If one of the copy of the superblock is zero it does not
confirm to us that btrfs isn't there on that disk. When
we are having more than one copy of superblock we should
rather let the for loop to continue to check other copies.
the following test case and results would justify the
fix
mkfs.btrfs /dev/sdb /dev/sdc -f
mount /dev/sdb /btrfs
dd if=/dev/zero bs=1 count=8 of=/dev/sdc seek=$((64*1024+64))
~/before/btrfs-select-super -s 1 /dev/sdc
using SB copy 1, bytenr 67108864
here btrfs-select-super just wrote superblock to a mounted btrfs
Why does not check_mounted() catch this in the first place? Ie. based on
the status in /proc/mounts not on random bytes in the superblock.
the reason is, as of now /proc/mounts just knows about the devid 1.
My oversight, it's mkfs on sdb and select-super on sdc, but then sdc is
already open and the open(O_EXCL) should prevent that, right? The same
way mkfs checks whether all the devices are available.
thanks for the comments.
checking for O_EXCL would help in a way to avoid this problem.
but it doesn't address the actual problem.
here, IMO this is wrong..
------
int btrfs_read_dev_super(int fd, struct btrfs_super_block *sb, u64
sb_bytenr, u64 flags)
{
::
/* if magic is NULL, the device was removed */ <----
if (buf.magic == 0 && i == 0)
return -1;
-----
since it would inhibits check for the backup superblock
when the primary superblock is wrongly overwritten with
zeros.
eg:
in general threads which set BTRFS_SCAN_BACKUP_SB flag
are affected.
such as btrfs-find-root should fail to work and it
does as in the below eg: with single disk.
--------
mkfs.btrfs /dev/dm-5 -f
dd if=/dev/zero bs=1 count=8 of=/dev/dm-5 seek=$((64*1024+64))
~/before/btrfs-find-root /dev/dm-5
No valid Btrfs found on /dev/dm-5
Open ctree failed
with the fix:
btrfs-find-root /dev/dm-5
Super think's the tree root is at 29364224, chunk root 20971520
Well block 4194304 seems great, but generation doesn't match, have=2, want=4
Well block 4206592 seems great, but generation doesn't match, have=3, want=4
Found tree root at 29364224
----------
Thanks, Anand
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