Re: [PATCH 1/2] ext4: fix bug_on ext4_mb_use_inode_pa

[Jan Kara <jack@suse.cz>]

On Sat 21-05-22 21:42:16, Baokun Li wrote:
> Hulk Robot reported a BUG_ON:
> ==================================================================
> kernel BUG at fs/ext4/mballoc.c:3211!
> [...]
> RIP: 0010:ext4_mb_mark_diskspace_used.cold+0x85/0x136f
> [...]
> Call Trace:
>  ext4_mb_new_blocks+0x9df/0x5d30
>  ext4_ext_map_blocks+0x1803/0x4d80
>  ext4_map_blocks+0x3a4/0x1a10
>  ext4_writepages+0x126d/0x2c30
>  do_writepages+0x7f/0x1b0
>  __filemap_fdatawrite_range+0x285/0x3b0
>  file_write_and_wait_range+0xb1/0x140
>  ext4_sync_file+0x1aa/0xca0
>  vfs_fsync_range+0xfb/0x260
>  do_fsync+0x48/0xa0
> [...]
> ==================================================================
> 
> Above issue may happen as follows:
> -------------------------------------
> do_fsync
>  vfs_fsync_range
>   ext4_sync_file
>    file_write_and_wait_range
>     __filemap_fdatawrite_range
>      do_writepages
>       ext4_writepages
>        mpage_map_and_submit_extent
>         mpage_map_one_extent
>          ext4_map_blocks
>           ext4_mb_new_blocks
>            ext4_mb_normalize_request
>             >>> start + size <= ac->ac_o_ex.fe_logical
>            ext4_mb_regular_allocator
>             ext4_mb_simple_scan_group
>              ext4_mb_use_best_found
>               ext4_mb_new_preallocation
>                ext4_mb_new_inode_pa
>                 ext4_mb_use_inode_pa
>                  >>> set ac->ac_b_ex.fe_len <= 0
>            ext4_mb_mark_diskspace_used
>             >>> BUG_ON(ac->ac_b_ex.fe_len <= 0);
> 
> we can easily reproduce this problem with the following commands:
> 	`fallocate -l100M disk`
> 	`mkfs.ext4 -b 1024 -g 256 disk`
> 	`mount disk /mnt`
> 	`fsstress -d /mnt -l 0 -n 1000 -p 1`
> 
> The size must be smaller than or equal to EXT4_BLOCKS_PER_GROUP.
> Therefore, "start + size <= ac->ac_o_ex.fe_logical" may occur
> when the size is truncated. So start should be the start position of
> the group where ac_o_ex.fe_logical is located after alignment.
> In addition, when the value of fe_logical or EXT4_BLOCKS_PER_GROUP
> is very large, the value calculated by start_off is more accurate.
> 
> Fixes: cd648b8a8fd5 ("ext4: trim allocation requests to group size")
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

Looks good. I'd just phrase the comment below a bit differently:

> +	/*
> +	 * Because size must be less than or equal to
> +	 * EXT4_BLOCKS_PER_GROUP, start should be the start position of
> +	 * the group where ac_o_ex.fe_logical is located after alignment.
> +	 * In addition, when the value of fe_logical or
> +	 * EXT4_BLOCKS_PER_GROUP is very large, the value calculated
> +	 * by start_off is more accurate.
> +	 */
> +	start = max(start, round_down(ac->ac_o_ex.fe_logical,
> +			EXT4_BLOCKS_PER_GROUP(ac->ac_sb)));
> +

Can we make the comment like:

	/*
	 * For tiny groups (smaller than 8MB) the chosen allocation
	 * alignment may be larger than group size. Make sure the alignment
	 * does not move allocation to a different group which makes mballoc
	 * fail assertions later.
	 */

With that feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR