On 19/02/16 15:08, Balbir Singh wrote:
On 19/02/16 12:55, Rashmica Gupta wrote:
Currently on PPC64 changing kernel pagesize from 4K to 64K leaves
FORCE_MAX_ZONEORDER set to 13 - which produces a compile error.
The error occurs because of the following constraint (from
include/linux/mmzone.h) being violated:
MAX_ORDER -1 + PAGESHIFT <= SECTION_SIZE_BITS.
IA64 has this cool hack
12 #ifdef CONFIG_FORCE_MAX_ZONEORDER
13 #if ((CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) >
SECTION_SIZE_BITS)
14 #undef SECTION_SIZE_BITS
15 #define SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 +
PAGE_SHIFT)
16 #endif
But coming back (we can revisit the SECTION_SIZE_BITS definition later)
I feel like someone more senior than me should weigh in on if this is
worth doing...
MAX_ORDER -1 + 16 <= 24 for 64 K
and
MAX_ORDER -1 + 12 < = 24 for 4K
Your calculations are correct
Expanding this out, we get:
FORCE_MAX_ZONEBITS <= 25 - PAGESHIFT,
which requires, for a 64K page, FORCE_MAX_ZONEBITS <= 9. Thus
set max value of FORCE_MAX_ZONEORDER for 64K pages to 9.
Also, check the minimum value:
In include/linux/huge_mm.h, we have the constraint HPAGE_PMD_ORDER <
MAX_ORDER which expands out to:
PTE_INDEX_SIZE < FORCE_MAX_ZONEORDER.
PTE_INDEX_SIZE is:
9 (4k hash or no hash 4K pgtable) or
8 (64K hash or no hash 64K pgtable).
Thus a min value of 9 for 64K pages is reasonable.
For 4K pages we end up with
9 < FORCE_MAX_ZONE_ORDER
FORCE_MAX_ZONE_ORDER -1 + 12 < = 24
The range is 9 to 13
For 64K we end up with
8 < FORCE_MAX_ZONE_ORDER
FORCE_MAX_ZONE_ORDER -1 + 16 <= 24 or FORCE_MAX_ZONE_ORDER <= 9
The range is really between 8 and 9 unless we tweak the SECTION_SIZE_BITS
Yup, you are right! Might have had a brain spaz...
So, update the range of FORCE_MAX_ZONEORDER from 9-64 to 9-9.
Signed-off-by: Rashmica Gupta <rashm...@gmail.com>
---
arch/powerpc/Kconfig | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index e4824fd04bb7..3bd3465b93ba 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -585,7 +585,7 @@ endchoice
config FORCE_MAX_ZONEORDER
int "Maximum zone order"
- range 9 64 if PPC64 && PPC_64K_PAGES
+ range 9 9 if PPC64 && PPC_64K_PAGES
range 8 9?
Agreed.
default "9" if PPC64 && PPC_64K_PAGES
range 13 64 if PPC64 && !PPC_64K_PAGES
Should this be fixed as well?
range 9 13?
Agreed.
default "13" if PPC64 && !PPC_64K_PAGES
Should the default values remain as is, or follow the trend and be equal
to the minimum value?
Please check my calculations
Reviewed-by: Balbir Singh <bsinghar...@gmail.com>
Balbir Singh
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