Re: [PATCH] Btrfs-progs: fsck: fix wrong return value in check_block()

[Wang Shilong <wangsl.fnst@cn.fujitsu.com>]

On 02/26/2014 05:39 AM, Mitch Harder wrote:
> On Mon, Feb 24, 2014 at 7:38 PM, Wang Shilong
> <wangsl.fnst@cn.fujitsu.com> wrote:
>> Hi Mitch,
>>
>>
>> On 02/25/2014 07:03 AM, Mitch Harder wrote:
>>> On Mon, Feb 24, 2014 at 5:55 AM, Wang Shilong
>>> <wangsl.fnst@cn.fujitsu.com> wrote:
>>>> We found btrfsck will output backrefs mismatch while the filesystem
>>>> is defenitely ok.
>>>>
>>>> The problem is that check_block() don't return right value,which
>>>> makes btrfsck won't walk all tree blocks thus we don't get a consistent
>>>> filesystem, we will fail to check extent refs etc.
>>>>
>>>> Reported-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
>>>> Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
>>>> ---
>>>>    cmds-check.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/cmds-check.c b/cmds-check.c
>>>> index a2afae6..253569f 100644
>>>> --- a/cmds-check.c
>>>> +++ b/cmds-check.c
>>>> @@ -2477,7 +2477,7 @@ static int check_block(struct btrfs_trans_handle
>>>> *trans,
>>>>           struct cache_extent *cache;
>>>>           struct btrfs_key key;
>>>>           enum btrfs_tree_block_status status;
>>>> -       int ret = 1;
>>>> +       int ret = 0;
>>>>           int level;
>>>>
>>>>           cache = lookup_cache_extent(extent_cache, buf->start, buf->len);
>>>> --
>>> I tried this fix on a broken btrfs volume I've been trying to repair,
>>> and it seemed to put me in an infinite loop.
>>>
>>> I agree that something seems wrong with the way the caller of
>>> check_block uses the return value, and I also noticed that it seemed
>>> to exit before walking all the tree blocks.
>>>
>>> But I think the problem is more subtle than flipping the default ret
>>> value from 1 to 0.
>> No, not really even though i know there are other problems with fsck repair
>> mode.
>> But this problem should be fixed and pushed into btrfs-progsv3.13.(Notice,
>> the below problem did not exist in btrfs-progsv3.12)
>>
>> An easy way to trigger this problem:
>>
>> # mkfs.btrfs -f /dev/sda9
>> # mount /dev/sda9 /mnt
>> # dd if=/dev/zero of=/mnt/data bs=4k count=10240 oflag=direct
>> # btrfs sub snapshot /mnt /mnt/snap1
>> # btrfs sub snapshot /mnt /mnt/snap2
>> # umount /mnt
>> # btrfs check /dev/sda9
>>
>> After applying this patch, the above problems did not exist.
>> Feel free to correct me if i miss something here.^_^
>>
> I took a closer look at the check_block function today, and it looks
> to me like the problem is that the return value is not modified when
> BTRFS_BLOCK_FLAG_FULL_BACKREF is set.
Hm, i'd say no.

Let's see what is check_block() doing.

It firstly check if there exists next block to deal, if not, return 1 
directly.
and then we do some checks with that block, and we only explictly set @ret
with error when we found an error.

So why we got such a regression when josef changed codes, it was because 
firstly
we set @ret with a none-zero value. So we had to take care of error and 
success
case both for the following codes!

I was considering your suggestion when i was writting patch, but IMO 
this patch
makes codes less error-prone.

I won't change the patch unless i am really missing something here.

Thanks,
Wang
>
> @@ -2521,14 +2521,17 @@ static int check_block(struct btrfs_trans_handle *trans,
>           }
>       } else {
>           rec->content_checked = 1;
> -        if (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF)
> +        if (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
>               rec->owner_ref_checked = 1;
> +            ret = 0;
> +        }
>           else {
>               ret = check_owner_ref(root, rec, buf);
>               if (!ret)
>                   rec->owner_ref_checked = 1;
>           }
>
> For me, in this function I would lean towards an initial return value
> that must be updated by having check_block() make an affirmative
> PASS/FAIL decision on the block.
>
> What do you think about something like this?
>
> diff --git a/cmds-check.c b/cmds-check.c
> index ffc5d3e..55070da 100644
> --- a/cmds-check.c
> +++ b/cmds-check.c
> @@ -2477,7 +2477,7 @@ static int check_block(struct btrfs_trans_handle *trans,
>       struct cache_extent *cache;
>       struct btrfs_key key;
>       enum btrfs_tree_block_status status;
> -    int ret = 1;
> +    int ret = -EINVAL;
>       int level;
>
>       cache = lookup_cache_extent(extent_cache, buf->start, buf->len);
> @@ -2521,14 +2521,17 @@ static int check_block(struct btrfs_trans_handle *trans,
>           }
>       } else {
>           rec->content_checked = 1;
> -        if (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF)
> +        if (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
>               rec->owner_ref_checked = 1;
> +            ret = 0;
> +        }
>           else {
>               ret = check_owner_ref(root, rec, buf);
>               if (!ret)
>                   rec->owner_ref_checked = 1;
>           }
>       }
> +    BUG_ON(ret == -EINVAL);
>       if (!ret)
>           maybe_free_extent_rec(extent_cache, rec);
>       return ret;
> --
>