Re: [PATCH v2 2/2] kselftest: mm: replace usage of /proc/self/smaps for check_huge_xxx() helper

[Zi Yan]

On Thu Aug 27, 2026 at 6:44 AM EDT, Yeoreum Yun wrote:
> Hi Baolin,
>
>> 
>> 
>> On 8/26/26 8:24 PM, Yeoreum Yun wrote:
>> > Since glibc commit 321e1fc73f (“malloc: Enable 2MB THP by default on AArch64”),
>> > glibc may call madvise(MADV_HUGEPAGE) for sufficiently large allocations
>> > made by memalign().
>> > 
>> > The underlying VMA may start at a different address from the aligned
>> > address returned by memalign(). Furthermore, a subsequent
>> > madvise(MADV_HUGEPAGE) call does not split the VMA because the flag is
>> > already set.
>> > 
>> > This causes split_huge_page_test to fail because the check_huge_xxx()
>> > helpers incorrectly require the address returned by memalign() to
>> > match the VMA start address reported in /proc/self/smaps.
>> > 
>> > Fix this by using /proc/self/pagemap and /proc/kpageflags instead of
>> > /proc/self/smaps to detect huge pages.
>> > 
>> > Reported-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>> > Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
>> > ---
>> >   tools/testing/selftests/mm/vm_util.c | 130 ++++++++++++++++++++---------------
>> >   tools/testing/selftests/mm/vm_util.h |   1 +
>> >   2 files changed, 77 insertions(+), 54 deletions(-)
>> > 
>> > diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
>> > index 4821a3563036..1d0959b3b9e8 100644
>> > --- a/tools/testing/selftests/mm/vm_util.c
>> > +++ b/tools/testing/selftests/mm/vm_util.c
>> > @@ -351,31 +351,13 @@ char *__get_smap_entry(void *addr, const char *pattern, char *buf, size_t len)
>> >   	return entry;
>> >   }
>> > -static bool __check_pmd_huge(void *addr, char *pattern, int nr_hpages,
>> > -		  uint64_t hpage_size)
>> > -{
>> > -	char buffer[MAX_LINE_LENGTH];
>> > -	uint64_t thp = -1;
>> > -	char *entry;
>> > -
>> > -	entry = __get_smap_entry(addr, pattern, buffer, sizeof(buffer));
>> > -	if (!entry)
>> > -		goto err_out;
>> > -
>> > -	if (sscanf(entry, "%9" SCNu64 " kB", &thp) != 1)
>> > -		ksft_exit_fail_msg("Reading smap error\n");
>> > -
>> > -err_out:
>> > -	return thp == (nr_hpages * (hpage_size >> 10));
>> > -}
>> > -
>> > -static bool check_large_folios(void *addr, size_t len, int nr_hpages,
>> > -		uint64_t hpage_size)
>> > +static bool check_large_folios(int pagemap_fd, int kpageflags_fd,
>> > +			       void *addr, size_t len, int nr_hpages,
>> > +			       uint64_t hpage_size)
>> >   {
>> >   	int order = 0, pagesize = getpagesize();
>> >   	unsigned int nr_pages = hpage_size / pagesize;
>> >   	int orders[MAX_NR_ORDERS], status;
>> > -	int pagemap_fd, kpageflags_fd;
>> >   	bool ret = false;
>> >   	if (!nr_pages)
>> > @@ -386,15 +368,6 @@ static bool check_large_folios(void *addr, size_t len, int nr_hpages,
>> >   		ksft_exit_fail_msg("invalid order\n");
>> >   	memset(orders, 0, sizeof(int) * MAX_NR_ORDERS);
>> > -	pagemap_fd = open(PAGEMAP_PATH, O_RDONLY);
>> > -	if (pagemap_fd == -1)
>> > -		ksft_exit_fail_msg("read pagemap fail\n");
>> > -
>> > -	kpageflags_fd = open(KPAGEFLAGS_PATH, O_RDONLY);
>> > -	if (kpageflags_fd == -1) {
>> > -		close(pagemap_fd);
>> > -		ksft_exit_fail_msg("read kpageflags fail\n");
>> > -	}
>> >   	status = gather_folio_orders(addr, len, pagemap_fd,
>> >   			kpageflags_fd, orders, MAX_NR_ORDERS);
>> > @@ -405,48 +378,97 @@ static bool check_large_folios(void *addr, size_t len, int nr_hpages,
>> >   		ret = true;
>> >   out:
>> > -	close(pagemap_fd);
>> > -	close(kpageflags_fd);
>> >   	return ret;
>> >   }
>> > -bool check_huge_anon(void *addr, size_t len, int nr_hpages, uint64_t hpage_size)
>> > +enum check_huge_type {
>> > +	CHECK_HUGE_ANON,
>> > +	CHECK_HUGE_FILE,
>> > +	CHECK_HUGE_SHMEM,
>> > +};
>> > +
>> > +static bool __check_pmd_huge(void *addr, size_t len, int nr_hpages,
>> > +			     uint64_t hpage_size, enum check_huge_type type)
>> 
>> The original __check_pmd_huge() is only for PMD-sized large folios, but now
>> it not only checks PMD-sized large folios but also mTHP large folios, which
>> I find confusing. Please keep its original semantics, and only check
>> PMD-sized large folios.
>
> But, It seems to valuable to check other page-flags than checking
> the large-folio only.
>
>> 
>> >   {
>> > -	uint64_t pmd_pagesize = read_pmd_pagesize();
>> > +	int pagemap_fd, kpageflags_fd;
>> > +	uint64_t pmd_pagesize, granule;
>> > +	uint64_t categories, kpf;
>> > +	unsigned long pfn;
>> > +	bool check_large, huge_mapped;
>> > +	char *start = addr;
>> > +	char *end = start + len;
>> > +	pmd_pagesize = read_pmd_pagesize();
>> >   	if (!pmd_pagesize)
>> >   		ksft_exit_fail_msg("reading PMD pagesize failed\n");
>> > -	if (hpage_size == pmd_pagesize)
>> > -		return __check_pmd_huge(addr, "AnonHugePages: ", nr_hpages, hpage_size);
>> > +	if (nr_hpages > 0) {
>> > +		check_large = true;
>> > +		granule = hpage_size;
>> > +	} else {
>> > +		check_large = false;
>> > +		granule = psize();
>> > +	}
>> 
>> This is incorrect for the mTHP large folio check. I already hit a selftest
>> failure. Please test your patches before sending them out.
>
> Since for a split case, large folio can be as-is but only remove
> the PMD mapping only, skipping the large_folio checking seems valid
> when nr_hpage is 0.

What split care are you referring to? split_huge_page_test() always
splits the folio.

>
> And the failure of test seems because of unmapped area after
> changinng the collapse-order. Therefore, it seems to fine with below
> patch:
>
> ---------------&<----------------------
>
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index 1d0959b3b9e8..f174a76d2310 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -387,14 +387,14 @@ enum check_huge_type {
>         CHECK_HUGE_SHMEM,
>  };
>
> -static bool __check_pmd_huge(void *addr, size_t len, int nr_hpages,
> -                            uint64_t hpage_size, enum check_huge_type type)
> +static bool __check_huge(void *addr, size_t len, int nr_hpages,
> +                        uint64_t hpage_size, enum check_huge_type type)
>  {
>         int pagemap_fd, kpageflags_fd;
>         uint64_t pmd_pagesize, granule;
>         uint64_t categories, kpf;
>         unsigned long pfn;
> -       bool check_large, huge_mapped;
> +       bool check_large, check_huge_mapped, allow_nomap;
>         char *start = addr;
>         char *end = start + len;
>
> @@ -405,11 +405,20 @@ static bool __check_pmd_huge(void *addr, size_t len, int nr_hpages,
>         if (nr_hpages > 0) {
>                 check_large = true;
>                 granule = hpage_size;
> +               if (granule == pmd_pagesize)
> +                       check_huge_mapped = true;
> +               else
> +                       check_huge_mapped = false;
>         } else {
>                 check_large = false;
>                 granule = psize();
>         }

The else is for nr_hpages == 0? But it looks like we allow negative
nr_hpages. Maybe add a bool expect_huge = nr_hpages > 0 to make it
explicit.

granule is an optimization for PAGE_IS_HUGE scanning? When we expect a
PMD mapping, we just scan at pmd_pagesize granularity, otherwise we
check every single page?

At the high level, the function looks good to me, the rules are:

1. if hpage_size == pmd_pagesize, we need to check PAGE_IS_HUGE and
check_large_folio() can be skipped, since we only care about mappings.
This checks for PMD mappings.

2. in other cases, check_large_folios() is always needed. This is for
mTHP checks.


I think the ifs at the beginning is confusing. Can we do something like
below to get rid of the ifs? I also moved KPF_* checks in a separate
function. Feel free to make changes if you find any issue there.

Thanks.



static bool check_huge_type(uint64_t categories, uint64_t kpageflags,
			    enum check_huge_type type)
{
	bool file = categories & PAGE_IS_FILE;
	bool swapbacked = kpageflags & KPF_SWAPBACKED;

	switch (type) {
	case CHECK_HUGE_ANON:
		return !file;
	case CHECK_HUGE_FILE:
		return file && !swapbacked;
	case CHECK_HUGE_SHMEM:
		return file && swapbacked;
	}

	return false;
}

static bool __check_huge(void *addr, size_t len, int nr_hpages,
			 uint64_t hpage_size, enum check_huge_type type)
{
	int pagemap_fd, kpageflags_fd;
	int nr_pmd_mappings = 0;
	uint64_t pmd_pagesize, scan_mapping_size;
	uint64_t categories, kpf;
	unsigned long pfn;
	bool check_pmd_mapping;
	bool allow_nonpresent;
	bool ret = false;
	char *start = addr;
	char *end = start + len;

	pmd_pagesize = read_pmd_pagesize();
	if (!pmd_pagesize)
		ksft_exit_fail_msg("reading PMD pagesize failed\n");

	check_pmd_mapping = hpage_size == pmd_pagesize;
	scan_mapping_size = nr_hpages > 0 ? hpage_size : psize();
	/* Some mTHP tests check a partially populated PMD-sized range. */
	allow_nonpresent = (uint64_t)nr_hpages * hpage_size < len;

	pagemap_fd = open(PAGEMAP_PATH, O_RDONLY);
	if (pagemap_fd < 0)
		ksft_exit_fail_msg("open pagemap fail\n");

	kpageflags_fd = open(KPAGEFLAGS_PATH, O_RDONLY);
	if (kpageflags_fd < 0) {
		close(pagemap_fd);
		ksft_exit_fail_msg("open kpageflags fail\n");
	}

	/* PTE-mapped large folios cannot be identified by PAGE_IS_HUGE. */
	if (!check_pmd_mapping &&
	    !check_large_folios(pagemap_fd, kpageflags_fd, addr, len,
				 nr_hpages, hpage_size))
		goto out;

	for (; start < end; start += scan_mapping_size) {
		categories = pagemap_scan_get_categories(pagemap_fd, start);
		pfn = pagemap_get_pfn(pagemap_fd, start);
		if (pfn == -1UL) {
			if (!allow_nonpresent)
				goto out;
			continue;
		}
		if (pageflags_get(pfn, kpageflags_fd, &kpf))
			ksft_exit_fail_msg("read kpageflags: %s\n", strerror(errno));
		if (check_pmd_mapping && (categories & PAGE_IS_HUGE))
			nr_pmd_mappings++;
		if (kpf & KPF_COMPOUND_TAIL)
			continue;
		if (!check_huge_type(categories, kpf, type))
			goto out;
	}
	if (check_pmd_mapping && nr_pmd_mappings != nr_hpages)
		goto out;
	ret = true;

out:
	close(pagemap_fd);
	close(kpageflags_fd);
	return ret;
}

>
> +       if (nr_hpages * hpage_size < len)
> +               allow_nomap = true;
> +       else
> +               allow_nomap = false;
> +
>         pagemap_fd = open(PAGEMAP_PATH, O_RDONLY);
>         if (pagemap_fd < 0)
>                 ksft_exit_fail_msg("open pagemap fail\n");
> @@ -428,18 +437,15 @@ static bool __check_pmd_huge(void *addr, size_t len, int nr_hpages,
>                 categories = pagemap_scan_get_categories(pagemap_fd, start);
>                 pfn = pagemap_get_pfn(pagemap_fd, start);
>                 if (pfn == -1UL) {
> -                       if (check_large)
> +                       if (!allow_nomap)
>                                 goto out;
>                         else
>                                 continue;
>                 }
>                 if (pageflags_get(pfn, kpageflags_fd, &kpf))
>                         ksft_exit_fail_msg("read kpageflags: %s\n", strerror(errno));
> -               huge_mapped = categories & PAGE_IS_HUGE;
> -               if (check_large != huge_mapped) {
> -                       if (!check_large || granule == pmd_pagesize)
> +               if (check_huge_mapped != !!(categories & PAGE_IS_HUGE))
>                                 goto out;
> -               }
>                 if (kpf & KPF_COMPOUND_TAIL)
>                         continue;
>                 if (!!(categories & PAGE_IS_FILE) != (type != CHECK_HUGE_ANON))





> @@ -458,17 +464,17 @@ static bool __check_pmd_huge(void *addr, size_t len, int nr_hpages,
>
>  bool check_huge_anon(void *addr, size_t len, int nr_hpages, uint64_t hpage_size)
>  {
> -       return __check_pmd_huge(addr, len, nr_hpages, hpage_size, CHECK_HUGE_ANON);
> +       return __check_huge(addr, len, nr_hpages, hpage_size, CHECK_HUGE_ANON);
>  }
>
>  bool check_huge_file(void *addr, size_t len, int nr_hpages, uint64_t hpage_size)
>  {
> -       return __check_pmd_huge(addr, len, nr_hpages, hpage_size, CHECK_HUGE_FILE);
> +       return __check_huge(addr, len, nr_hpages, hpage_size, CHECK_HUGE_FILE);
>  }
>
>  bool check_huge_shmem(void *addr, size_t len, int nr_hpages, uint64_t hpage_size)
>  {
> -       return __check_pmd_huge(addr, len, nr_hpages, hpage_size, CHECK_HUGE_SHMEM);
> +       return __check_huge(addr, len, nr_hpages, hpage_size, CHECK_HUGE_SHMEM);
>  }
>
>
> If it's good for you, I'll repost again.
>
> Thanks!




-- 
Best Regards,
Yan, Zi