linux-kernel - Re: [PATCH v2 2/3] selftests/mm: add check_folio

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Message-ID: <20250811212802.gsvbeqmo62v4tqly@master>
Date: Mon, 11 Aug 2025 21:28:02 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: Wei Yang <richard.weiyang@...il.com>, wang lian <lianux.mm@...il.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>, Shuah Khan <shuah@...nel.org>,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 2/3] selftests/mm: add check_folio_orders() helper.

On Mon, Aug 11, 2025 at 02:39:08PM -0400, Zi Yan wrote:
[...]
>>> +static int gather_folio_orders(char *vaddr_start, size_t len,
>>> +			       int pagemap_file, int kpageflags_file,
>>> +			       int orders[], int nr_orders)
>>> +{
>>> +	uint64_t page_flags = 0;
>>> +	int cur_order = -1;
>>> +	char *vaddr;
>>> +
>>> +	if (!pagemap_file || !kpageflags_file)
>>> +		return -1;
>>> +	if (nr_orders <= 0)
>>> +		return -1;
>>> +
>>> +	for (vaddr = vaddr_start; vaddr < vaddr_start + len; ) {
>>> +		char *next_folio_vaddr;
>>> +		int status;
>>> +
>>> +		if (get_page_flags(vaddr, pagemap_file, kpageflags_file, &page_flags))
>>> +			return -1;
>>> +
>>> +		/* all order-0 pages with possible false postive (non folio) */
>>> +		if (!(page_flags & (KPF_COMPOUND_HEAD | KPF_COMPOUND_TAIL))) {
>>> +			orders[0]++;
>>> +			vaddr += psize();
>>> +			continue;
>>> +		}
>>> +
>>> +		/* skip non thp compound pages */
>>> +		if (!(page_flags & KPF_THP)) {
>>> +			vaddr += psize();
>>> +			continue;
>>> +		}
>>> +
>>> +		/* vpn points to part of a THP at this point */
>>> +		if (page_flags & KPF_COMPOUND_HEAD)
>>> +			cur_order = 1;
>>> +		else {
>>> +			/* not a head nor a tail in a THP? */
>>> +			if (!(page_flags & KPF_COMPOUND_TAIL))
>>> +				return -1;
>>> +			continue;
>>> +		}
>>> +
>>> +		next_folio_vaddr = vaddr + (1UL << (cur_order + pshift()));
>>> +
>>> +		if (next_folio_vaddr >= vaddr_start + len)
>>> +			break;
>>
>> Would we skip order 1 folio at the last position?
>>
>> For example, vaddr_start is 0x2000, len is 0x2000 and the folio at vaddr_start
>> is an order 1 folio, whose size is exactly 0x2000.
>>
>> Then we will get next_folio_vaddr == vaddr_start + len.
>>
>> Could that happen?
>
>No. After the loop, there is code checking cur_order and updating orders[].
>

Oh, I missed this.

-- 
Wei Yang
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