Re: [PATCH -V5 17/25] powerpc/THP: Implement transparent hugepages for ppc64

David Gibson [off-list ref] writes:

On Thu, Apr 11, 2013 at 01:10:29PM +0530, Aneesh Kumar K.V wrote:

quoted

David Gibson [off-list ref] writes:

quoted

On Thu, Apr 04, 2013 at 11:27:55AM +0530, Aneesh Kumar K.V wrote:

quoted

From: "Aneesh Kumar K.V" <redacted>

We now have pmd entries covering to 16MB range. To implement THP on powerpc,
we double the size of PMD. The second half is used to deposit the pgtable (PTE page).
We also use the depoisted PTE page for tracking the HPTE information. The information
include [ secondary group | 3 bit hidx | valid ]. We use one byte per each HPTE entry.
With 16MB hugepage and 64K HPTE we need 256 entries and with 4K HPTE we need
4096 entries. Both will fit in a 4K PTE page.

Signed-off-by: Aneesh Kumar K.V <redacted>
---
 arch/powerpc/include/asm/page.h              |    2 +-
 arch/powerpc/include/asm/pgtable-ppc64-64k.h |    3 +-
 arch/powerpc/include/asm/pgtable-ppc64.h     |    2 +-
 arch/powerpc/include/asm/pgtable.h           |  240 ++++++++++++++++++++
 arch/powerpc/mm/pgtable.c                    |  314 ++++++++++++++++++++++++++
 arch/powerpc/mm/pgtable_64.c                 |   13 ++
 arch/powerpc/platforms/Kconfig.cputype       |    1 +
 7 files changed, 572 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
index 38e7ff6..b927447 100644
--- a/arch/powerpc/include/asm/page.h
+++ b/arch/powerpc/include/asm/page.h

@@ -40,7 +40,7 @@
 #ifdef CONFIG_HUGETLB_PAGE
 extern unsigned int HPAGE_SHIFT;
 #else
-#define HPAGE_SHIFT PAGE_SHIFT
+#define HPAGE_SHIFT PMD_SHIFT

That looks like it could break everything except the 64k page size
64-bit base.

How about 

It seems very dubious to me to have transparent hugepages enabled
without explicit hugepages in the first place.

IMHO once we have THP, we will not be using explicit hugepages unless we
want 16GB pages.

-aneesh