Re: [PATCH 1/1] Only use ARCH_PFN_OFFSET once during boot
From: Mel Gorman
Date: Thu Jul 06 2006 - 12:07:09 EST
On Thu, 6 Jul 2006, Franck Bui-Huu wrote:
Hi Mel !
Hi.
Mel Gorman wrote:
The FLATMEM memory model assumes that memory is one contiguous region based
at PFN 0 and uses the NODE_DATA(0)->node_mem_map as the global mem_map. As
[snip]
diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.17-mm6-clean/include/asm-generic/memory_model.h linux-2.6.17-mm6-archpfnoffset_optimise/include/asm-generic/memory_model.h
--- linux-2.6.17-mm6-clean/include/asm-generic/memory_model.h 2006-07-05 14:31:17.000000000 +0100
+++ linux-2.6.17-mm6-archpfnoffset_optimise/include/asm-generic/memory_model.h 2006-07-05 14:36:04.000000000 +0100
@@ -28,9 +28,8 @@
*/
#if defined(CONFIG_FLATMEM)
-#define __pfn_to_page(pfn) (mem_map + ((pfn) - ARCH_PFN_OFFSET))
-#define __page_to_pfn(page) ((unsigned long)((page) - mem_map) + \
- ARCH_PFN_OFFSET)
+#define __pfn_to_page(pfn) (mem_map + (pfn))
+#define __page_to_pfn(page) ((unsigned long)((page) - mem_map))
#elif defined(CONFIG_DISCONTIGMEM)
ok for that part.
Grand.
#define __pfn_to_page(pfn) \
diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.17-mm6-clean/mm/page_alloc.c linux-2.6.17-mm6-archpfnoffset_optimise/mm/page_alloc.c
--- linux-2.6.17-mm6-clean/mm/page_alloc.c 2006-07-05 14:31:18.000000000 +0100
+++ linux-2.6.17-mm6-archpfnoffset_optimise/mm/page_alloc.c 2006-07-05 17:01:01.000000000 +0100
@@ -2157,10 +2157,14 @@ static void __init alloc_node_mem_map(st
}
#ifdef CONFIG_FLATMEM
/*
- * With no DISCONTIG, the global mem_map is just set as node 0's
+ * With FLATMEM, the global mem_map is just set as node 0's. The
+ * FLATMEM memory model assumes that memory is in one contiguous area
+ * starting at PFN 0. Architectures that do not start NODE 0 at PFN 0
+ * must define ARCH_PFN_OFFSET as the offset between
+ * NODE_DATA(0)->node_mem_map and PFN 0.
*/
if (pgdat == NODE_DATA(0))
- mem_map = NODE_DATA(0)->node_mem_map;
+ mem_map = NODE_DATA(0)->node_mem_map - ARCH_PFN_OFFSET;
is mem_map always aligned on MAX_ORDER ?
The start of mem_map is at PFN 0 so it's always aligned. node_mem_map is
MAX_ORDER_NR_PAGES aligned in alloc_node_mem_map().
Bear in mind that the only difference between the patched and unpatched
kernel is that the unpatched kernel offsets NODE_DATA(0)->node_mem_map by
NODE_DATA(0)->node_mem_map at every call to __pfn_to_page() and
__page_to_pfn() and the patched kernel offsets just once. I'm not looking
to alter anything fundamental here.
#endif
#endif /* CONFIG_FLAT_NODE_MEM_MAP */
}
I'm not sure of that part. We basically make incoherent the use of
free_area_init_node()'s fourth parameter by doing this change (for
FLATMEM model of course).
When using free_area_init() which is defined as follow:
void __init free_area_init(unsigned long *zones_size)
{
free_area_init_node(0, NODE_DATA(0), zones_size,
__pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
}
we will end up to have 2 definitions for the mem start:
- __pa(PAGE_OFFSET) >> PAGE_SHIFT
- ARCH_PFN_OFFSET
The former will be used to calculate the size of mem_map
The size of the node_mem_map is determined by zones_size. mem_map does not
have a size as such because it's not necessarily a fully populated array.
The location of mem_map is determined by the location of
NODE_DATA(0)->node_mem_map and ARCH_PFN_OFFSET. Neither the former nor the
latter affect the size of mem_map in other words.
and the
latter will be used to calculate the offset between
NODE_DATA(0)->node_mem_map and PFN 0. I don't think that will result
in any problem since:
ARCH_PFN_OFFSET == __pa(PAGE_OFFSET) >> PAGE_SHIFT
That just makes the code is harder to follow.
But if some platforms were doing something like (which is unlikely)
[...]
free_area_init_node(0, NODE_DATA(0), zone_size, FOO_PFN_OFFSET, NULL);
[...]
and ARCH_PFN_OFFSET != FOO_PFN_OFFSET then they may have some troubles.
I'm not aware of an architecture that would run into such a problem but
that is not saying a lot.
What about this patch for page_alloc.c ? I think it makes more obvious
what is ARCH_PFN_OFFSET. And if someone doesn't want to use
ARCH_PFN_OFFSET, he still can use:
free_area_init_node(0, NODE_DATA(0), zone_size, FOO_PFN_OFFSET, NULL);
I think my patch does the job of moving ARCH_PFN_OFFSET out of the hot
path in a less risky fashion. However, if you are sure that callers to
free_area_init() and ARCH_PFN_OFFSET are ok after your patch, I'd be happy
to go with it. If you're not sure, I reckon my patch would be the way to
go.
Franck
-- >8 --
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 253a450..9daee06 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2147,7 +2147,7 @@ #ifdef CONFIG_FLATMEM
* With no DISCONTIG, the global mem_map is just set as node 0's
*/
if (pgdat == NODE_DATA(0))
- mem_map = NODE_DATA(0)->node_mem_map;
+ mem_map = NODE_DATA(0)->node_mem_map - pgdat->node_start_pfn;
#endif
#endif /* CONFIG_FLAT_NODE_MEM_MAP */
}
@@ -2172,10 +2172,13 @@ struct pglist_data contig_page_data = {
EXPORT_SYMBOL(contig_page_data);
#endif
+/*
+ * This function is used only by FLATMEM. In that case the
+ * start of physical mem is always given by ARCH_PFN_OFFSET.
+ */
void __init free_area_init(unsigned long *zones_size)
{
- free_area_init_node(0, NODE_DATA(0), zones_size,
- __pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
+ free_area_init_node(0, NODE_DATA(0), zones_size, ARCH_PFN_OFFSET, NULL);
}
#ifdef CONFIG_PROC_FS
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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