[PATCH 0/3] Make PMD_ORDER generically available

These three architectures each define PMD_ORDER to mean "the order of
an allocation for a PMD table", but logically PMD_ORDER should be the
order of a PMD allocation, ie (PMD_SHIFT - PAGE_SHIFT) as DAX defines it.

Could each architecture maintainer please apply the appropriate patch
to their respective trees?

Matthew Wilcox (Oracle) (3):
  arm: Rename PMD_ORDER to PMD_TABLE_ORDER
  mips: Rename PMD_ORDER to PMD_TABLE_ORDER
  parisc: Rename PMD_ORDER to PMD_TABLE_ORDER

 arch/arm/kernel/head.S             | 34 +++++++++++++++---------------
 arch/mips/include/asm/pgalloc.h    |  2 +-
 arch/mips/include/asm/pgtable-32.h |  2 +-
 arch/mips/include/asm/pgtable-64.h | 18 ++++++++--------
 arch/mips/kernel/asm-offsets.c     |  2 +-
 arch/parisc/include/asm/pgalloc.h  |  6 +++---
 arch/parisc/include/asm/pgtable.h  |  4 ++--
 arch/parisc/mm/init.c              |  4 ++--
 8 files changed, 36 insertions(+), 36 deletions(-)

-- 
2.30.2


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This is the order of the page table allocation, not the order of a PMD.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 arch/arm/kernel/head.S | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
index 9eb0b4dbcc12..6da39a1d70ba 100644
--- a/arch/arm/kernel/head.S
+++ b/arch/arm/kernel/head.S

@@ -38,10 +38,10 @@
 #ifdef CONFIG_ARM_LPAE
 	/* LPAE requires an additional page for the PGD */
 #define PG_DIR_SIZE	0x5000
-#define PMD_ORDER	3
+#define PMD_TABLE_ORDER	3
 #else
 #define PG_DIR_SIZE	0x4000
-#define PMD_ORDER	2
+#define PMD_TABLE_ORDER	2
 #endif
 
 	.globl	swapper_pg_dir

@@ -237,7 +237,7 @@ __create_page_tables:
 	mov	r6, r6, lsr #SECTION_SHIFT
 
 1:	orr	r3, r7, r5, lsl #SECTION_SHIFT	@ flags + kernel base
-	str	r3, [r4, r5, lsl #PMD_ORDER]	@ identity mapping
+	str	r3, [r4, r5, lsl #PMD_TABLE_ORDER]	@ identity mapping
 	cmp	r5, r6
 	addlo	r5, r5, #1			@ next section
 	blo	1b

@@ -247,13 +247,13 @@ __create_page_tables:
 	 * set two variables to indicate the physical start and end of the
 	 * kernel.
 	 */
-	add	r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	ldr	r6, =(_end - 1)
 	adr_l	r5, kernel_sec_start		@ _pa(kernel_sec_start)
 	str	r8, [r5]			@ Save physical start of kernel
 	orr	r3, r8, r7			@ Add the MMU flags
-	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)
-1:	str	r3, [r0], #1 << PMD_ORDER
+	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_TABLE_ORDER)
+1:	str	r3, [r0], #1 << PMD_TABLE_ORDER
 	add	r3, r3, #1 << SECTION_SHIFT
 	cmp	r0, r6
 	bls	1b

@@ -269,14 +269,14 @@ __create_page_tables:
 	mov	r3, pc
 	mov	r3, r3, lsr #SECTION_SHIFT
 	orr	r3, r7, r3, lsl #SECTION_SHIFT
-	add	r0, r4,  #(XIP_START & 0xff000000) >> (SECTION_SHIFT - PMD_ORDER)
-	str	r3, [r0, #((XIP_START & 0x00f00000) >> SECTION_SHIFT) << PMD_ORDER]!
+	add	r0, r4,  #(XIP_START & 0xff000000) >> (SECTION_SHIFT - PMD_TABLE_ORDER)
+	str	r3, [r0, #((XIP_START & 0x00f00000) >> SECTION_SHIFT) << PMD_TABLE_ORDER]!
 	ldr	r6, =(_edata_loc - 1)
-	add	r0, r0, #1 << PMD_ORDER
-	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)
+	add	r0, r0, #1 << PMD_TABLE_ORDER
+	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_TABLE_ORDER)
 1:	cmp	r0, r6
 	add	r3, r3, #1 << SECTION_SHIFT
-	strls	r3, [r0], #1 << PMD_ORDER
+	strls	r3, [r0], #1 << PMD_TABLE_ORDER
 	bls	1b
 #endif
 

@@ -286,10 +286,10 @@ __create_page_tables:
 	 */
 	mov	r0, r2, lsr #SECTION_SHIFT
 	cmp	r2, #0
-	ldrne	r3, =FDT_FIXED_BASE >> (SECTION_SHIFT - PMD_ORDER)
+	ldrne	r3, =FDT_FIXED_BASE >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	addne	r3, r3, r4
 	orrne	r6, r7, r0, lsl #SECTION_SHIFT
-	strne	r6, [r3], #1 << PMD_ORDER
+	strne	r6, [r3], #1 << PMD_TABLE_ORDER
 	addne	r6, r6, #1 << SECTION_SHIFT
 	strne	r6, [r3]
 

@@ -308,7 +308,7 @@ __create_page_tables:
 	addruart r7, r3, r0
 
 	mov	r3, r3, lsr #SECTION_SHIFT
-	mov	r3, r3, lsl #PMD_ORDER
+	mov	r3, r3, lsl #PMD_TABLE_ORDER
 
 	add	r0, r4, r3
 	mov	r3, r7, lsr #SECTION_SHIFT

@@ -338,7 +338,7 @@ __create_page_tables:
 	 * If we're using the NetWinder or CATS, we also need to map
 	 * in the 16550-type serial port for the debug messages
 	 */
-	add	r0, r4, #0xff000000 >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #0xff000000 >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	orr	r3, r7, #0x7c000000
 	str	r3, [r0]
 #endif

@@ -348,10 +348,10 @@ __create_page_tables:
 	 * Similar reasons here - for debug.  This is
 	 * only for Acorn RiscPC architectures.
 	 */
-	add	r0, r4, #0x02000000 >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #0x02000000 >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	orr	r3, r7, #0x02000000
 	str	r3, [r0]
-	add	r0, r4, #0xd8000000 >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #0xd8000000 >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	str	r3, [r0]
 #endif
 #endif

-- 
2.30.2


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This is the order of the page table allocation, not the order of a PMD.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 arch/mips/include/asm/pgalloc.h    |  2 +-
 arch/mips/include/asm/pgtable-32.h |  2 +-
 arch/mips/include/asm/pgtable-64.h | 18 +++++++++---------
 arch/mips/kernel/asm-offsets.c     |  2 +-
 4 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/arch/mips/include/asm/pgalloc.h b/arch/mips/include/asm/pgalloc.h
index 4b2567d6b2df..795e9e2219c9 100644
--- a/arch/mips/include/asm/pgalloc.h
+++ b/arch/mips/include/asm/pgalloc.h

@@ -61,7 +61,7 @@ static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long address)
 	pmd_t *pmd = NULL;
 	struct page *pg;
 
-	pg = alloc_pages(GFP_KERNEL | __GFP_ACCOUNT, PMD_ORDER);
+	pg = alloc_pages(GFP_KERNEL | __GFP_ACCOUNT, PMD_TABLE_ORDER);
 	if (pg) {
 		pgtable_pmd_page_ctor(pg);
 		pmd = (pmd_t *)page_address(pg);

diff --git a/arch/mips/include/asm/pgtable-32.h b/arch/mips/include/asm/pgtable-32.h
index 95df9c293d8d..8d57bd5b0b94 100644
--- a/arch/mips/include/asm/pgtable-32.h
+++ b/arch/mips/include/asm/pgtable-32.h

@@ -82,7 +82,7 @@ extern int add_temporary_entry(unsigned long entrylo0, unsigned long entrylo1,
 
 #define PGD_ORDER	(__PGD_ORDER >= 0 ? __PGD_ORDER : 0)
 #define PUD_ORDER	aieeee_attempt_to_allocate_pud
-#define PMD_ORDER	aieeee_attempt_to_allocate_pmd
+#define PMD_TABLE_ORDER	aieeee_attempt_to_allocate_pmd
 #define PTE_ORDER	0
 
 #define PTRS_PER_PGD	(USER_PTRS_PER_PGD * 2)

diff --git a/arch/mips/include/asm/pgtable-64.h b/arch/mips/include/asm/pgtable-64.h
index 41921acdc9d8..ae0d5a09064d 100644
--- a/arch/mips/include/asm/pgtable-64.h
+++ b/arch/mips/include/asm/pgtable-64.h

@@ -51,12 +51,12 @@
 #define PMD_MASK	(~(PMD_SIZE-1))
 
 # ifdef __PAGETABLE_PUD_FOLDED
-# define PGDIR_SHIFT	(PMD_SHIFT + (PAGE_SHIFT + PMD_ORDER - 3))
+# define PGDIR_SHIFT	(PMD_SHIFT + (PAGE_SHIFT + PMD_TABLE_ORDER - 3))
 # endif
 #endif
 
 #ifndef __PAGETABLE_PUD_FOLDED
-#define PUD_SHIFT	(PMD_SHIFT + (PAGE_SHIFT + PMD_ORDER - 3))
+#define PUD_SHIFT	(PMD_SHIFT + (PAGE_SHIFT + PMD_TABLE_ORDER - 3))
 #define PUD_SIZE	(1UL << PUD_SHIFT)
 #define PUD_MASK	(~(PUD_SIZE-1))
 #define PGDIR_SHIFT	(PUD_SHIFT + (PAGE_SHIFT + PUD_ORDER - 3))

@@ -91,13 +91,13 @@
 #  define PGD_ORDER		1
 #  define PUD_ORDER		aieeee_attempt_to_allocate_pud
 # endif
-#define PMD_ORDER		0
+#define PMD_TABLE_ORDER		0
 #define PTE_ORDER		0
 #endif
 #ifdef CONFIG_PAGE_SIZE_8KB
 #define PGD_ORDER		0
 #define PUD_ORDER		aieeee_attempt_to_allocate_pud
-#define PMD_ORDER		0
+#define PMD_TABLE_ORDER		0
 #define PTE_ORDER		0
 #endif
 #ifdef CONFIG_PAGE_SIZE_16KB

@@ -107,22 +107,22 @@
 #define PGD_ORDER               0
 #endif
 #define PUD_ORDER		aieeee_attempt_to_allocate_pud
-#define PMD_ORDER		0
+#define PMD_TABLE_ORDER		0
 #define PTE_ORDER		0
 #endif
 #ifdef CONFIG_PAGE_SIZE_32KB
 #define PGD_ORDER		0
 #define PUD_ORDER		aieeee_attempt_to_allocate_pud
-#define PMD_ORDER		0
+#define PMD_TABLE_ORDER		0
 #define PTE_ORDER		0
 #endif
 #ifdef CONFIG_PAGE_SIZE_64KB
 #define PGD_ORDER		0
 #define PUD_ORDER		aieeee_attempt_to_allocate_pud
 #ifdef CONFIG_MIPS_VA_BITS_48
-#define PMD_ORDER		0
+#define PMD_TABLE_ORDER		0
 #else
-#define PMD_ORDER		aieeee_attempt_to_allocate_pmd
+#define PMD_TABLE_ORDER		aieeee_attempt_to_allocate_pmd
 #endif
 #define PTE_ORDER		0
 #endif

@@ -132,7 +132,7 @@
 #define PTRS_PER_PUD	((PAGE_SIZE << PUD_ORDER) / sizeof(pud_t))
 #endif
 #ifndef __PAGETABLE_PMD_FOLDED
-#define PTRS_PER_PMD	((PAGE_SIZE << PMD_ORDER) / sizeof(pmd_t))
+#define PTRS_PER_PMD	((PAGE_SIZE << PMD_TABLE_ORDER) / sizeof(pmd_t))
 #endif
 #define PTRS_PER_PTE	((PAGE_SIZE << PTE_ORDER) / sizeof(pte_t))
 

diff --git a/arch/mips/kernel/asm-offsets.c b/arch/mips/kernel/asm-offsets.c
index 04ca75278f02..d6b89080d245 100644
--- a/arch/mips/kernel/asm-offsets.c
+++ b/arch/mips/kernel/asm-offsets.c

@@ -198,7 +198,7 @@ void output_mm_defines(void)
 	BLANK();
 	DEFINE(_PGD_ORDER, PGD_ORDER);
 #ifndef __PAGETABLE_PMD_FOLDED
-	DEFINE(_PMD_ORDER, PMD_ORDER);
+	DEFINE(_PMD_TABLE_ORDER, PMD_TABLE_ORDER);
 #endif
 	DEFINE(_PTE_ORDER, PTE_ORDER);
 	BLANK();

-- 
2.30.2


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This is the order of the page table allocation, not the order of a PMD.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 arch/parisc/include/asm/pgalloc.h | 6 +++---
 arch/parisc/include/asm/pgtable.h | 4 ++--
 arch/parisc/mm/init.c             | 4 ++--
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/parisc/include/asm/pgalloc.h b/arch/parisc/include/asm/pgalloc.h
index 6a7e98e71f1d..54b63374579b 100644
--- a/arch/parisc/include/asm/pgalloc.h
+++ b/arch/parisc/include/asm/pgalloc.h

@@ -48,15 +48,15 @@ static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long address)
 {
 	pmd_t *pmd;
 
-	pmd = (pmd_t *)__get_free_pages(GFP_PGTABLE_KERNEL, PMD_ORDER);
+	pmd = (pmd_t *)__get_free_pages(GFP_PGTABLE_KERNEL, PMD_TABLE_ORDER);
 	if (likely(pmd))
-		memset ((void *)pmd, 0, PAGE_SIZE << PMD_ORDER);
+		memset ((void *)pmd, 0, PAGE_SIZE << PMD_TABLE_ORDER);
 	return pmd;
 }
 
 static inline void pmd_free(struct mm_struct *mm, pmd_t *pmd)
 {
-	free_pages((unsigned long)pmd, PMD_ORDER);
+	free_pages((unsigned long)pmd, PMD_TABLE_ORDER);
 }
 #endif
 

diff --git a/arch/parisc/include/asm/pgtable.h b/arch/parisc/include/asm/pgtable.h
index 43937af127b1..7badd872f05a 100644
--- a/arch/parisc/include/asm/pgtable.h
+++ b/arch/parisc/include/asm/pgtable.h

@@ -112,7 +112,7 @@ static inline void purge_tlb_entries(struct mm_struct *mm, unsigned long addr)
 #define KERNEL_INITIAL_SIZE	(1 << KERNEL_INITIAL_ORDER)
 
 #if CONFIG_PGTABLE_LEVELS == 3
-#define PMD_ORDER	1
+#define PMD_TABLE_ORDER	1
 #define PGD_ORDER	0
 #else
 #define PGD_ORDER	1

@@ -131,7 +131,7 @@ static inline void purge_tlb_entries(struct mm_struct *mm, unsigned long addr)
 #define PMD_SHIFT       (PLD_SHIFT + BITS_PER_PTE)
 #define PMD_SIZE	(1UL << PMD_SHIFT)
 #define PMD_MASK	(~(PMD_SIZE-1))
-#define BITS_PER_PMD	(PAGE_SHIFT + PMD_ORDER - BITS_PER_PMD_ENTRY)
+#define BITS_PER_PMD	(PAGE_SHIFT + PMD_TABLE_ORDER - BITS_PER_PMD_ENTRY)
 #define PTRS_PER_PMD    (1UL << BITS_PER_PMD)
 #else
 #define BITS_PER_PMD	0

diff --git a/arch/parisc/mm/init.c b/arch/parisc/mm/init.c
index 591a4e939415..3f7d6d5b56ac 100644
--- a/arch/parisc/mm/init.c
+++ b/arch/parisc/mm/init.c

@@ -378,8 +378,8 @@ static void __init map_pages(unsigned long start_vaddr,
 
 #if CONFIG_PGTABLE_LEVELS == 3
 		if (pud_none(*pud)) {
-			pmd = memblock_alloc(PAGE_SIZE << PMD_ORDER,
-					     PAGE_SIZE << PMD_ORDER);
+			pmd = memblock_alloc(PAGE_SIZE << PMD_TABLE_ORDER,
+					     PAGE_SIZE << PMD_TABLE_ORDER);
 			if (!pmd)
 				panic("pmd allocation failed.\n");
 			pud_populate(NULL, pud, pmd);

-- 
2.30.2


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On 7/15/21 3:46 PM, Matthew Wilcox (Oracle) wrote:

These three architectures each define PMD_ORDER to mean "the order of
an allocation for a PMD table", but logically PMD_ORDER should be the
order of a PMD allocation, ie (PMD_SHIFT - PAGE_SHIFT) as DAX defines it.

Some architectures do have PGD_ORDER, PUD_ORDER and PTE_ORDER as well.
If you rename PMD_ORDER, IMHO the others should be renamed too.

Why not simply rename "PMD_ORDER" in fs/dax.c to e.g.
#define DAX_PMD_SHIFT   (PMD_SHIFT - PAGE_SHIFT)
and use that inside the dax filesystem code?

Helge

Could each architecture maintainer please apply the appropriate patch
to their respective trees?

Matthew Wilcox (Oracle) (3):
   arm: Rename PMD_ORDER to PMD_TABLE_ORDER
   mips: Rename PMD_ORDER to PMD_TABLE_ORDER
   parisc: Rename PMD_ORDER to PMD_TABLE_ORDER

  arch/arm/kernel/head.S             | 34 +++++++++++++++---------------
  arch/mips/include/asm/pgalloc.h    |  2 +-
  arch/mips/include/asm/pgtable-32.h |  2 +-
  arch/mips/include/asm/pgtable-64.h | 18 ++++++++--------
  arch/mips/kernel/asm-offsets.c     |  2 +-
  arch/parisc/include/asm/pgalloc.h  |  6 +++---
  arch/parisc/include/asm/pgtable.h  |  4 ++--
  arch/parisc/mm/init.c              |  4 ++--
  8 files changed, 36 insertions(+), 36 deletions(-)

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On Thu, Jul 15, 2021 at 06:28:42PM +0200, Helge Deller wrote:

On 7/15/21 3:46 PM, Matthew Wilcox (Oracle) wrote:

quoted

These three architectures each define PMD_ORDER to mean "the order of
an allocation for a PMD table", but logically PMD_ORDER should be the
order of a PMD allocation, ie (PMD_SHIFT - PAGE_SHIFT) as DAX defines it.

Some architectures do have PGD_ORDER, PUD_ORDER and PTE_ORDER as well.
If you rename PMD_ORDER, IMHO the others should be renamed too.

Why not simply rename "PMD_ORDER" in fs/dax.c to e.g.
#define DAX_PMD_SHIFT   (PMD_SHIFT - PAGE_SHIFT)
and use that inside the dax filesystem code?

Because that's wrong.  PMD order is clearly the order of a PMD page,
not the order of a PMD table.

I'm sure the others should also be renamed, but this is the one which
is causing me pain today.

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On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:

I think PMD_ENTRY_ORDER would make more sense here - this is the
power-of-2 of an individual PMD entry, not of the entire table.

-- 
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On Thu, Jul 15, 2021 at 05:47:41PM +0100, Russell King (Oracle) wrote:

On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:

quoted

This is the order of the page table allocation, not the order of a PMD.
-#define PMD_ORDER	3
+#define PMD_TABLE_ORDER	3
 #else
 #define PG_DIR_SIZE	0x4000
-#define PMD_ORDER	2
+#define PMD_TABLE_ORDER	2

I think PMD_ENTRY_ORDER would make more sense here - this is the
power-of-2 of an individual PMD entry, not of the entire table.

But ... we have two kinds of PMD entries.  We have the direct entry that
points to a 1-16MB sized chunk of memory, and we have the table entry that
points to a 4k-32k chunk of memory that contains PTEs.  So I don't think
calling it 'entry' order actually disambiguates anything.  That's why
I went with 'table' -- I can't think of anything else to call it!
PMD_PTE_ARRAY_ORDER doesn't seem like an improvement to me ...

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On Thu, Jul 15, 2021 at 07:10:54PM +0100, Matthew Wilcox wrote:

On Thu, Jul 15, 2021 at 05:47:41PM +0100, Russell King (Oracle) wrote:

quoted

On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:

quoted

This is the order of the page table allocation, not the order of a PMD.
-#define PMD_ORDER	3
+#define PMD_TABLE_ORDER	3
 #else
 #define PG_DIR_SIZE	0x4000
-#define PMD_ORDER	2
+#define PMD_TABLE_ORDER	2

I think PMD_ENTRY_ORDER would make more sense here - this is the
power-of-2 of an individual PMD entry, not of the entire table.

But ... we have two kinds of PMD entries.  We have the direct entry that
points to a 1-16MB sized chunk of memory, and we have the table entry that
points to a 4k-32k chunk of memory that contains PTEs.  So I don't think
calling it 'entry' order actually disambiguates anything.  That's why
I went with 'table' -- I can't think of anything else to call it!
PMD_PTE_ARRAY_ORDER doesn't seem like an improvement to me ...

There may be two kinds of PMD entries, but that isn't relevant here.
Going back to the original terminology, 1 << PMD_ORDER here is the
size of each PMD entry. It doesn't have anything to do with how much
memory is being mapped by each entry.

I think what is confusing you is stuff like:

        add     r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_ORDER)

r4 is the base address of the page tables, and r0 is the address of
the entry we want to manipulate for "KERNEL_OFFSET" - which is the
virtual address. 1 << SECTION_SHIFT is how much memory each entry
maps (and this is fixed here - there's no variability as you suggest
above.)

Effectively, the calculation above is:

	index = KERNEL_OFFSET >> SECTION_SHIFT;
	pmd_entry_size = 1 << PMD_ORDER;
	r0 = base + index * pmd_entry_size;

but in a single instruction as we can be sure that KERNEL_OFFSET will
have zeros as the low bits after shifting by SECTION_SHIFT - PMD_ORDER.

Hope this helps to explain what this PMD_ORDER is actually doing here.

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On Thu, Jul 15, 2021 at 07:37:27PM +0100, Russell King (Oracle) wrote:

On Thu, Jul 15, 2021 at 07:10:54PM +0100, Matthew Wilcox wrote:

quoted

On Thu, Jul 15, 2021 at 05:47:41PM +0100, Russell King (Oracle) wrote:

quoted

On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:

quoted

This is the order of the page table allocation, not the order of a PMD.
-#define PMD_ORDER	3
+#define PMD_TABLE_ORDER	3
 #else
 #define PG_DIR_SIZE	0x4000
-#define PMD_ORDER	2
+#define PMD_TABLE_ORDER	2

I think PMD_ENTRY_ORDER would make more sense here - this is the
power-of-2 of an individual PMD entry, not of the entire table.

But ... we have two kinds of PMD entries.  We have the direct entry that
points to a 1-16MB sized chunk of memory, and we have the table entry that
points to a 4k-32k chunk of memory that contains PTEs.  So I don't think
calling it 'entry' order actually disambiguates anything.  That's why
I went with 'table' -- I can't think of anything else to call it!
PMD_PTE_ARRAY_ORDER doesn't seem like an improvement to me ...

There may be two kinds of PMD entries, but that isn't relevant here.
Going back to the original terminology, 1 << PMD_ORDER here is the
size of each PMD entry. It doesn't have anything to do with how much
memory is being mapped by each entry.

Oh.  Oh!  So, 'order' is usually a shift that is _added on to_ the
PAGE_SHIFT in order to find how many bytes are in question.  See
include/asm-generic/getorder.h.

Now, PMD_SHIFT is already in use, but perhaps what is meant here is
PMD_ENTRY_SHIFT?

I think what is confusing you is stuff like:

        add     r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_ORDER)

r4 is the base address of the page tables, and r0 is the address of
the entry we want to manipulate for "KERNEL_OFFSET" - which is the
virtual address. 1 << SECTION_SHIFT is how much memory each entry
maps (and this is fixed here - there's no variability as you suggest
above.)

(the variability I intended above was more to accommodate architectural
differences; I hate to use x86-specific numbers like 4KiB and 2MiB)

Effectively, the calculation above is:

	index = KERNEL_OFFSET >> SECTION_SHIFT;
	pmd_entry_size = 1 << PMD_ORDER;
	r0 = base + index * pmd_entry_size;

but in a single instruction as we can be sure that KERNEL_OFFSET will
have zeros as the low bits after shifting by SECTION_SHIFT - PMD_ORDER.

Hope this helps to explain what this PMD_ORDER is actually doing here.

Thank you, yes, I was terminally confused.

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