linux-ext4 - Re: BUG: unable to handle kernel NULL pointer dereference at 00000000 [ext4_new_meta

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Date:	Wed, 17 Dec 2008 21:55:54 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference at
	00000000 [ext4_new_meta_blocks+0x7c/0xb7]

On Wed, Dec 17, 2008 at 06:47:11AM -0500, Theodore Tso wrote:
> On Wed, Dec 17, 2008 at 01:26:35PM +0530, Aneesh Kumar K.V wrote:
> > > 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 extents, yes.  With normal indirect block-based files, no.  I
> agree that for consistency's sake, it should be the same, but at the
> moment, it isn't.

Hmm. May be I am missing something. This is the call chain i followed
with the Linus tree.
ext4_get_block
 ext4_get_blocks_wrap
  ext4_get_blocks_handle
   ext4_alloc_branch
    ext4_alloc_blocks
     ext4_new_meta_blocks
      do_blk_alloc -> which set ar.flags = 0 for meta data.
     ext4_new_blocks
      do_blk_alloc -> which set ar.flags = 0 for !S_ISREG(inode->i_mode)

So the current Linus kernel using mballoc for non extent format doesn't
set EXT4_MB_HINT_DATA for directories.


-aneesh
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