linux-kernel - Re: [Xen-devel] [PATCH 1/3] x86: calculate precisely the memory needed by init_memory

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Message-ID: <alpine.DEB.2.00.1106202008390.12963@kaball-desktop>
Date:	Mon, 20 Jun 2011 20:16:01 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Ian Jackson <Ian.Jackson@...citrix.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	"yinghai@...nel.org" <yinghai@...nel.org>
Subject: Re: [Xen-devel] [PATCH 1/3] x86: calculate precisely the memory
 needed by init_memory_mapping

On Fri, 17 Jun 2011, Ian Jackson wrote:
> stefano.stabellini@...citrix.com writes ("[Xen-devel] [PATCH 1/3] x86: calculate precisely the memory needed by init_memory_mapping"):
> > +	if (pgt_buf_end != pgt_buf_top)
> > +		printk(KERN_DEBUG "initial kernel pagetable allocation wasted %lx"
> > +				" pages\n", pgt_buf_top - pgt_buf_end);
> 
> If (due to a bug) pgt_buf_end > pgt_buf_top, this will printk a
> message about wasting a negative number of pages, rather than
> crashing.  Is there something else that will catch this case ?

Thanks for reviewing this patch!

Yes, there is something else that catches this case: both the 32 bit
and the 64 bit versions of alloc_low_page contain this code:


unsigned long pfn = pgt_buf_end++;

if (pfn >= pgt_buf_top)
		panic("alloc_low_page: ran out of memory");


so we are safe from that point of view.
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