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

[Gao Xiang]

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@xxxxxxxxxx>
> > Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> > ---
> > Â fs/f2fs/recovery.c | 14 ++++++++++++++
> > Â 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,
> > ÂÂÂÂÂ struct curseg_info *curseg;
> > ÂÂÂÂÂ struct page *page = NULL;
> > ÂÂÂÂÂ block_t blkaddr;
> > +ÂÂÂ unsigned int loop_cnt = 0;
> > +ÂÂÂ unsigned int free_blocks = sbi->user_block_count -
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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)
> ÂÂ 1 Â 2Â 3Â 4ÂÂ 5 Â 6 Â Â 7
> ÂÂ |Â ÂÂÂÂÂÂÂÂ Â \ÂÂÂÂÂÂÂÂÂÂÂÂ /
> ÂÂ | ÂÂÂÂÂÂÂÂÂÂÂ ÂÂ \------/
> Â A, B
> 2)
> ÂÂ 1Â 2Â 3Â 4ÂÂ 5 Â 6 Â Â 7
> Â Â |ÂÂ |ÂÂÂÂÂ Â \ÂÂÂÂÂÂÂÂÂÂÂÂ /
> ÂÂ BÂÂ AÂÂÂÂ ÂÂ \------/
> Â3)
> ÂÂ 1Â 2Â 3Â 4ÂÂ 5 Â 6 Â Â 7
> Â ÂÂÂÂ | ÂÂ |ÂÂÂÂ \ÂÂÂÂÂÂÂÂÂÂÂÂ /
> ÂÂÂÂÂ BÂÂ AÂÂ ÂÂ \------/
> Â 4)
> ÂÂ 1Â 2Â 3Â 4ÂÂ 5 Â 6 Â Â 7
> Â ÂÂÂÂ | ÂÂÂÂÂ |\ÂÂÂÂÂÂÂÂÂÂÂÂ /
> ÂÂÂÂÂ BÂ Â Â A \------/
> 5)....


Sorry, it seems the encoded diagram is in a mess, I try again.
1)
ÂÂ 1 -> 2 -> 3 -> 4 -> 5 ->Â 6 -> 7
ÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ \ÂÂÂÂÂÂÂÂÂÂÂÂ /
ÂÂ |ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ \-----<-----/
 A, B
2)
ÂÂ 1 -> 2 -> 3 -> 4 -> 5 ->Â 6 -> 7
ÂÂ |ÂÂÂ |ÂÂÂÂÂÂÂÂÂ \ÂÂÂÂÂÂÂÂÂÂÂÂ /
ÂÂ |ÂÂÂ |ÂÂÂÂÂÂÂÂÂÂ \-----<-----/
ÂÂ BÂÂÂ A
3)
ÂÂ 1 -> 2 -> 3 -> 4 -> 5 ->Â 6 -> 7
ÂÂÂÂÂÂÂ |ÂÂÂ |ÂÂÂÂ \ÂÂÂÂÂÂÂÂÂÂÂÂ /
ÂÂÂÂÂÂÂ |ÂÂÂ |ÂÂÂÂÂ \-----<-----/
ÂÂÂÂÂÂÂ BÂÂÂ A
4)
ÂÂ 1 -> 2 -> 3 -> 4 -> 5 ->Â 6 -> 7
ÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂ |\ÂÂÂÂÂÂÂÂÂÂÂÂ /
ÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂ | \-----<-----/
ÂÂÂÂÂÂÂ BÂÂÂÂÂÂÂÂ 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,
>
> > ÂÂÂÂÂ int err = 0;
> > Â ÂÂÂÂÂ /* 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,
> > ÂÂÂÂÂÂÂÂÂ if (IS_INODE(page) && is_dent_dnode(page))
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂ entry->last_dentry = blkaddr;
> > Â next:
> > +ÂÂÂÂÂÂÂ /* sanity check in order to detect looped node chain */
> > +ÂÂÂÂÂÂÂ if (++loop_cnt >= free_blocks ||
> > +ÂÂÂÂÂÂÂÂÂÂÂ blkaddr == next_blkaddr_of_node(page)) {
> > +ÂÂÂÂÂÂÂÂÂÂÂ f2fs_msg(sbi->sb, KERN_NOTICE,
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ "%s: detect looped node chain, "
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ "blkaddr:%u, next:%u",
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ __func__, blkaddr, next_blkaddr_of_node(page));
> > +ÂÂÂÂÂÂÂÂÂÂÂ err = -EINVAL;
> > +ÂÂÂÂÂÂÂÂÂÂÂ break;
> > +ÂÂÂÂÂÂÂ }
> > +
> > ÂÂÂÂÂÂÂÂÂ /* check next segment */
> > ÂÂÂÂÂÂÂÂÂ blkaddr = next_blkaddr_of_node(page);
> > ÂÂÂÂÂÂÂÂÂ f2fs_put_page(page, 1);