Re: [PATCH] ext4: Fix kernel BUG at fs/ext4/mballoc.c:910!

[Valerie Clement <valerie.clement@bull.net>]

Andreas Dilger wrote:
> On Feb 13, 2008  18:19 +0100, Valerie Clement wrote:
>> From: Valerie Clement <valerie.clement@bull.net>
>>
>> With the flex_bg feature enabled, a large file creation oopses the
>> kernel.
>> The BUG_ON is:
>> 	BUG_ON(len >= EXT4_BLOCKS_PER_GROUP(sb));
>>
>> As the allocation of the bitmaps and the inode table can be done
>> outside the block group with flex_bg, this allows to allocate up to
>> EXT4_BLOCKS_PER_GROUP blocks in a group.
> 
> Caution is needed here.  In the past we were limited to BLOCKS_PER_GROUP()
> blocks per extent (32768 blocks at most, regardless of blocksize I think)
> but now an extent might be larger.
> 
> Can you please verify that the extent-length limits for "initialized" vs.
> "uninitialized" extents are being hit so that extents don't accidentally
> grow to be > 32768 blocks long and suddenly get marked as short uninitialized
> extents.
> 
> Note that the assertion can still be hit if groups are created with fewer
> blocks, or with blocksize < 4096.  For example, if we have blocksize = 1024
> this gives BLOCKS_PER_GROUP=8192, but an extent can be up to 32768 blocks.
> 
> I think the right assertion is now:
> 
> 	BUG_ON(len > EXT4_INIT_MAX_LEN);
> 
> if FLEX_BG is active.  I'm not sure if we want to keep the stricter assertion:
> 
> 	BUG_ON(len > EXT4_HAS_INCOMPAT_FEATURE_FLEX_BG(sb) ? EXT4_INIT_MAX_LEN :
> 						EXT4_BLOCKS_PER_GROUP(sb));
> 
> but it might be worthwhile at least initially, and I don't think the CPU cost
> is very high.

I agree. I'll do the changes.

> 
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index b0f84b4..0275150 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -907,7 +907,7 @@ static void ext4_mb_mark_free_simple(struct super_block *sb,
>>  	unsigned short chunk;
>>  	unsigned short border;
>>  
>> -	BUG_ON(len >= EXT4_BLOCKS_PER_GROUP(sb));
>> +	BUG_ON(len > EXT4_BLOCKS_PER_GROUP(sb));
>>  
>>  	border = 2 << sb->s_blocksize_bits;
>>  
>> @@ -3286,7 +3286,7 @@ static void ext4_mb_normalize_request(struct ext4_allocation_context *ac,
>>  	}
>>  	BUG_ON(start + size <= ac->ac_o_ex.fe_logical &&
>>  			start > ac->ac_o_ex.fe_logical);
>> -	BUG_ON(size <= 0 || size >= EXT4_BLOCKS_PER_GROUP(ac->ac_sb));
>> +	BUG_ON(size <= 0 || size > EXT4_BLOCKS_PER_GROUP(ac->ac_sb));
> 
> Please separate this into two BUG_ON() statements, so it is clear which
> one is being hit.

OK.
Thanks for review,
   Valerie