[RFC] mm: Do not look up the next level table when large page exist.

[RFC] mm: Do not look up the next level table when large page exist.

[zhong jiang]

show_pte can show the kernel page table and user page table. user
page table is totally four level table. but kernel page table can
be a large page. Therefore, it need to check the page table.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
---
 arch/arm64/mm/fault.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 1b40676..a397a54 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -169,13 +169,13 @@ void show_pte(unsigned long addr)
 		pudp = pud_offset(pgdp, addr);
 		pud = READ_ONCE(*pudp);
 		pr_cont(", pud=%016llx", pud_val(pud));
-		if (pud_none(pud) || pud_bad(pud))
+		if (pud_none(pud) || pud_bad(pud) || pud_sect(pud))
 			break;
 
 		pmdp = pmd_offset(pudp, addr);
 		pmd = READ_ONCE(*pmdp);
 		pr_cont(", pmd=%016llx", pmd_val(pmd));
-		if (pmd_none(pmd) || pmd_bad(pmd))
+		if (pmd_none(pmd) || pmd_bad(pmd) || pmd_sect(pmd))
 			break;
 
 		ptep = pte_offset_map(pmdp, addr);
-- 
1.7.12.4

[RFC] mm: Do not look up the next level table when large page exist.

[Catalin Marinas]

On Wed, Jul 25, 2018 at 11:21:59PM +0800, zhong jiang wrote:
> show_pte can show the kernel page table and user page table. user
> page table is totally four level table.

Well, you can have huge pages in user space.

> but kernel page table can
> be a large page. Therefore, it need to check the page table.
> 
> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> ---
>  arch/arm64/mm/fault.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 1b40676..a397a54 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -169,13 +169,13 @@ void show_pte(unsigned long addr)
>  		pudp = pud_offset(pgdp, addr);
>  		pud = READ_ONCE(*pudp);
>  		pr_cont(", pud=%016llx", pud_val(pud));
> -		if (pud_none(pud) || pud_bad(pud))
> +		if (pud_none(pud) || pud_bad(pud) || pud_sect(pud))

Have you actually hit a problem with this code?

pud_sect() checks that the bottom two bits of the pud are 0b01.
pud_bad() is true when bit 1 is 0, which is the case with a section
mapping.

-- 
Catalin

[RFC] mm: Do not look up the next level table when large page exist.

[zhong jiang]

On 2018/7/26 0:53, Catalin Marinas wrote:
> On Wed, Jul 25, 2018 at 11:21:59PM +0800, zhong jiang wrote:
>> show_pte can show the kernel page table and user page table. user
>> page table is totally four level table.
> Well, you can have huge pages in user space.
 No, I have not.
>> but kernel page table can
>> be a large page. Therefore, it need to check the page table.
>>
>> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
>> ---
>>  arch/arm64/mm/fault.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 1b40676..a397a54 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -169,13 +169,13 @@ void show_pte(unsigned long addr)
>>  		pudp = pud_offset(pgdp, addr);
>>  		pud = READ_ONCE(*pudp);
>>  		pr_cont(", pud=%016llx", pud_val(pud));
>> -		if (pud_none(pud) || pud_bad(pud))
>> +		if (pud_none(pud) || pud_bad(pud) || pud_sect(pud))
> Have you actually hit a problem with this code?
  Not yet.  Maybe I use the function mistake.  It is only used for user and vmalloc
 pagefault.  It should be not handled with large level table.  thank you for time.

 Thanks,
 zhong jiang
> pud_sect() checks that the bottom two bits of the pud are 0b01.
> pud_bad() is true when bit 1 is 0, which is the case with a section
> mapping.
>