On 26.03.2018 11:27, Anand Jain wrote:
> During the btrfs dev scan make sure that other copies of superblock
> contain the same fsid as the primary SB. So that we bring to the
> user notice if the superblock has been overwritten.
>
> mkfs.btrfs -fq /dev/sdc
> mkfs.btrfs -fq /dev/sdb
> dd if=/dev/sdb of=/dev/sdc count=4K skip=64K seek=64K obs=1 ibs=1
> mount /dev/sdc /btrfs
>
> Caveat: Pls note that older btrfs-progs do not wipe the non-overwriting
> stale superblock like copy2 if a smaller mkfs.btrfs -b <size> is created.
> So this patch in the kernel will report error. The workaround is to wipe
> the superblock manually, like
> dd if=/dev/zero of=<dev> seek=274877906944 ibs=1 obs=1 count4K
> OR apply btrfs-progs patch
> btrfs-progs: wipe copies of the stale superblock beyond -b size
> which shall find and wipe the non overwriting superblock.
>
> Signed-off-by: Anand Jain <anand.j...@oracle.com>
> ---
> fs/btrfs/volumes.c | 60
> ++++++++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 47 insertions(+), 13 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index ed22f0a3d239..45dd0674571b 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1198,40 +1198,74 @@ static int btrfs_read_disk_super(struct block_device
> *bdev, u64 bytenr,
> int btrfs_scan_one_device(const char *path, fmode_t flags, void *holder,
> struct btrfs_fs_devices **fs_devices_ret)
> {
> + struct btrfs_super_block *disk_super_primary;
> struct btrfs_super_block *disk_super;
> struct btrfs_device *device;
> struct block_device *bdev;
> struct page *page;
> int ret = 0;
> - u64 bytenr;
> + int i;
>
> - /*
> - * we would like to check all the supers, but that would make
> - * a btrfs mount succeed after a mkfs from a different FS.
> - * So, we need to add a special mount option to scan for
> - * later supers, using BTRFS_SUPER_MIRROR_MAX instead
> - */
> - bytenr = btrfs_sb_offset(0);
> flags |= FMODE_EXCL;
>
> bdev = blkdev_get_by_path(path, flags, holder);
> if (IS_ERR(bdev))
> return PTR_ERR(bdev);
>
> - ret = btrfs_read_disk_super(bdev, bytenr, &page, &disk_super);
> - if (ret)
> + disk_super_primary = kzalloc(sizeof(*disk_super_primary), GFP_KERNEL);
> + if (!disk_super_primary) {
> + ret = -ENOMEM;
> goto error_bdev_put;
> + }
> +
> + /*
> + * We would like to check all the supers and use one good copy,
> + * but that would make a btrfs mount succeed after a mkfs from
> + * a different FS.
> + * So, we need to add a special mount option to scan for
> + * later supers, using BTRFS_SUPER_MIRROR_MAX instead.
> + * So, just validate if all copies of the superblocks are ok
> + * and have the same fsid.
> + */
> + for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; i++) {
> + u64 bytenr = btrfs_sb_offset(i);
> +
> + ret = btrfs_read_disk_super(bdev, bytenr, &page, &disk_super);
> + if (ret) {
> + if (i == 0)
> + goto error_kfree;
> + /* copy2 is optional */
> + ret = 0;
> + continue;
> + }
> +
> + if (i == 0) {
> + memcpy(disk_super_primary, disk_super,
> + sizeof(*disk_super_primary));
> + btrfs_release_disk_super(page);
> + continue;
Doing the memcpy is enough here, the bottom of the loop already releases
the disk page and continues on the next iteration.
> + } else if (memcmp(disk_super_primary->fsid, disk_super->fsid,
> + BTRFS_FSID_SIZE)) {
> + pr_err("BTRFS (device %pg): superblock fsid missmatch "\
> + "primary %pU copy%d %pU", bdev,
> + disk_super_primary->fsid, i, disk_super->fsid);
> + ret = -EINVAL;
> + btrfs_release_disk_super(page);
> + goto error_kfree;
> + }
> + btrfs_release_disk_super(page);
I'd say split the "read first sb" from the loop, because alway want to
read it and return an error if it fails. And then have the loop begin at
i = 1 and handle only the possible mirrors of the sb. That would clean
up the nested 'if' in handling the ret.
Also you could introduce another struct *page primary_page where you
read the first super block. That way you save a memcpy + kzalloc but
you'd have to always free it on function exit so I am not sure how much
value it brings in terms of readability.
> + }
>
> mutex_lock(&uuid_mutex);
> - device = device_list_add(path, disk_super);
> + device = device_list_add(path, disk_super_primary);
> if (IS_ERR(device))
> ret = PTR_ERR(device);
> else
> *fs_devices_ret = device->fs_devices;
> mutex_unlock(&uuid_mutex);
>
> - btrfs_release_disk_super(page);
> -
> +error_kfree:
> + kfree(disk_super_primary);
> error_bdev_put:
> blkdev_put(bdev, flags);
>
>
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