Re: [PATCH] ext4: Fix a BUG in mb_mark_used during trim.

[Tao Ma <tm@tao.ma>]

On 03/03/2011 06:01 PM, Lukas Czerner wrote:
> On Thu, 3 Mar 2011, Tao Ma wrote:
>
>    
>> From: Tao Ma<boyu.mt@taobao.com>
>>
>> In a bs=4096 volume, if we call FITRIM with the following parameter as
>> fstrim_range(start = 102400, len = 134144000, minlen = 10240),
>> we will trigger a BUG_ON.
>> BUG_ON(start + len>  (e4b->bd_sb->s_blocksize<<  3));
>>
>> Mar  4 00:55:52 boyu-tm kernel: ------------[ cut here ]------------
>> Mar  4 00:55:52 boyu-tm kernel: kernel BUG at fs/ext4/mballoc.c:1506!
>> Mar  4 01:21:09 boyu-tm kernel: Code: d4 00 00 00 00 49 89 fe 8b 56 0c 44 8b 7e 04 89 55 c4 48 8b 4f 28 89 d6 44 01 fe 48 63 d6 48 8b 41 18 48 c1 e0 03 48 39 c2 76 04<0f>  0b eb fe 48 8b 55 b0 8b 47 34 3b 42 08 74 04 0f 0b eb fe 48
>> Mar  4 01:21:09 boyu-tm kernel: RIP  [<ffffffffa053eb42>] mb_mark_used+0x47/0x26c [ext4]
>> Mar  4 01:21:09 boyu-tm kernel:  RSP<ffff880121e45c38>
>> Mar  4 01:21:09 boyu-tm kernel: ---[ end trace 9f461696f6a9dcf2 ]---
>>
>> The reason is that in ext4_trim_fs, the last_block is checked wrongly.
>> if (len>= EXT4_BLOCKS_PER_GROUP(sb))
>> 	len -= (EXT4_BLOCKS_PER_GROUP(sb) - first_block);
>> else
>> 	last_block = first_block + len;
>>
>> So if len<  EXT4_BLOCKS_PER_GROUP while first_block + len>  EXT4_BLOCKS_PER_GROUP,
>> last_block will be set to a overflow value which exceeds EXT4_BLOCKS_PER_GROUP.
>>
>> This patch fixes it and adjusts len accordingly.
>>      
> Oh, thanks for spotting this. One comment though.
>
>    
>> Cc: "Theodore Ts'o"<tytso@mit.edu>
>> Signed-off-by: Tao Ma<boyu.mt@taobao.com>
>> ---
>>   fs/ext4/mballoc.c |    5 +++--
>>   1 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index 29d7d17..ed63c3e 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -4889,10 +4889,11 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
>>   			break;
>>   		}
>>
>> -		if (len>= EXT4_BLOCKS_PER_GROUP(sb))
>> -			len -= (EXT4_BLOCKS_PER_GROUP(sb) - first_block);
>> +		if (first_block + len>= EXT4_BLOCKS_PER_GROUP(sb))
>> +			last_block = EXT4_BLOCKS_PER_GROUP(sb);
>>   		else
>>   			last_block = first_block + len;
>>      
> Since last_block would not change until the last group, can we simplify
> the condition ?
>
> 		if (first_block + len<  EXT4_BLOCKS_PER_GROUP(sb))
> 			last_block = first_block + len;
>    
yes, it should work. But it is a bit tricky. And I guess we need a 
comment here
to describe the situation. Otherwise, the people who read the code would 
consider
why last_block won't be reset in case first _block + len >= 
EXT4_BLOCKS_PER_GROUP
every time. Agree?
>    
>> +		len -= last_block - first_block;
>>
>>   		if (e4b.bd_info->bb_free>= minlen) {
>>   			cnt = ext4_trim_all_free(sb,&e4b, first_block,
>>
>>      
> Thanks!
> -Lukas
>    
Regards,
Tao