Re: [PATCH v2 1/5] ARC: mm: remove __ARCH_USE_5LEVEL_HACK

[Vineet Gupta]

On 10/15/19 2:48 PM, Linus Torvalds wrote:
> On Tue, Oct 15, 2019 at 12:19 PM Vineet Gupta
>  wrote:
>> This is a non-functional change anyways since ARC has software page walker
>> with 2 lookup levels (pgd -> pte)
>
> Could we encourage other architectures to do the same, and get rid of
> all uses of __ARCH_USE_5LEVEL_HACK?

IMHO this should have been done at the onset. The actual changes don't seem that
difficult, just need to add the missing p4d calls although I can sympathize with
hassles of coordinating/building/testing/yadi yada cross arch.

OTOH, the [45]LEVEL_HACK seem like a nice way to "fold" the levels: the
skipped/folded level vanishes completely. Among others it does things like

 #define p4d_t  pgd_t   

On ARC 2-level code:

   free_pte_range
pmd_pgtable(*pmd);

((*pmd).pud).pgd))) & PAGE_MASK));   <-- 5LEVEL_HACK
vs.
*pmd).pud).p4d).pgd & PAGE_MASK ));  <-- w/o 5LEVEL_HACK

pmd_clear(pmd);

*(pmd)).pud).pgd = 0
vs.
*(pmd)).pud).p4d).pgd = 0


So we may not be able to fix all he historical misgivigs, but this might 
alleviate
the pain a bit. I'll try to dabble a bit.

Thx for taking a look and te ACKs.
-Vineet

Re: [PATCH v2 1/5] ARC: mm: remove __ARCH_USE_5LEVEL_HACK

[Linus Torvalds]

On Tue, Oct 15, 2019 at 12:19 PM Vineet Gupta
 wrote:
>
> This is a non-functional change anyways since ARC has software page walker
> with 2 lookup levels (pgd -> pte)

Could we encourage other architectures to do the same, and get rid of
all uses of __ARCH_USE_5LEVEL_HACK?

Linus

[PATCH v2 1/5] ARC: mm: remove __ARCH_USE_5LEVEL_HACK

[Vineet Gupta]

Add the intermedivmlinux-A-baseline vmlinux-B-elide-ARCH_USE_5LEVEL_HACK

This is a non-functional change anyways since ARC has software page walker
with 2 lookup levels (pgd -> pte)

There is slight code bloat due to pulling in needless p*d_free_tlb()
macros which needs to be addressed seperately.

| bloat-o-meter2 vmlinux-with-5LEVEL_HACK vmlinux-patched
| add/remove: 0/0 grow/shrink: 2/0 up/down: 128/0 (128)
| function old new   delta
| free_pgd_range   546 656+110
| p4d_clear_bad  2  20 +18
| Total: Before=4137148, After=4137276, chg 0.00%

Cc: Kirill A. Shutemov 
Signed-off-by: Vineet Gupta 
---
 arch/arc/include/asm/pgtable.h |  1 -
 arch/arc/mm/fault.c| 10 --
 arch/arc/mm/highmem.c  |  4 +++-
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/arc/include/asm/pgtable.h b/arch/arc/include/asm/pgtable.h
index 7addd0301c51..b917b596f7fb 100644
--- a/arch/arc/include/asm/pgtable.h
+++ b/arch/arc/include/asm/pgtable.h
@@ -33,7 +33,6 @@
 #define _ASM_ARC_PGTABLE_H
 
 #include 
-#define __ARCH_USE_5LEVEL_HACK
 #include 
 #include 
 #include/* to propagate CONFIG_ARC_MMU_VER  */
diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index 3861543b66a0..fb86bc3e9b35 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -30,6 +30,7 @@ noinline static int handle_kernel_vaddr_fault(unsigned long 
address)
 * with the 'reference' page table.
 */
pgd_t *pgd, *pgd_k;
+   p4d_t *p4d, *p4d_k;
pud_t *pud, *pud_k;
pmd_t *pmd, *pmd_k;
 
@@ -39,8 +40,13 @@ noinline static int handle_kernel_vaddr_fault(unsigned long 
address)
if (!pgd_present(*pgd_k))
goto bad_area;
 
-   pud = pud_offset(pgd, address);
-   pud_k = pud_offset(pgd_k, address);
+   p4d = p4d_offset(pgd, address);
+   p4d_k = p4d_offset(pgd_k, address);
+   if (!p4d_present(*p4d_k))
+   goto bad_area;
+
+   pud = pud_offset(p4d, address);
+   pud_k = pud_offset(p4d_k, address);
if (!pud_present(*pud_k))
goto bad_area;
 
diff --git a/arch/arc/mm/highmem.c b/arch/arc/mm/highmem.c
index a4856bfaedf3..fc8849e4f72e 100644
--- a/arch/arc/mm/highmem.c
+++ b/arch/arc/mm/highmem.c
@@ -111,12 +111,14 @@ EXPORT_SYMBOL(__kunmap_atomic);
 static noinline pte_t * __init alloc_kmap_pgtable(unsigned long kvaddr)
 {
pgd_t *pgd_k;
+   p4d_t *p4d_k;
pud_t *pud_k;
pmd_t *pmd_k;
pte_t *pte_k;
 
pgd_k = pgd_offset_k(kvaddr);
-   pud_k = pud_offset(pgd_k, kvaddr);
+   p4d_k = p4d_offset(pgd_k, kvaddr);
+   pud_k = pud_offset(p4d_k, kvaddr);
pmd_k = pmd_offset(pud_k, kvaddr);
 
pte_k = (pte_t *)memblock_alloc_low(PAGE_SIZE, PAGE_SIZE);
-- 
2.20.1