linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix to handle looped node chain during recovery

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Date:   Sun, 4 Feb 2018 23:14:21 +0800
From:   Chao Yu <chao@...nel.org>
To:     Gao Xiang <hsiangkao@....com>, Chao Yu <yuchao0@...wei.com>,
        jaegeuk@...nel.org, Yunlei He <heyunlei@...wei.com>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix to handle looped node chain during
 recovery

Well, that's classic algorithm for checking loop in a list, as well as
other one we know, in order to decrease time complexity of these algorithms,
their implementations are a little more complex.

But IMO, the issue we are trying to fix is really a corner case, and the
performance in that path is not such critical, so I just intend to fix it
with fewest codes.

On 2018/2/3 19:36, Gao Xiang via Linux-f2fs-devel wrote:
> 
> Sorry, I saw the related code entirely, please ignore these replies.
> 
> 
> On 2018/2/3 18:45, Gao Xiang wrote:
>>
>>
>> On 2018/2/3 18:35, Gao Xiang wrote:
>>> Hi Chao and YunLei,
>>>
>>>
>>> On 2018/2/3 17:44, Chao Yu wrote:
>>>> There is no checksum in node block now, so bit-transition from hardware
>>>> can make node_footer.next_blkaddr being corrupted w/o any detection,
>>>> result in node chain becoming looped one.
>>>>
>>>> For this condition, during recovery, in order to avoid running into dead
>>>> loop, let's detect it and just skip out.
>>>>
>>>> Signed-off-by: Yunlei He <heyunlei@...wei.com>
>>>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
>>>> ---
>>>>   fs/f2fs/recovery.c | 14 ++++++++++++++
>>>>   1 file changed, 14 insertions(+)
>>>>
>>>> diff --git a/fs/f2fs/recovery.c b/fs/f2fs/recovery.c
>>>> index b6d1ec620a8c..60dd0cee4820 100644
>>>> --- a/fs/f2fs/recovery.c
>>>> +++ b/fs/f2fs/recovery.c
>>>> @@ -243,6 +243,9 @@ static int find_fsync_dnodes(struct f2fs_sb_info *sbi, struct list_head *head,
>>>>       struct curseg_info *curseg;
>>>>       struct page *page = NULL;
>>>>       block_t blkaddr;
>>>> +    unsigned int loop_cnt = 0;
>>>> +    unsigned int free_blocks = sbi->user_block_count -
>>>> +                    valid_user_blocks(sbi);
>>> There exists another way to detect loop more faster but only using two variables.
>>> The algorithm is described as simply "B goes forward a steps only A goes forwards 2 steps".
>> "B goes forward a step only when A goes forward 2(or constant x, more than 1) steps".
>>
>>> For example:
>>> 1)
>>>    1   2  3  4   5   6     7
>>>    |             \             /
>>>    |                \------/
>>>   A, B
>>> 2)
>>>    1  2  3  4   5   6     7
>>>     |   |        \             /
>>>    B   A        \------/
>>>  3)
>>>    1  2  3  4   5   6     7
>>>        |    |     \             /
>>>       B   A      \------/
>>>   4)
>>>    1  2  3  4   5   6     7
>>>        |       |\             /
>>>       B      A \------/
>>> 5)....
>>>
>>
>>
>> Sorry, it seems the encoded diagram is in a mess, I try again.
>> 1)
>>    1 -> 2 -> 3 -> 4 -> 5 ->  6 -> 7
>>    |               \             /
>>    |                \-----<-----/
>>   A, B
>> 2)
>>    1 -> 2 -> 3 -> 4 -> 5 ->  6 -> 7
>>    |    |          \             /
>>    |    |           \-----<-----/
>>    B    A
>> 3)
>>    1 -> 2 -> 3 -> 4 -> 5 ->  6 -> 7
>>         |    |     \             /
>>         |    |      \-----<-----/
>>         B    A
>> 4)
>>    1 -> 2 -> 3 -> 4 -> 5 ->  6 -> 7
>>         |         |\             /
>>         |         | \-----<-----/
>>         B         A
>> 5)....
>> if B catchs up A, there exists a cycle.
>>
>>
>> Thanks,
>>> B will equal A or beyoud A if and only if there has a cycle.
>>> It's a more faster algorithm. :D
>>>
>>> Thanks,
>>>
>>>>       int err = 0;
>>>>         /* get node pages in the current segment */
>>>> @@ -295,6 +298,17 @@ static int find_fsync_dnodes(struct f2fs_sb_info *sbi, struct list_head *head,
>>>>           if (IS_INODE(page) && is_dent_dnode(page))
>>>>               entry->last_dentry = blkaddr;
>>>>   next:
>>>> +        /* sanity check in order to detect looped node chain */
>>>> +        if (++loop_cnt >= free_blocks ||
>>>> +            blkaddr == next_blkaddr_of_node(page)) {
>>>> +            f2fs_msg(sbi->sb, KERN_NOTICE,
>>>> +                "%s: detect looped node chain, "
>>>> +                "blkaddr:%u, next:%u",
>>>> +                __func__, blkaddr, next_blkaddr_of_node(page));
>>>> +            err = -EINVAL;
>>>> +            break;
>>>> +        }
>>>> +
>>>>           /* check next segment */
>>>>           blkaddr = next_blkaddr_of_node(page);
>>>>           f2fs_put_page(page, 1);
>>>
>>
> 
> 
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