linux-kernel - Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE

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Date:   Wed, 11 Nov 2020 16:57:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>, mingo@...hat.com,
        acme@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, eranian@...gle.com, ak@...ux.intel.com,
        dave.hansen@...el.com, kirill.shutemov@...ux.intel.com,
        benh@...nel.crashing.org, paulus@...ba.org,
        David Miller <davem@...emloft.net>, vbabka@...e.cz
Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

On Wed, Nov 11, 2020 at 03:30:22PM +0000, Matthew Wilcox wrote:
> This confuses me.  Why only special-case hugetlbfs pages here?  Should
> they really be treated differently from THP?  If you want to consider
> that we might be mapping a page that's twice as big as a PUD entry and
> this is only half of it, then the simple way is:
> 
> 	if (pud_leaf(pud)) {
> #ifdef pud_page
> 		page = compound_head(pud_page(*pud));
> 		return page_size(page);

Also; this is 'wrong'. The purpose of this function is to return the
hardware TLB size of a given address. The above will return the compound
size, for any random compound page, which would be myrads of reasons.

So the PageHuge() thing tells us it is a HugeTLB page and those are
(barring hardware TLB promotion/demotion) guaranteed to reflect the
actual TLB size.

> #else
> 		return 1ULL << PUD_SHIFT;
> #endif
> 	}