linux-kernel - Re: [PATCH V4 01/13] perf/core, x86: Add PERF_SAMPLE_DATA_PAGE

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Date:   Fri, 1 Feb 2019 11:16:51 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     acme@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        linux-kernel@...r.kernel.org, eranian@...gle.com, jolsa@...hat.com,
        namhyung@...nel.org, ak@...ux.intel.com, luto@...capital.net,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH V4 01/13] perf/core, x86: Add PERF_SAMPLE_DATA_PAGE_SIZE



On 2/1/2019 7:43 AM, Peter Zijlstra wrote:
> On Fri, Feb 01, 2019 at 01:36:00PM +0300, Kirill A. Shutemov wrote:
>> On Fri, Feb 01, 2019 at 11:03:58AM +0100, Peter Zijlstra wrote:
> 
>>> 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.
> 
> Surely not for PageHuge() ?! I thought the point of hugetlbfs was to
> guarantee page granularity.
> 
> How is the below?
> 
> 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;
> 	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 (p4d_large(*p4d)) {

This one looks like x86 specific?

Kan

> 		struct page *page = p4d_page(*p4d);
> 		int shift = P4D_SHIFT;
> 
> 		if (PageHuge(page)) {
> 			page = compound_head(page);
> 			shift = PAGE_SHIFT + compound_order(page);
> 		}
> 
> 		return 1ULL << shift;
> 	}
> 
> 	if (!p4d_present(*p4d))
> 		return 0;
> 
> 	pud = pud_offset(p4d, addr);
> 	if (!pud_present(*pud))
> 		return 0;
> 
> 	if (pud_large(*pud)) {
> 		struct page *page = pud_page(*pud);
> 		int shift = P4D_SHIFT;
> 
> 		if (PageHuge(page)) {
> 			page = compound_head(page);
> 			shift = PAGE_SHIFT + compound_order(page);
> 		}
> 
> 		return 1ULL << shift;
> 	}
> 
> 	pmd = pmd_offset(pud, addr);
> 	if (!pmd_present(*pmd))
> 		return 0;
> 
> 	if (pmd_large(*pmd)) {
> 		struct page *page = pud_page(*pud);
> 		int shift = P4D_SHIFT;
> 
> 		if (PageHuge(page)) {
> 			page = compound_head(page);
> 			shift = PAGE_SHIFT + compound_order(page);
> 		}
> 
> 		return 1ULL << shift;
> 	}
> 
> 	pte = pte_offset_map(pmd, addr);
> 	if (!pte_present(*pte)) {
> 		pte_unmap(pte);
> 		return 0;
> 	}
> 
> 	pte_unmap(pte);
> 	return PAGE_SIZE;
> }
>