Re: [PATCH V6 01/16] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

Dave Hansen
2020-08-10T21:47:15Z

On 8/10/20 2:24 PM, Kan Liang wrote:
> +static u64 __perf_get_page_size(struct mm_struct *mm, unsigned long addr)
> +{
> + struct page *page;
> + 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 (defined(CONFIG_HUGETLB_PAGE) || defined(CONFIG_TRANSPARENT_HUGEPAGE))
> + if (p4d_leaf(*p4d)) {
> + page = p4d_page(*p4d);
> +
> + if (PageCompound(page))
> + return page_size(compound_head(page));
> +
> + return P4D_SIZE;
> + }
> +#endif
> +
> + pud = pud_offset(p4d, addr);
> + if (!pud_present(*pud))
> + return 0;
> +
> +#if (defined(CONFIG_HUGETLB_PAGE) || defined(CONFIG_TRANSPARENT_HUGEPAGE))
> + if (pud_leaf(*pud)) {
> + page = pud_page(*pud);
> +
> + if (PageCompound(page))
> + return page_size(compound_head(page));
> +
> + return PUD_SIZE;
> + }
> +#endif
> +
> + pmd = pmd_offset(pud, addr);
> + if (!pmd_present(*pmd))
> + return 0;
> +
> +#if (defined(CONFIG_HUGETLB_PAGE) || defined(CONFIG_TRANSPARENT_HUGEPAGE))
> + if (pmd_leaf(*pmd)) {
> + page = pmd_page(*pmd);
> +
> + if (PageCompound(page))
> + return page_size(compound_head(page));
> +
> + return PMD_SIZE;
> + }
> +#endif
> +
> + pte = pte_offset_map(pmd, addr);
> + if (!pte_present(*pte)) {
> + pte_unmap(pte);
> + return 0;
> + }
> +
> + pte_unmap(pte);
> + return PAGE_SIZE;
> +}

It's probably best if we very carefully define up front what is getting
reported here. For instance, I believe we already have some fun cases
with huge tmpfs where a compound page is mapped with 4k PTEs. Kirill
also found a few drivers doing this as well. I think there were also
some weird cases for ARM hugetlbfs where there were multiple hardware
page table entries mapping a single hugetlbfs page. These would be
cases where compound_head() size would be greater than the size of the
leaf paging structure entry.

This is also why we have KerelPageSize and MMUPageSize in /proc/$pid/smaps.

So, is this returning the kernel software page size or the MMU size?