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

[Peter Zijlstra]

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)) {
		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;
}