linux-ext4 - Re: [PATCH][RFC]JBD2: Fix journal checksum kernel oops on NUMA

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Date:	Sat, 3 Nov 2007 09:36:21 +0800
From:	Andreas Dilger <adilger@....com>
To:	Badari Pulavarty <pbadari@...il.com>
Cc:	Mingming Cao <cmm@...ibm.com>,
	Girish Shilamkar <girish@...sterfs.com>,
	Avantika Mathur <mathur@...ux.vnet.ibm.com>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH][RFC]JBD2: Fix journal checksum kernel oops on NUMA

On Nov 02, 2007  08:31 -0800, Badari Pulavarty wrote:
> 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.

But...  this implies that every user of bh->b_data needs to kmap, and I
don't see that in the code anywhere else.  That makes me think something
else is going wrong here.

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

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