On 2012?01?31? 20:25, Catalin Marinas wrote:

On 31 January 2012 09:58, Russell King - ARM Linux
[off-list ref]  wrote:

quoted

On Mon, Jan 30, 2012 at 03:57:13PM +0800, bill4carson at gmail.com wrote:

quoted

diff --git a/arch/arm/include/asm/pgtable-2level.h b/arch/arm/include/asm/pgtable-2level.h
index 2317a71..062c93c 100644
--- a/arch/arm/include/asm/pgtable-2level.h
+++ b/arch/arm/include/asm/pgtable-2level.h

@@ -123,6 +123,11 @@
  #define L_PTE_USER           (_AT(pteval_t, 1)<<  8)
  #define L_PTE_XN             (_AT(pteval_t, 1)<<  9)
  #define L_PTE_SHARED         (_AT(pteval_t, 1)<<  10)        /* shared(v6), coherent(xsc3) */
+#ifdef CONFIG_ARM_HUGETLB_SUPPORT
+#define L_PTE_HUGEPAGE       (_AT(pteval_t, 1)<<  11) /* mark hugepage */
+#define L_PTE_HPAGE_2M  (_AT(pteval_t, 1)<<  12) /* only when HUGEPAGE set */
+#define L_PTE_HPAGE_16M (_AT(pteval_t, 1)<<  13) /* only when HUGEPAGE set */
+#endif

(1) How does this work when normal pages can have bit 11 set if they're an
odd PFN?

Isn't that bit 12?

quoted

(2) How do we even get to PTE level when a 2 or 16MB section doesn't have
a pte table (as the L1 entry is used for the section or supersection
mapping) ?

We don't, that's why I think we don't even need this bit defined.

First, thanks for Russel/Catalin to take time review this patch:)


a:
By pte used in here, I mean linux pte. linux pte is needed by generic mm 
layer
whether this mapping is 4K, 1MB or 16MB.


b:
Why L_PTE_HUGEPAGE is needed?

hugetlb subsystem will call pte_page to derive the corresponding page 
struct from
a given pte, and pte_pfn is used first to convert pte into a page frame 
number.
This is where need to be careful. Normal page based pte upper [31:12] 
bits is pfn,
huge page(1MB) based pte upper[31:20] is pfn, so one bit MUST 
distinguish normal
page based pte with huge page based pte.

This one bit will *only* be BIT11, one last unused PTE flag within 
normal page.
that's why L_PTE_HUGEPAGE is defined.

If L_PTE_HUGEPAGE is set, then BIT19:12 is enough to be used to mark 
this pte is 1MB
or 16MB huge page.



-- 
I am a slow learner
but I will keep trying to fight for my dreams!

--bill