Re: [PATCH] fix bogus BUG_ONs in in mballoc code

[Eric Sandeen <sandeen@redhat.com>]

Theodore Tso wrote:
> On Thu, Mar 12, 2009 at 01:46:57PM -0500, Eric Sandeen wrote:
>> Thiemo Nagel reported that:
>>
>> # dd if=/dev/zero of=image.ext4 bs=1M count=2
>> # mkfs.ext4 -v -F -b 1024 -m 0 -g 512 -G 4 -I 128 -N 1 \
>>   -O large_file,dir_index,flex_bg,extent,sparse_super image.ext4
>> # mount -o loop image.ext4 mnt/
>> # dd if=/dev/zero of=mnt/file
>>
>> oopsed, with a BUG_ON in ext4_mb_normalize_request because
>> size == EXT4_BLOCKS_PER_GROUP
>>
>> It appears to me (esp. after talking to Andreas) that the BUG_ON
>> is bogus; a request of exactly EXT4_BLOCKS_PER_GROUP should
>> be allowed, though larger sizes do indicate a problem.
>>
>> Fix that an another (apparently rare) codepath with a similar check.
> 
> Hmm.... is this at all likely to happen with a standard ext4
> filesystem parameters?  Or was this triggered because of the
> artifially set -g 512 parameter?  The question is whether we should
> try pushing this to Linus at this point, or let this wait until the
> merge window opens.
> 
> Opinions?
> 
> 						= Ted
> <

I wondered the same thing, and will admit to probably not digging deep
enough on this one.  I think the fix is ok as is but you are asking the
right questions.  Maybe a clusterfs mballoc expert can chime in and save
us some time? :)

-=Eric