[QUESTION] BUG_ON in ext4_mb_simple_scan_group

yangerkun
2020-04-16T14:19:38Z

Nowadays, we trigger the a bug that has been reported before[1](trigger
the bug with read block bitmap error before). After search the patch,
I found some related patch which has not been included in our kernel.

eb5760863fc2 ext4: mark block bitmap corrupted when found instead of BUGON
736dedbb1a7d ext4: mark block bitmap corrupted when found
206f6d552d0c ext4: mark inode bitmap corrupted when found
db79e6d1fb1f ext4: add new ext4_mark_group_bitmap_corrupted() helper
0db9fdeb347c ext4: fix wrong return value in ext4_read_inode_bitmap()

Maybe this patch can fix the problem, but I am a little confused with
the explain from Ted described in the mail:

> What probably happened is that the page containing actual allocation
> bitmap was pushed out of memory due to memory pressure. However, the
> buddy bitmap was still cached in memory. That's actually quite
> possible since the buddy bitmap will often be referenced more
> frequently than the allocation bitmap (for example, while searching
> for free space of a specific size, and then having that block group
> skipped when it's not available).

> Since there was an I/O error reading the allocation bitmap, the buffer
> is not valid. So it's not surprising that the BUG_ON(k >= max) is
> getting triggered.

(Our machine: x86, 4K page size, 4K block size)

After check the related code, we found that once we get a IO error from
ext4_wait_block_bitmap, ext4_mb_init_cache will return directly with a
error number, so the latter ext4_mb_simple_scan_group may never been
called! So any other scene will trigger this BUG_ON?

Thanks,
Kun.

-----
[1] https://www.spinics.net/lists/linux-ext4/msg60329.html