linux-kernel - Re: [PATCH 5/5] add ksm kernel shared memory driver.

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Message-Id: <20090513161739.d801ab67.akpm@linux-foundation.org>
Date:	Wed, 13 May 2009 16:17:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Izik Eidus <ieidus@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm@...r.kernel.org, avi@...hat.com, aarcange@...hat.com,
	chrisw@...hat.com, mtosatti@...hat.com, hugh@...itas.com,
	ieidus@...hat.com
Subject: Re: [PATCH 5/5] add ksm kernel shared memory driver.

On Mon, 20 Apr 2009 04:36:06 +0300
Izik Eidus <ieidus@...hat.com> wrote:

> Ksm is driver that allow merging identical pages between one or more
> applications in way unvisible to the application that use it.
> Pages that are merged are marked as readonly and are COWed when any
> application try to change them.
> 
> Ksm is used for cases where using fork() is not suitable,
> one of this cases is where the pages of the application keep changing
> dynamicly and the application cannot know in advance what pages are
> going to be identical.
> 
> Ksm works by walking over the memory pages of the applications it
> scan in order to find identical pages.
> It uses a two sorted data strctures called stable and unstable trees
> to find in effective way the identical pages.
> 
> When ksm finds two identical pages, it marks them as readonly and merges
> them into single one page,
> after the pages are marked as readonly and merged into one page, linux
> will treat this pages as normal copy_on_write pages and will fork them
> when write access will happen to them.
> 
> Ksm scan just memory areas that were registred to be scanned by it.
> 
> ...
> +	copy_user_highpage(kpage, page1, addr1, vma);
> ...

Breaks ppc64 allmodcofnig because that architecture doesn't export its
copy_user_page() to modules.

Architectures are inconsistent about this.  x86 _does_ export it,
because it bounces it to the exported copy_page().

So can I ask that you sit down and work out upon which architectures it
really makes sense to offer KSM?  Disallow the others in Kconfig and
arrange for copy_user_highpage() to be available on the allowed architectures?

Thanks.
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