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SubjectRe: [f2fs-dev] [PATCH] f2fs: fix to handle looped node chain during recovery
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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@huawei.com>
> Signed-off-by: Chao Yu <yuchao0@huawei.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".
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)....

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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