linux-kernel - Re: [PATCH 10/28] i386: map enough initial memory to create lowmem mappings

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Message-ID: <462CEB55.9070508@zytor.com>
Date:	Mon, 23 Apr 2007 10:22:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Chuck Ebbert <cebbert@...hat.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	virtualization@...ts.osdl.org, lkml <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 10/28] i386: map enough initial memory to create lowmem
 mappings

Jeremy Fitzhardinge wrote:
> H. Peter Anvin wrote:
>> Since we allocate the maximum possible memory statically, I fail to
>> see how holes could make the situation any worse, or better. 
> 
> No, we map enough space to map 4G (~4 pages), but we don't actually map
> 4G.  If a hole happened to start within that 4 page mapping, then the
> memory still wouldn't be available for allocation.
> 
> I think this is a bit of a spurious argument though, since if it were
> really a problem we'd have to worry about holes hitting the kernel image
> too.  As far as I can see, that's not considered to be a problem.
> 
> I think the real point is that there's currently a subtle dependency
> between head.S and bootmem allocation which happens between start_kernel
> and pagetable_init.  Your patch preventing over-mapping should make them
> easier to smoke out as it currently stands, but eliminating the problem
> by making alloc_bootmem create the mappings for itself does have
> appeal.  There would still be the dependency on head.S to map the kernel
> itself and the bootmem allocator bitmap.
> 

Agreed.  However, saying that your patch shouldn't go into the mainline 
kernel until that has been fixed is spurious and wrong.  It fixes a real 
problem with minimal risk.

	-hpa
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