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

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Date:   Wed, 11 Nov 2020 10:57:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     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>
Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

On Wed, Nov 04, 2020 at 12:11:16PM -0500, Liang, Kan wrote:
> On 10/13/2020 12:34 PM, Peter Zijlstra wrote:

> > @@ -7037,13 +7057,20 @@ static u64 __perf_get_page_size(struct m
> >   		return 0;
> >   	}
> > +	page = pte_page(*pte);
> > +	if (PageHuge(page)) {
> > +		u64 size = page_size(compound_head(page));
> > +		pte_unmap(pte);
> > +		return size;
> > +	}
> > +
> 
> The PageHuge() check for PTE crashes my machine when I did page size test.
> (Sorry, I didn't find the issue earlier. I just found some time to re-run
> the page size test.)
> 
> It seems we don't need the check for PTE here. The size should be always
> PAGE_SIZE, no? After I remove the check, everything looks good.

That's the thing, an architecture could have non-page-table aligned
huge-pages. For example using 4 consecutive 4k pages to create 16k
pages. In that case the above code would trigger and find a 16k compound
page with HUGETLB_PAGE_DTOR (assuming it was created through hugetlbfs).

What is this page size test; I'd like to reproduce.