linux-ext4 - Re: EXT4: kernel BUG at fs/ext4/mballoc.c:1721!

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Date:	Mon, 7 Sep 2009 15:05:20 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Andreas Dilger <adilger@....com>
Cc:	Sachin Sant <sachinp@...ibm.com>, linux-ext4@...r.kernel.org,
	Theodore Tso <tytso@....edu>
Subject: Re: EXT4: kernel BUG at fs/ext4/mballoc.c:1721!

On Fri, Sep 04, 2009 at 06:52:33AM -0600, Andreas Dilger wrote:
> On Sep 04, 2009  14:19 +0530, Aneesh Kumar wrote:
> > Ok i am running test with the below patch. It is more invasive in that it
> > moves the need init flag check into load buddy. I guess we need to do that,
> > otherwise we will be operating with stale buddy information when
> > we have resize happening parallel. Also with the patch i posted before
> > we still have issues as explained below
> > 
> > a) we check for init flag we find it doesn't need an cache init
> > b) we resize and mark the group in need for init
> > c) in load buddy we look at the pageuptodate flag and find it uptodate
> >    and continue using the old buddy cache information.
> 
> Why not have the resize code do the update of the buddy bitmap also?
> When we were just using the block bitmap for allocation the resize
> code would clear the bits in the bitmap just like deleting a file,
> so that it was totally coherent with any other bitmap user.  Having
> the resize code do the same with the buddy (instead of only marking
> it stale and leaving it for another process to refresh) should avoid
> the race condition entirely.
> 

We have EXT4_GROUP_INFO_NEED_INIT_BIT used at mutliple places. So
having ext4_mb_load_buddy check for EXT4_GROUP_INFO_NEED_INIT_BIT
flag make sense. It also allows us to consolidate the group init
in one location. Another advantage is, with ext4_mb_load_buddy checking
for EXT4_GROUP_INFO_NEED_INIT_BIT flag, we don't do reinit the buddy
cache each time we add few blocks to the group. 

-aneesh
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