On 28.03.2018 01:06, Anand Jain wrote:
>
>
> On 03/27/2018 04:49 PM, Nikolay Borisov wrote:
>>
>>
>> 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.
>
> The page map happens inside btrfs_read_disk_super(), we
> need unmap before going for the next superblock.
You already have btrfs_release_disk_super(page); called at the end of
the iteration, right after the closing bracket for the else, see below...
>
>
>>> + } 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);
^^^^^^^^^^^^^^^^^^^^^^^
Here
>>
>> 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.
>
> Right. Also with this approach I don't have to kzallo() anymore. Will fix.
>
> Thanks, Anand
>
>>> + }
>>> 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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>
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