Re: BUG: unable to handle kernel NULL pointer dereference at 00000000 [ext4_new_meta_blocks+0x7c/0xb7]

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Fri, Dec 12, 2008 at 09:56:09AM -0500, Theodore Tso wrote:
> On Tue, Dec 09, 2008 at 04:11:22PM +0530, Aneesh Kumar K.V wrote:
> > The problem is due to remove-do_blk_alloc patch.
> > 
> > The patch below should fix the crash. 
> > 
> > -		EXT4_I(inode)->i_allocated_meta_blocks += *count;
> > +		EXT4_I(inode)->i_allocated_meta_blocks += ar.len;
> 
> 
> Good catch, thanks.  I'll add it to the patch queue.
> 
> > I have one question regarding the patch. What about blocks allocated for
> > directories for the  ext3 format.  With extent format we are not
> > setting EXT4_MB_HINT_DATA for non regular files. So i guess we also
> > need the below patch .
> 
> One of the good things about getting rid of too many layers of
> abstractions is that it makes bugs like this easier to spot.  We've
> been sending allocating directory and symlinks using EXT4_MB_HINT_DATA
> if extents haven't been enabled, and no one noticed before we
> simplified out things....

We had always sent the directory allocation request with
EXT4_MB_HINT_DATA not set.  With Linus kernel I have this

671 static ext4_fsblk_t do_blk_alloc(handle_t *handle, struct inode *inode,
.....
....
686         if (S_ISREG(inode->i_mode) && !(flags & EXT4_META_BLOCK))
687                 /* enable in-core preallocation for data block allocation */
688                 ar.flags = EXT4_MB_HINT_DATA;
689         else
690                 /* disable in-core preallocation for non-regular files */
691                 ar.flags = 0;


That means if the request for block allocation is not on regular files
set ar.flags = 0; For regular files if the request is for meta-data
blocks set ar.glags = 0.



> 
> Actually, I wonder if maybe we should set EXT4_MB_HINT_DATA for
> directories as well.  Making directories contiguous does speed up
> certain workloads, and it does speed up fsck.  It may be though that
> the mballoc algorithms should be tuned specifically for directories,
> and what we should do is to define a new flag, EXT4_MB_HINT_DIRECTORY,
> and pass it in for that case. 
> 
> Some experimentation is clearly called for, here....
> 

True. But with the changes to do do_blk_alloc I guess we need to make
sure we request directories with EXT4_MB_HINT_DATA not set.

-aneesh