To: <c7715d09a67f212e0ecb5fea2d598513912092f4.1619443900.git.anand.jain@oracle.com>

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(8) 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(8) 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.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reported-by: Chris Murphy <lists@colorremedies.com>
---
 fs/btrfs/extent-tree.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 7a28314189b4..f1d15b68994a 100644
--- a/fs/btrfs/extent-tree.c
+++ b/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;
-- 
2.29.2

