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

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