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Date:   Wed, 11 Nov 2020 16:52:51 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        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>, vbabka@suse.cz
Subject: Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE
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On Wed, Nov 11, 2020 at 03:30:22PM +0000, Matthew Wilcox wrote:
> 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?

Do we have non-pagetable aligned THP ? I thought THP was always PUD
sized.

> 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.

Well, the PTE code we have today (in tip/perf/core) is:

	pte = pte_offset_map(pmd, addr);
	if (!pte_present(*pte)) {
		pte_unmap(pte);
		return 0;
	}

	page = pte_page(*pte);
	if (PageHuge(page)) {
		u64 size = page_size(compound_head(page));
		pte_unmap(pte);
		return size;
	}

	pte_unmap(pte);
	return PAGE_SIZE;

and that's crashing in PageHuge()'s PageCompound() test. Clearly I
should be checking pte_special() here (as well as all the READ_ONCE()s I
added in the patch you just commented on). But I wasn't quite sure about
devmap and paranoia won.

You're saying devmap should be valid compound pages? Then I can remove
all that and only keep pte_special().