linux-kernel - Re: [PATCH 3/5] lib: lockless generic and arch independent page table (gpt) v2.

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Date:	Fri, 14 Nov 2014 02:58:33 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jerome Glisse <j.glisse@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Joerg Roedel <joro@...tes.org>,
	Mel Gorman <mgorman@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Airlie <airlied@...hat.com>,
	Brendan Conoboy <blc@...hat.com>,
	Joe Donohue <jdonohue@...hat.com>,
	Duncan Poole <dpoole@...dia.com>,
	Sherry Cheung <SCheung@...dia.com>,
	Subhash Gutti <sgutti@...dia.com>,
	John Hubbard <jhubbard@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Lucien Dunning <ldunning@...dia.com>,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Arvind Gopalakrishnan <arvindg@...dia.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Liran Liss <liranl@...lanox.com>,
	Roland Dreier <roland@...estorage.com>,
	Ben Sander <ben.sander@....com>,
	Greg Stoner <Greg.Stoner@....com>,
	John Bridgman <John.Bridgman@....com>,
	Michael Mantor <Michael.Mantor@....com>,
	Paul Blinzer <Paul.Blinzer@....com>,
	Laurent Morichetti <Laurent.Morichetti@....com>,
	Alexander Deucher <Alexander.Deucher@....com>,
	Oded Gabbay <Oded.Gabbay@....com>,
	Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH 3/5] lib: lockless generic and arch independent page
 table (gpt) v2.

On Thu, Nov 13, 2014 at 03:50:02PM -0800, Linus Torvalds wrote:
> +/*
> + * The 'tree_level' data only describes one particular level
> + * of the tree. The upper levels are totally invisible to the
> + * user of the tree walker, since the tree walker will walk
> + * those using the tree definitions.
> + *
> + * NOTE! "struct tree_entry" is an opaque type, and is just a
> + * used as a pointer to the particular level. You can figure
> + * out which level you are at by looking at the "tree_level",
> + * but even better is to just use different "lookup()"
> + * functions for different levels, at which point the
> + * function is inherent to the level.

Please, don't.

We will end up with the same last-level centric code as we have now in mm
subsystem: all code only cares about pte. It makes implementing variable
page size support really hard and lead to copy-paste approach. And to
hugetlb parallel world...

It would be nice to have tree_level description generic enough to get rid
of pte_present()/pte_dirty()/pte_* and implement generic helpers instead.

Apart from variable page size problem, we could get one day support
different CPU page table format supported in runtime: PAE/non-PAE on
32-bit x86 or LPAE/non-LPAE on ARM in one binary kernel image.

The big topic is how to get it done without significant runtime cost :-/

-- 
 Kirill A. Shutemov
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