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Date:   Wed, 11 Nov 2020 10:57:50 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Liang, Kan" <kan.liang@linux.intel.com>
Cc:     Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>, mingo@redhat.com,
        acme@kernel.org, linux-kernel@vger.kernel.org,
        mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
        jolsa@redhat.com, eranian@google.com, ak@linux.intel.com,
        dave.hansen@intel.com, kirill.shutemov@linux.intel.com,
        benh@kernel.crashing.org, paulus@samba.org,
        David Miller <davem@davemloft.net>
Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
Message-ID: <20201111095750.GS2594@hirez.programming.kicks-ass.net>
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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.