On Wednesday, June 15, 2016 09:12:28 AM Chandan Rajendra wrote:
> Hello Liu Bo,
>
> We have to fix the following check in check_super() as well,
>
> if (btrfs_super_stripesize(sb) != 4096) {
> error("invalid stripesize %u", btrfs_super_stripesize(sb));
> goto error_out;
> }
>
> i.e. btrfs_super_stripesize(sb) must be equal to
> btrfs_super_sectorsize(sb).
>
> However in btrfs-progs (mkfs.c to be precise) since we had stripesize
> hardcoded to 4096, setting stripesize to the value of sectorsize in
> mkfs.c will cause the following to occur when mkfs.btrfs is invoked for
> devices with existing Btrfs filesystem instances,
>
> NOTE: Assume we have changed the stripesize validation in btrfs-progs'
> check_super() to,
>
> if (btrfs_super_stripesize(sb) != btrfs_super_sectorsize(sb)) {
> error("invalid stripesize %u", btrfs_super_stripesize(sb));
> goto error_out;
> }
>
>
> main()
> for each device file passed as an argument,
> test_dev_for_mkfs()
> check_mounted
> check_mounted_where
> btrfs_scan_one_device
> btrfs_read_dev_super
> check_super() call will fail for existing filesystems which
> have stripesize set to 4k. All existing filesystem instances will fall into
> this category.
>
> This error value is pushed up the call stack and this causes the device to
> not get added to the fs_devices_mnt list in check_mounted_where(). Hence we
> would fail to correctly check the mount status of the multi-device btrfs
> filesystems.
>
We can end up in the following scenario,
- /dev/loop0, /dev/loop1 and /dev/loop2 are mounted as a single
filesystem. The filesystem was created by an older version of mkfs.btrfs
which set stripesize to 4k.
- losetup -a
/dev/loop0: [0030]:19477 (/root/disk-imgs/file-0.img)
/dev/loop1: [0030]:16577 (/root/disk-imgs/file-1.img)
/dev/loop2: [64770]:3423229 (/root/disk-imgs/file-2.img)
- /etc/mtab lists only /dev/loop0
- "losetup /dev/loop4 /root/disk-imgs/file-1.img"
The new mkfs.btrfs invoked as 'mkfs.btrfs -f /dev/loop4' succeeds even
though /dev/loop1 has already been mounted and has
/root/disk-imgs/file-1.img as its backing file.
So IMHO the only solution is to have the stripesize check in check_super() to
allow both '4k' and 'sectorsize' as valid values i.e.
if ((btrfs_super_stripesize(sb) != 4096)
&& (btrfs_super_stripesize(sb) != btrfs_super_sectorsize(sb))) {
error("invalid stripesize %u", btrfs_super_stripesize(sb));
goto error_out;
}
--
chandan
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