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

[Anshuman Khandual <anshuman.khandual@arm.com>]

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.