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

["Liang, Kan" <kan.liang@linux.intel.com>]

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