From: Badari Pulavarty <pbadari@gmail.com>
Subject: Re: [PATCH][RFC]JBD2: Fix journal checksum kernel oops on NUMA
Date: Fri, 02 Nov 2007 08:31:48 -0800
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	 <20071102052031.GC18505@webber.adilger.int>
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Cc: Mingming Cao <cmm@us.ibm.com>,
	Girish Shilamkar <girish@clusterfs.com>,
	Avantika Mathur <mathur@linux.vnet.ibm.com>,
	ext4 <linux-ext4@vger.kernel.org>
To: Andreas Dilger <adilger@sun.com>
In-Reply-To: <20071102052031.GC18505@webber.adilger.int>
Sender: linux-ext4-owner@vger.kernel.org

On Fri, 2007-11-02 at 13:20 +0800, Andreas Dilger wrote:
> On Nov 01, 2007  17:40 -0700, Mingming Cao wrote:
> > Current journal checksumming patch failed fsstress test on NUMA. The 
> > bh->b_data passed to the crc32_be () function could be NULL pointer, 
> > which caused kernel oops immediately when running fsstress with -o 
> > journal_checksum. It is because the page is part of highmem on NUMA box.
> > We need to kmap the page before access the bh->b_data to calculate
> > the checksums.
> 
> I have no objection to the patch, per-se, but I'm surprised that there
> would ever be a buffer head pointing at a page in high memory?  That
> seems contrary to what I would expect...

I was surprised to see that too while helping Mingming/Avantika track
this issue. I was under impression that we are checksumming only
metadata and it should be lowmem. But only "buffer_head"s are in lowmem.
Pages that point to can be in Highmem.

Thanks,
Badari