linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Date:	Fri, 16 Jul 2010 07:43:37 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

On Fri, 2010-07-16 at 12:47 +0200, Frederic Weisbecker wrote:
> > Thus the kernel maps the physical memory with the fewest TLB entries as
> > needed (large pages and large TLB entries). If we can map 64K pages, we
> > do that. Then kmalloc just allocates within this range, it does not need
> > to map any pages. They are already mapped.
> > 
> > Does this make a bit more sense?
> 
> 
> 
> Totally! You've made it very clear to me.
> Moreover I did not know we can have such variable page size. I mean I thought
> we can have variable page size but that would apply to every pages.

In x86_64, if bit 7 in the PDE (Page Directory Entry) is set then it
points to a 2 Meg page. Otherwise it points to a page table which will
have 512 PTE's pointing to 4K pages.

Download:

http://support.amd.com/us/Processor_TechDocs/24593.pdf

It has nice diagrams that explains this. Check out page 207 (fig 5-17)
and 210 (fig 5-22).

The phys_pmd_init() in arch/x86/mm/init_64.c will try to map memory
using 2M pages if it can, otherwise it falls back to 4K pages.

-- Steve

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