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Date:	Wed, 23 Jan 2008 15:40:10 -0700
From:	Andreas Dilger <adilger@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Theodore Ts'o <tytso@....EDU>, linux-kernel@...r.kernel.org,
	girish@...sterfs.com, adilger@...sterfs.com,
	shaggy@...ux.vnet.ibm.com, cmm@...ibm.com,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 33/49] ext4: Add the journal checksum feature

On Jan 23, 2008  14:07 -0800, Andrew Morton wrote:
> > +{
> > +	struct page *page = bh->b_page;
> > +	char *addr;
> > +	__u32 checksum;
> > +
> > +	addr = kmap_atomic(page, KM_USER0);
> > +	checksum = crc32_be(crc32_sum,
> > +		(void *)(addr + offset_in_page(bh->b_data)), bh->b_size);
> > +	kunmap_atomic(addr, KM_USER0);
> > +
> > +	return checksum;
> > +}
> 
> Can this buffer actually be in highmem?

Yes, this was found during system testing.  While ext3/4 will only allocate
buffer heads in lowmem, the jbd/jbd2 code can allocate buffers in highmem.
I was surprised about this also.

Please see the thread in ext4-devel:
[PATCH][RFC]JBD2: Fix journal checksum kernel oops on NUMA

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

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