linux-kernel - Re: + powerpc-fix-code-for-reserved-memory-spanning-across-nodes.patch added to -mm tree

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Message-ID: <20090210175915.GA2541@cmpxchg.org>
Date:	Tue, 10 Feb 2009 18:59:15 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Chandru <chandru@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, mm-commits@...r.kernel.org,
	benh@...nel.crashing.org, chandru@...ux.vnet.ibm.com,
	paulus@...ba.org, akpm@...ux-foundation.org
Subject: Re: + powerpc-fix-code-for-reserved-memory-spanning-across-nodes.patch added to -mm tree

On Tue, Feb 10, 2009 at 05:05:45PM +0530, Chandru wrote:
> Johannes Weiner wrote:
> >---
> >From: Johannes Weiner <hannes@...xchg.org>
> >Subject: powerpc: fix rounding error in teaching bootmem about LMB
> >
> >If the reserved LMB does not exactly span complete pages, treating
> >(start + size) >> PAGE_SHIFT as the ending PFN is an off by one error.
> >
> >The subsequent check for whether the region needs to be trimmed to fit
> >the underlying node can now fail if the range exceeds the node by 1 to
> >PAGE_SIZE - 1 bytes.  The excessive range is then passed to bootmem
> >which BUG()s out on it correctly.
> >
> >Fix up the rounding to include all pages the LMB spans, even partial
> >ones.
> >
> >Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> >---
> 
> Hello Hannes,
> 
> Dave Hansen gave a fix for this problem which looks similar
> to the changes that you have made here , but has an additional
> one line change too. Here is the patch ( probably may not apply
> cleanly to the latest kernel ) . I don' t know if Dave
> submitted this patch to lkml for it's inclusion into the
> latest tree. Thanks for looking into this issue. We may also have
> to remove the 
> powerpc-fix-code-for-reserved-memory-spanning-across-nodes.patch
> from the -mm tree.
> 
> Thanks,
> Chandru
> 
> =================
> Snippet from Dave's patch without change log
> 
> ---
> 
> linux-2.6.git-dave/arch/powerpc/mm/numa.c |    5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff -puN arch/powerpc/mm/numa.c~reserve-over-fix arch/powerpc/mm/numa.c
> --- linux-2.6.git/arch/powerpc/mm/numa.c~reserve-over-fix	2009-01-26 
> 10:17:20.000000000 -0800
> +++ linux-2.6.git-dave/arch/powerpc/mm/numa.c	2009-01-26 
> 10:17:30.000000000 -0800
> @@ -19,6 +19,7 @@
> #include <linux/notifier.h>
> #include <linux/lmb.h>
> #include <linux/of.h>
> +#include <linux/pfn.h>
> #include <asm/sparsemem.h>
> #include <asm/prom.h>
> #include <asm/system.h>
> @@ -882,7 +883,7 @@ static void mark_reserved_regions_for_ni
> 		unsigned long physbase = lmb.reserved.region[i].base;
> 		unsigned long size = lmb.reserved.region[i].size;
> 		unsigned long start_pfn = physbase >> PAGE_SHIFT;
> -		unsigned long end_pfn = ((physbase + size) >> PAGE_SHIFT);
> +		unsigned long end_pfn = PFN_UP(physbase + size);
> 		struct node_active_region node_ar;
> 		unsigned long node_end_pfn = node->node_start_pfn +
> 					     node->node_spanned_pages;
> @@ -908,7 +909,7 @@ static void mark_reserved_regions_for_ni
> 			 */
> 			if (end_pfn > node_ar.end_pfn)
> 				reserve_size = (node_ar.end_pfn << 
> 				PAGE_SHIFT)
> -					- (start_pfn << PAGE_SHIFT);
> +					- physbase;
> 			/*
> 			 * Only worry about *this* node, others may not
> 			 * yet have valid NODE_DATA().
> diff -puN arch/powerpc/kernel/prom_init.c~reserve-over-fix 
> arch/powerpc/kernel/prom_init.c

This is fine, too.  Subtracting physbase yields a smaller reserve_size
if not page aligned (bootmem will round it up again, no problem there)
and at the end of the loop, size is not zero.  This has no practical
impacts, though, as far as I can see.

The interesting question, however, is whether this patch actually
fixes the problem you encountered?  If so, I would be glad if we could
drop the workaround we currently have in -mm.

	Hannes
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