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From:   "Alex Zhu (Kernel)" <alexlzhu@fb.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Kernel Team <Kernel-team@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>
Subject: Re: [PATCH v2] mm: add thp_utilization metrics to debugfs
Thread-Topic: [PATCH v2] mm: add thp_utilization metrics to debugfs
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Date:   Wed, 10 Aug 2022 19:14:11 +0000
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> On Aug 9, 2022, at 6:33 AM, Matthew Wilcox <willy@infradead.org> wrote:
>=20
> >=20
> On Mon, Aug 08, 2022 at 06:49:50PM -0700, alexlzhu@fb.com wrote:
>> +int thp_number_utilized_pages(struct page *page)
>> +{
>> +	unsigned long page_index, page_offset, value;
>> +	int thp_nr_utilized_pages =3D HPAGE_PMD_NR;
>> +	int step_size =3D sizeof(unsigned long);
>> +	bool is_all_zeroes;
>> +	void *kaddr;
>> +
>> +	if (!page || !is_anon_transparent_hugepage(page))
>> +		return -1;
>> +
>> +	kaddr =3D kmap_local_page(compound_head(page));
>> +	for (page_index =3D 0; page_index < HPAGE_PMD_NR; page_index++) {
>> +		is_all_zeroes =3D true;
>> +		for (page_offset =3D 0; page_offset < PAGE_SIZE; page_offset +=3D ste=
p_size) {
>> +			value =3D *(unsigned long *)(kaddr + page_index * PAGE_SIZE + page_o=
ffset);
>> +			if (value !=3D 0) {
>> +				is_all_zeroes =3D false;
>> +				break;
>> +			}
>> +		}
>> +		if (is_all_zeroes)
>> +			thp_nr_utilized_pages--;
>> +	}
>> +	kunmap_local(kaddr);
>=20
> You haven't tested this on 32-bit maachines with HIGHMEM, have you?
> kmap_local_page() only maps PAGE_SIZE bytes, so you need to map it and
> unmap each page.  Also, why are you calling compound_head()?  Surely
> you're already assuming that page is a head page here?
>=20
> (this is why I introduced the struct folio type, so you know whether
> a pointer is already to a head page and you don't need to call
> compound_head() again).
>=20

I see. Thanks for catching this! I have reimplemented with folio in a way t=
hat will work on 32 bit
architectures with HIGHMEM. Will send out v3.=20