Re: Do not depend on MAX_ORDER when grouping pages by mobility

On (12/11/07 13:21), Stephen Rothwell didst pronounce:

I discovered recently that a kernel built with ppc64_defconfig no longer
boots on legacy iSeries.  It did in 2.6.23.  I bisected down the commit
d9c2340052278d8eb2ffb16b0484f8f794def4de ("Do not depend on MAX_ORDER
when grouping pages by mobility") which fails while its parent is ok.
Also, an iseries_defconfig kernel will boot.  The reason it seem is
because on PowerPC 64 with CONFIG_HUGETLB_PAGE, HPAGE_SHIFT is not
constant and its value is determined at runtime early.

Ok, that in itself is ok. IA-64 does something similar.

For legacy iSeries HPAGE_SHIFT remains 0 which means that
HUGETLB_PAGE_ORDER becomes -PAGE_SHIFT and things degenerate badly.

D'oh. That would have problems for sure.

I can enable CONFIG_HUGETLB_PAGE_SIZE_VARIABLE for PowerPC 64, but I
still need to know a good value for HPAGE_SHIFT.  Do you have a
suggestion? Is there a better way to fix this problem?  There are places
in the PowerPC code that assume that HPAGE_SHIFT == 0 means that we have
no huge pages.

How about the following both in terms of taste and whether it works or
not?

===

Ordinarily, the size of a pageblock is determined from the hugepage size.
On PPC64, the hugepage size is determined at runtime based on the ability
of the machine. If the machine does not support hugepages, HPAGE_SHIFT is
0. This results in pageblock_order being set to -PAGE_SHIFT and a crash
results shortly afterwards.

This patch checks that HPAGE_SHIFT is a sensible value before using the
hugepage size. If it is 0, MAX_ORDER-1 is used instead as this is a sensible
value of pageblock_order.

Signed-off-by: Mel Gorman <redacted>
---

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 18f397c..232c298 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -187,6 +187,11 @@ config FORCE_MAX_ZONEORDER
 	default "9" if PPC_64K_PAGES
 	default "13"
 
+config HUGETLB_PAGE_SIZE_VARIABLE
+	bool
+	depends on HUGETLB_PAGE
+	default y
+
 config MATH_EMULATION
 	bool "Math emulation"
 	depends on 4xx || 8xx || E200 || PPC_MPC832x || E500

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index da69d83..14e0ac3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -3386,7 +3386,16 @@ static void __meminit free_area_init_core(struct pglist_data *pgdat,
 		if (!size)
 			continue;
 
-		set_pageblock_order(HUGETLB_PAGE_ORDER);
+		/*
+		 * If HPAGE_SHIFT is a sensible value, base the size of a
+		 * pageblock on the hugepage size. Otherwise MAX_ORDER-1
+		 * is a sensible choice
+		 */
+		if (HPAGE_SHIFT > PAGE_SHIFT)
+			set_pageblock_order(HUGETLB_PAGE_ORDER);
+		else
+			set_pageblock_order(MAX_ORDER-1);
+
 		setup_usemap(pgdat, zone, size);
 		ret = init_currently_empty_zone(zone, zone_start_pfn,
 						size, MEMMAP_EARLY);

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab