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

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Date:   Wed, 11 Nov 2020 15:30:22 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...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 01:43:57PM +0100, Peter Zijlstra wrote:
> +	if (pud_leaf(pud)) {
>  #ifdef pud_page
> -		page = pud_page(*pud);
> -		if (PageHuge(page))
> -			return page_size(compound_head(page));
> +		if (!pud_devmap(pud)) {
> +			page = pud_page(pud);
> +			if (PageHuge(page))
> +				return page_size(compound_head(page));
> +		}
>  #endif
>  		return 1ULL << PUD_SHIFT;

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

Also, what's up with the special-casing of devmap pages here?  Did the
devmap people fuck up their compound pages?  If so, they should fix their
shit, not expect the rest of the kernel to work around this brokenness.