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

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Date:	Mon, 23 Apr 2007 12:54:47 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...source.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

Jeremy Fitzhardinge <jeremy@...source.com> writes:

> 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.

If (cpu_has_pse) it may only be an additional two pages.
INIT_MAP_BEYOND is currently mapping a lot more then that.

>> 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?

Catch-22.  If the page is not mapped yet you can't use it where it lies.

Eric
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