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

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Message-ID: <AAB35EEE-D506-4DE1-9AD6-534EAAE68EC0@nvidia.com>
Date: Mon, 11 Aug 2025 14:40:03 -0400
From: Zi Yan <ziy@...dia.com>
To: Donet Tom <donettom@...ux.ibm.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 10 Aug 2025, at 12:49, Donet Tom wrote:

> On 8/9/25 12:31 AM, Zi Yan wrote:
>> The helper gathers an folio order statistics of folios within a virtual
>> address range and checks it against a given order list. It aims to provide
>> a more precise folio order check instead of just checking the existence of
>> PMD folios.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>   .../selftests/mm/split_huge_page_test.c       |   4 +-
>>   tools/testing/selftests/mm/vm_util.c          | 133 ++++++++++++++++++
>>   tools/testing/selftests/mm/vm_util.h          |   7 +
>>   3 files changed, 141 insertions(+), 3 deletions(-)
>>
>> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
>> index cb364c5670c6..5ab488fab1cd 100644
>> --- a/tools/testing/selftests/mm/split_huge_page_test.c
>> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
>> @@ -34,8 +34,6 @@ uint64_t pmd_pagesize;
>>   #define PID_FMT_OFFSET "%d,0x%lx,0x%lx,%d,%d"
>>   #define PATH_FMT "%s,0x%lx,0x%lx,%d"
>>  -#define PFN_MASK     ((1UL<<55)-1)
>> -#define KPF_THP      (1UL<<22)
>>   #define GET_ORDER(nr_pages)    (31 - __builtin_clz(nr_pages))
>>    int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file)
>> @@ -49,7 +47,7 @@ int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file)
>>    		if (kpageflags_file) {
>>   			pread(kpageflags_file, &page_flags, sizeof(page_flags),
>> -				(paddr & PFN_MASK) * sizeof(page_flags));
>> +				PAGEMAP_PFN(paddr) * sizeof(page_flags));
>>    			return !!(page_flags & KPF_THP);
>>   		}
>> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
>> index 6a239aa413e2..41d50b74b2f6 100644
>> --- a/tools/testing/selftests/mm/vm_util.c
>> +++ b/tools/testing/selftests/mm/vm_util.c
>> @@ -338,6 +338,139 @@ int detect_hugetlb_page_sizes(size_t sizes[], int max)
>>   	return count;
>>   }
>>  +static int get_page_flags(char *vaddr, int pagemap_file, int kpageflags_file,
>> +			  uint64_t *flags)
>> +{
>> +	unsigned long pfn;
>> +	size_t count;
>> +
>> +	pfn = pagemap_get_pfn(pagemap_file, vaddr);
>> +	/*
>> +	 * Treat non-present page as a page without any flag, so that
>> +	 * gather_folio_orders() just record the current folio order.
>> +	 */
>> +	if (pfn == -1UL) {
>> +		*flags = 0;
>> +		return 0;
>> +	}
>> +
>> +	count = pread(kpageflags_file, flags, sizeof(*flags),
>> +		      pfn * sizeof(*flags));
>> +
>> +	if (count != sizeof(*flags))
>> +		return -1;
>> +
>> +	return 0;
>> +}
>> +
>> +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;
>
> If KPF_COMPOUND_TAIL is set, do we use the same vaddr, or should we advance to the next vaddr before continuing?

Yeah, I missed a vaddr += psize() here. Thank you for pointing this out.

Best Regards,
Yan, Zi