linux-kernel - Re: [PATCH] Tight check of pfn_valid on sparsemem

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Message-ID: <20100731153031.GE27064@n2100.arm.linux.org.uk>
Date:	Sat, 31 Jul 2010 16:30:31 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Milton Miller <miltonm@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	Kukjin Kim <kgene.kim@...sung.com>
Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

On Fri, Jul 30, 2010 at 07:48:00AM -0500, Christoph Lameter wrote:
> On Thu, 29 Jul 2010, Dave Hansen wrote:
> 
> > SPARSEMEM_EXTREME would be a bit different.  It's a 2-level lookup.
> > You'd have 16 "section roots", each representing 256MB of address space.
> > Each time we put memory under one of those roots, we'd fill in a
> > 512-section second-level table, which is designed to always fit into one
> > page.  If you start at 256MB, you won't waste all those entries.
> 
> That is certain a solution to the !MMU case and it would work very much
> like a page table. If you have an MMU then the vmemmap sparsemem
> configuration can take advantage of of that to avoid the 2 level lookup.

Looking at vmemmap sparsemem, we need to fix it as the page table
allocation in there bypasses the arch defined page table setup.

This causes a problem if you have 256-entry L2 page tables with no
room for the additional Linux VM PTE support bits (such as young,
dirty, etc), and need to glue two 256-entry L2 hardware page tables
plus a Linux version to store its accounting in each page.  See
arch/arm/include/asm/pgalloc.h.

So this causes a problem with vmemmap:

                pte_t entry;
                void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node);
                if (!p)
                        return NULL;
                entry = pfn_pte(__pa(p) >> PAGE_SHIFT, PAGE_KERNEL);

Are you willing for this stuff to be replaced by architectures as
necessary?
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