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

Liang, Kan
2020-08-10T22:38:40Z

On 8/10/2020 5:47 PM, Dave Hansen wrote:
> 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?
>

This tries to return the kernel software page size. I will add a commit
to the function. For the above cases, I think they can be detected by
PageCompound(page). The current code should already cover them. Is my
understanding correct?

Thanks,
Kan