diff mbox series

[v9,12/21] mm: pagewalk: Allow walking without vma

Message ID 20190722154210.42799-13-steven.price@arm.com
State New, archived
Headers show
Series Generic page walk and ptdump | expand

Commit Message

Steven Price July 22, 2019, 3:42 p.m. UTC
Since 48684a65b4e3: "mm: pagewalk: fix misbehavior of walk_page_range
for vma(VM_PFNMAP)", page_table_walk() will report any kernel area as
a hole, because it lacks a vma.

This means each arch has re-implemented page table walking when needed,
for example in the per-arch ptdump walker.

Remove the requirement to have a vma except when trying to split huge
pages.

Signed-off-by: Steven Price <steven.price@arm.com>
---
 mm/pagewalk.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

Comments

Anshuman Khandual July 28, 2019, 2:20 p.m. UTC | #1
On 07/22/2019 09:12 PM, Steven Price wrote:
> Since 48684a65b4e3: "mm: pagewalk: fix misbehavior of walk_page_range
> for vma(VM_PFNMAP)", page_table_walk() will report any kernel area as
> a hole, because it lacks a vma.
> 
> This means each arch has re-implemented page table walking when needed,
> for example in the per-arch ptdump walker.
> 
> Remove the requirement to have a vma except when trying to split huge
> pages.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
>  mm/pagewalk.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 98373a9f88b8..1cbef99e9258 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -36,7 +36,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>  	do {
>  again:
>  		next = pmd_addr_end(addr, end);
> -		if (pmd_none(*pmd) || !walk->vma) {
> +		if (pmd_none(*pmd)) {
>  			if (walk->pte_hole)
>  				err = walk->pte_hole(addr, next, walk);
>  			if (err)
> @@ -59,9 +59,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>  		if (!walk->pte_entry)
>  			continue;
>  
> -		split_huge_pmd(walk->vma, pmd, addr);
> -		if (pmd_trans_unstable(pmd))
> -			goto again;
> +		if (walk->vma) {
> +			split_huge_pmd(walk->vma, pmd, addr);

Check for a PMD THP entry before attempting to split it ?

> +			if (pmd_trans_unstable(pmd))
> +				goto again;
> +		} else if (pmd_leaf(*pmd)) {
> +			continue;
> +		}
> +
>  		err = walk_pte_range(pmd, addr, next, walk);
>  		if (err)
>  			break;
> @@ -81,7 +86,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>  	do {
>   again:
>  		next = pud_addr_end(addr, end);
> -		if (pud_none(*pud) || !walk->vma) {
> +		if (pud_none(*pud)) {
>  			if (walk->pte_hole)
>  				err = walk->pte_hole(addr, next, walk);
>  			if (err)
> @@ -95,9 +100,13 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>  				break;
>  		}
>  
> -		split_huge_pud(walk->vma, pud, addr);
> -		if (pud_none(*pud))
> -			goto again;
> +		if (walk->vma) {
> +			split_huge_pud(walk->vma, pud, addr);

Check for a PUD THP entry before attempting to split it ?

> +			if (pud_none(*pud))
> +				goto again;
> +		} else if (pud_leaf(*pud)) {
> +			continue;
> +		}

This is bit cryptic. walk->vma check should be inside a helper is_user_page_table()
or similar to make things clear. p4d_leaf() check missing in walk_p4d_range() for
kernel page table walk ? Wondering if p?d_leaf() test should be moved earlier while
calling p?d_entry() for kernel page table walk.
Steven Price July 29, 2019, 12:29 p.m. UTC | #2
On 28/07/2019 15:20, Anshuman Khandual wrote:
> 
> 
> On 07/22/2019 09:12 PM, Steven Price wrote:
>> Since 48684a65b4e3: "mm: pagewalk: fix misbehavior of walk_page_range
>> for vma(VM_PFNMAP)", page_table_walk() will report any kernel area as
>> a hole, because it lacks a vma.
>>
>> This means each arch has re-implemented page table walking when needed,
>> for example in the per-arch ptdump walker.
>>
>> Remove the requirement to have a vma except when trying to split huge
>> pages.
>>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>>  mm/pagewalk.c | 25 +++++++++++++++++--------
>>  1 file changed, 17 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>> index 98373a9f88b8..1cbef99e9258 100644
>> --- a/mm/pagewalk.c
>> +++ b/mm/pagewalk.c
>> @@ -36,7 +36,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>>  	do {
>>  again:
>>  		next = pmd_addr_end(addr, end);
>> -		if (pmd_none(*pmd) || !walk->vma) {
>> +		if (pmd_none(*pmd)) {
>>  			if (walk->pte_hole)
>>  				err = walk->pte_hole(addr, next, walk);
>>  			if (err)
>> @@ -59,9 +59,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>>  		if (!walk->pte_entry)
>>  			continue;
>>  
>> -		split_huge_pmd(walk->vma, pmd, addr);
>> -		if (pmd_trans_unstable(pmd))
>> -			goto again;
>> +		if (walk->vma) {
>> +			split_huge_pmd(walk->vma, pmd, addr);
> 
> Check for a PMD THP entry before attempting to split it ?

split_huge_pmd does the check for us:
> #define split_huge_pmd(__vma, __pmd, __address)				\
> 	do {								\
> 		pmd_t *____pmd = (__pmd);				\
> 		if (is_swap_pmd(*____pmd) || pmd_trans_huge(*____pmd)	\
> 					|| pmd_devmap(*____pmd))	\
> 			__split_huge_pmd(__vma, __pmd, __address,	\
> 						false, NULL);		\
> 	}  while (0)

And this isn't a change from the previous code - only that the entry is
no longer split when walk->vma==NULL.

>> +			if (pmd_trans_unstable(pmd))
>> +				goto again;
>> +		} else if (pmd_leaf(*pmd)) {
>> +			continue;
>> +		}
>> +
>>  		err = walk_pte_range(pmd, addr, next, walk);
>>  		if (err)
>>  			break;
>> @@ -81,7 +86,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>  	do {
>>   again:
>>  		next = pud_addr_end(addr, end);
>> -		if (pud_none(*pud) || !walk->vma) {
>> +		if (pud_none(*pud)) {
>>  			if (walk->pte_hole)
>>  				err = walk->pte_hole(addr, next, walk);
>>  			if (err)
>> @@ -95,9 +100,13 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>  				break;
>>  		}
>>  
>> -		split_huge_pud(walk->vma, pud, addr);
>> -		if (pud_none(*pud))
>> -			goto again;
>> +		if (walk->vma) {
>> +			split_huge_pud(walk->vma, pud, addr);
> 
> Check for a PUD THP entry before attempting to split it ?

Same as above.

>> +			if (pud_none(*pud))
>> +				goto again;
>> +		} else if (pud_leaf(*pud)) {
>> +			continue;
>> +		}
> 
> This is bit cryptic. walk->vma check should be inside a helper is_user_page_table()
> or similar to make things clear. p4d_leaf() check missing in walk_p4d_range() for
> kernel page table walk ? Wondering if p?d_leaf() test should be moved earlier while
> calling p?d_entry() for kernel page table walk.

I wasn't sure if it was worth putting p4d_leaf() and pgd_leaf() checks
in (yet). No architecture that I know of uses such large pages.

I'm not sure what you mean by moving the p?d_leaf() test earlier? Can
you explain with an example?

Thanks,

Steve
Anshuman Khandual Aug. 1, 2019, 6:41 a.m. UTC | #3
On 07/29/2019 05:59 PM, Steven Price wrote:
> On 28/07/2019 15:20, Anshuman Khandual wrote:
>>
>>
>> On 07/22/2019 09:12 PM, Steven Price wrote:
>>> Since 48684a65b4e3: "mm: pagewalk: fix misbehavior of walk_page_range
>>> for vma(VM_PFNMAP)", page_table_walk() will report any kernel area as
>>> a hole, because it lacks a vma.
>>>
>>> This means each arch has re-implemented page table walking when needed,
>>> for example in the per-arch ptdump walker.
>>>
>>> Remove the requirement to have a vma except when trying to split huge
>>> pages.
>>>
>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>> ---
>>>  mm/pagewalk.c | 25 +++++++++++++++++--------
>>>  1 file changed, 17 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>>> index 98373a9f88b8..1cbef99e9258 100644
>>> --- a/mm/pagewalk.c
>>> +++ b/mm/pagewalk.c
>>> @@ -36,7 +36,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>>>  	do {
>>>  again:
>>>  		next = pmd_addr_end(addr, end);
>>> -		if (pmd_none(*pmd) || !walk->vma) {
>>> +		if (pmd_none(*pmd)) {
>>>  			if (walk->pte_hole)
>>>  				err = walk->pte_hole(addr, next, walk);
>>>  			if (err)
>>> @@ -59,9 +59,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>>>  		if (!walk->pte_entry)
>>>  			continue;
>>>  
>>> -		split_huge_pmd(walk->vma, pmd, addr);
>>> -		if (pmd_trans_unstable(pmd))
>>> -			goto again;
>>> +		if (walk->vma) {
>>> +			split_huge_pmd(walk->vma, pmd, addr);
>>
>> Check for a PMD THP entry before attempting to split it ?
> 
> split_huge_pmd does the check for us:
>> #define split_huge_pmd(__vma, __pmd, __address)				\
>> 	do {								\
>> 		pmd_t *____pmd = (__pmd);				\
>> 		if (is_swap_pmd(*____pmd) || pmd_trans_huge(*____pmd)	\
>> 					|| pmd_devmap(*____pmd))	\
>> 			__split_huge_pmd(__vma, __pmd, __address,	\
>> 						false, NULL);		\
>> 	}  while (0)
> 
> And this isn't a change from the previous code - only that the entry is
> no longer split when walk->vma==NULL.

Does it make sense to name walk->vma check to differentiate between user
and kernel page tables. IMHO that will help make things clear and explicit
during page table walk.

> 
>>> +			if (pmd_trans_unstable(pmd))
>>> +				goto again;
>>> +		} else if (pmd_leaf(*pmd)) {
>>> +			continue;
>>> +		}
>>> +
>>>  		err = walk_pte_range(pmd, addr, next, walk);
>>>  		if (err)
>>>  			break;
>>> @@ -81,7 +86,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>>  	do {
>>>   again:
>>>  		next = pud_addr_end(addr, end);
>>> -		if (pud_none(*pud) || !walk->vma) {
>>> +		if (pud_none(*pud)) {
>>>  			if (walk->pte_hole)
>>>  				err = walk->pte_hole(addr, next, walk);
>>>  			if (err)
>>> @@ -95,9 +100,13 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>>>  				break;
>>>  		}
>>>  
>>> -		split_huge_pud(walk->vma, pud, addr);
>>> -		if (pud_none(*pud))
>>> -			goto again;
>>> +		if (walk->vma) {
>>> +			split_huge_pud(walk->vma, pud, addr);
>>
>> Check for a PUD THP entry before attempting to split it ?
> 
> Same as above.
> 
>>> +			if (pud_none(*pud))
>>> +				goto again;
>>> +		} else if (pud_leaf(*pud)) {
>>> +			continue;
>>> +		}
>>
>> This is bit cryptic. walk->vma check should be inside a helper is_user_page_table()
>> or similar to make things clear. p4d_leaf() check missing in walk_p4d_range() for
>> kernel page table walk ? Wondering if p?d_leaf() test should be moved earlier while
>> calling p?d_entry() for kernel page table walk.
> 
> I wasn't sure if it was worth putting p4d_leaf() and pgd_leaf() checks
> in (yet). No architecture that I know of uses such large pages.

Just to be complete it does make sense to add the remaining possible leaf
entry checks but will leave it upto you.

> 
> I'm not sure what you mean by moving the p?d_leaf() test earlier? Can
> you explain with an example?

In case its a kernel p?d_leaf() entry, then there is nothing to be done
after calling respective walk->p?d_entry() functions. Hence this check
should not complement user page table check (walk->vma) later in the
function but instead be checked right after walk->p?d_entry(). But its
not a big deal I guess.
diff mbox series

Patch

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 98373a9f88b8..1cbef99e9258 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -36,7 +36,7 @@  static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
 	do {
 again:
 		next = pmd_addr_end(addr, end);
-		if (pmd_none(*pmd) || !walk->vma) {
+		if (pmd_none(*pmd)) {
 			if (walk->pte_hole)
 				err = walk->pte_hole(addr, next, walk);
 			if (err)
@@ -59,9 +59,14 @@  static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
 		if (!walk->pte_entry)
 			continue;
 
-		split_huge_pmd(walk->vma, pmd, addr);
-		if (pmd_trans_unstable(pmd))
-			goto again;
+		if (walk->vma) {
+			split_huge_pmd(walk->vma, pmd, addr);
+			if (pmd_trans_unstable(pmd))
+				goto again;
+		} else if (pmd_leaf(*pmd)) {
+			continue;
+		}
+
 		err = walk_pte_range(pmd, addr, next, walk);
 		if (err)
 			break;
@@ -81,7 +86,7 @@  static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
 	do {
  again:
 		next = pud_addr_end(addr, end);
-		if (pud_none(*pud) || !walk->vma) {
+		if (pud_none(*pud)) {
 			if (walk->pte_hole)
 				err = walk->pte_hole(addr, next, walk);
 			if (err)
@@ -95,9 +100,13 @@  static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
 				break;
 		}
 
-		split_huge_pud(walk->vma, pud, addr);
-		if (pud_none(*pud))
-			goto again;
+		if (walk->vma) {
+			split_huge_pud(walk->vma, pud, addr);
+			if (pud_none(*pud))
+				goto again;
+		} else if (pud_leaf(*pud)) {
+			continue;
+		}
 
 		if (walk->pmd_entry || walk->pte_entry)
 			err = walk_pmd_range(pud, addr, next, walk);