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

Kirill A. Shutemov
2019-02-01T10:36:08Z

On Fri, Feb 01, 2019 at 11:03:58AM +0100, Peter Zijlstra wrote:
> On Fri, Feb 01, 2019 at 10:22:40AM +0100, Peter Zijlstra wrote:
>
> > +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;
> > +
> > + pgd = pgd_offset(mm, addr);
> > + if (pgd_none(*pgd))
> > + return 0;
> > +
> > + p4d = p4d_offset(pgd, addr);
> > + if (p4d_none(*p4d))
> > + return 0;
> > +
> > + if (p4d_large(*p4d));
> > + return 1ULL << P4D_SHIFT;
> > +
> > + if (!p4d_present(*p4d))
> > + return 0;
> > +
> > + pud = pud_offset(p4d, addr);
> > + if (pud_none(*pud))
> > + return 0;
> > +
> > + if (pud_large(*pud))
> > + return 1ULL << PUD_SHIFT;
>
> 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.

--
Kirill A. Shutemov