linux-kernel - Re: [00/17] Large Blocksize Support V3

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Message-ID: <20070425175335.GG4730@kernel.dk>
Date:	Wed, 25 Apr 2007 19:53:35 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Christoph Lameter <clameter@....com>
Cc:	linux-kernel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

On Wed, Apr 25 2007, Christoph Lameter wrote:
> On Wed, 25 Apr 2007, Jens Axboe wrote:
> 
> > I need this patch to actually boot the thing, or it bombs with a NULL
> > deref in page_cache_size().
> 
> Yeah on 32 bit which I disabled....

Yep, on the grounds that devices may not cover the full address space.
On my box they do.

> > It then boots, doing a little test with 8kb ext2 quickly dies though:
> > 
> > BUG: unable to handle kernel NULL pointer dereference at virtual address
> 
> > EIP: [<d8149519>] 0xd8149519 SS:ESP 0068:e9049d1c
> > 
> > That's here:
> > 
> >         page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, order,
> >                                 zonelist, ALLOC_WMARK_LOW|ALLOC_CPUSET);
> > ->     if (page)
> >                 goto got_pg;
> 
> Impossible. page is not dereferenced. Cannot cause a NULL pointer 
> deference.

Sure I know that, hence the oops happening there is likely fallout from
something scribbling where it should not have.

> > which doesn't look healthy. Note that this is a 32-bit machine, I
> > removed the 32-bit check (devices in this box are fine).
> 
> 
> > 
> > --- fs/libfs.c~	2007-04-25 13:30:50.000000000 +0200
> > +++ fs/libfs.c	2007-04-25 13:31:00.000000000 +0200
> > @@ -330,7 +330,7 @@
> >  int simple_prepare_write(struct file *file, struct page *page,
> >  			unsigned from, unsigned to)
> >  {
> > -	unsigned int page_size = page_cache_size(file->f_mapping);
> > +	unsigned int page_size = page_cache_size(page->mapping);
> 
> We do a write operation on a file that has no mapping (and thus no radix 
> tree etc etc)? Is that legit?

Probably not.

-- 
Jens Axboe

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