linux-kernel - Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux

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Date:	Thu, 2 Apr 2009 12:39:20 -0700
From:	Chris Wright <chrisw@...hat.com>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, avi@...hat.com,
	aarcange@...hat.com, chrisw@...hat.com, riel@...hat.com,
	jeremy@...p.org, mtosatti@...hat.com, hugh@...itas.com,
	corbet@....net, yaniv@...hat.com, dmonakhov@...nvz.org
Subject: Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux

* Jesper Juhl (jj@...osbits.net) wrote:
> Do you rely only on the checksum or do you actually compare pages to check 
> they are 100% identical before sharing?

Checksum has absolutely nothing to do w/ finding if two pages match.
It's only used as a heuristic to suggest whether a single page has
changed.  If that page is changing we won't bother trying to find a
match for it.  Here's an example of the life of a page w.r.t checksum.

1. checksum = uninitialized
2. first time page is found, checksum it (checksum = A).
   if checksum has changed (uninitialize != A) don't go any further w/ that page
3. next time page is found, checksum it (checksum = B).
   if checksum has change (A != B) don't go any further w/ that page
4. next time page is found, checksum it (checksum = B).
   if checksum has changed (B == B)...it hasn't, continue processing the
   page

later if a match is found in the tree (which is sorted by _contents_,
i.e. memcmp) we'll attempt to merge the pages which at it's very core
does:

	if (pages_identical(oldpage, newpage))
		ret = replace_page(vma, oldpage, newpage, orig_pte, newprot);

pages_identical?  you guessed it...just does:

	r = memcmp(addr1, addr2, PAGE_SIZE)

thanks,
-chris
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