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

[Andreas Dilger <adilger@sun.com>]

On Mar 12, 2009  20:38 -0400, Theodore Ts'o 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.
> 
> Clearly we should make this change to avoid the BUG_ON; but stupid
> question, why shouldn't we allow sizes larger than
> EXT4_BLOCKS_PER_GROUP?  
> 
> Especially with flex_bg, it is possible for an allocation size >
> EXT4_BLOCKS_PER_GROUP to be satisifed, especially if the filesystem
> isn't that full yet, and it might even make sense to request a larger
> allocation for video files that are getting preallocated, for
> example....

There are two reasons that we can't have too-large mballoc allocations:
- mballoc works on a per-group basis, so the most blocks that it can
  allocate at a time is BLOCKS_PER_GROUP.
- the on-disk extent format cannot map more than 128MB at a time, which
  is equal to the group size at 4kB blocksize.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.