Re: [PATCH V4 01/13] perf/core, x86: Add PERF_SAMPLE_DATA_PAGE_SIZE

Liang, Kan
2019-02-01T16:16:57Z

On 2/1/2019 7:43 AM, Peter Zijlstra wrote:
> On Fri, Feb 01, 2019 at 01:36:00PM +0300, Kirill A. Shutemov wrote:
>> On Fri, Feb 01, 2019 at 11:03:58AM +0100, Peter Zijlstra wrote:
>
>>> Will just mentioned a lovely feature where some archs have multi entry
>>> large pages.
>>>
>>> Possible something like:
>>>
>>> if (pud_large(*pud)) {
>>> struct page *page = pud_page(*pud);
>>> int order = PUD_SHIFT;
>>>
>>> if (PageHuge(page)) {
>>> page = compound_head(page);
>>> order += compound_order(page);
>>> }
>>>
>>> return 1ULL << order;
>>> }
>>>
>>> works correctly.
>>
>> For more fun: some compound pages can be mapped withe page table entries
>> not matching it's compound size, i.e. 2M pages mapped with PTE.
>
> Surely not for PageHuge() ?! I thought the point of hugetlbfs was to
> guarantee page granularity.
>
> How is the below?
>
> static u64 __perf_get_page_size(struct mm_struct *mm, unsigned long addr)
> {
> pgd_t *pgd;
> p4d_t *p4d;
> pud_t *pud;
> pmd_t *pmd;
> pte_t *pte;
>
> pgd = pgd_offset(mm, addr);
> if (pgd_none(*pgd))
> return 0;
>
> p4d = p4d_offset(pgd, addr);
> if (!p4d_present(*p4d))
> return 0;
>
> if (p4d_large(*p4d)) {

This one looks like x86 specific?

Kan

> struct page *page = p4d_page(*p4d);
> int shift = P4D_SHIFT;
>
> if (PageHuge(page)) {
> page = compound_head(page);
> shift = PAGE_SHIFT + compound_order(page);
> }
>
> return 1ULL << shift;
> }
>
> if (!p4d_present(*p4d))
> return 0;
>
> pud = pud_offset(p4d, addr);
> if (!pud_present(*pud))
> return 0;
>
> if (pud_large(*pud)) {
> struct page *page = pud_page(*pud);
> int shift = P4D_SHIFT;
>
> if (PageHuge(page)) {
> page = compound_head(page);
> shift = PAGE_SHIFT + compound_order(page);
> }
>
> return 1ULL << shift;
> }
>
> pmd = pmd_offset(pud, addr);
> if (!pmd_present(*pmd))
> return 0;
>
> if (pmd_large(*pmd)) {
> struct page *page = pud_page(*pud);
> int shift = P4D_SHIFT;
>
> if (PageHuge(page)) {
> page = compound_head(page);
> shift = PAGE_SHIFT + compound_order(page);
> }
>
> return 1ULL << shift;
> }
>
> pte = pte_offset_map(pmd, addr);
> if (!pte_present(*pte)) {
> pte_unmap(pte);
> return 0;
> }
>
> pte_unmap(pte);
> return PAGE_SIZE;
> }
>