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

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Message-ID: <462CF5B8.4070708@xensource.com>
Date:	Mon, 23 Apr 2007 11:06:48 -0700
From:	Jeremy Fitzhardinge <jeremy@...source.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	virtualization@...ts.osdl.org, Chuck Ebbert <cebbert@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 10/28] i386: map enough initial memory to create lowmem
 mappings

Eric W. Biederman wrote:
> Consider a memory hole of size 8M immediately after our bootmem bitmap.
> head.S which knows nothing of holes will map the pages of the hole
> into the initial page tables assuming that is where the page tables
> will live.
>   

Sure, but considering we're only talking about mapping an additional *2*
pages, the chances are that it will affect a kernel as much as the early
head.S mapping.  It's well within the variability of fairly similar
kernel configs.

> As for how to fix this, we need to call boot_ioremap or better
> bt_ioremap on the address returned from alloc_bootmem_pages_low to
> force the allocated page into the page table.  After we have setup
> the page table page we can call unmap the page to free up the scarce
> early mapping resource.
>   

Would that be necessary?  Is there any need to remap it?  Couldn't you
just do the straightforward phys->virt mapping and use the page where it
lies?

    J
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