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

[Liang, Kan]

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