linux-kernel - Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges

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Date:	Tue, 29 Jan 2008 16:22:46 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
cc:	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>, Nick Piggin <npiggin@...e.de>,
	kvm-devel@...ts.sourceforge.net,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges

On Wed, 30 Jan 2008, Andrea Arcangeli wrote:

> On Wed, Jan 30, 2008 at 01:00:39AM +0100, Andrea Arcangeli wrote:
> > get_user_pages, regular linux writes don't fault unless it's
> > explicitly writeprotect, which is mandatory in a few archs, x86 not).
> 
> actually get_user_pages doesn't fault either but it calls into
> set_page_dirty, however get_user_pages (unlike a userland-write) at
> least requires mmap_sem in read mode and the PT lock as serialization,
> userland writes don't, they just go ahead and mark the pte in hardware
> w/o faults. Anyway anonymous memory these days always mapped with
> dirty bit set regardless, even for read-faults, after Nick finally
> rightfully cleaned up the zero-page trick.

That is only partially true. pte are created wronly in order to track 
dirty state these days. The first write will lead to a fault that switches 
the pte to writable. When the page undergoes writeback the page again 
becomes write protected. Thus our need to effectively deal with 
page_mkclean.

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