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

[Yeoreum Yun <yeoreum.yun@arm.com>]

On Thu, Aug 27, 2026 at 11:03:17AM -0400, Zi Yan wrote:
> 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@kernel.org>
> >> > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> >> > ---
> >> >   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.

Not for split_huge_page_test case but for khugepage testcase like
collapse_full_of_compound() test.

> 
> >
> > 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.

Fair enough. I'll change.

> 
> 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.

Exactly, but for some testcase where use this function, doesn't
trigger split of large _folio but only check the HUGE MAP is removed
(nr_hpage == 0), it skips the chekc_large_folio(). 

> 
> 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.
> 
> 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;

Again, because of collapse_full_of_compound() testcase,
it would be failed for this. so, it would better to skip
check_large_folioes() when nr_hpages is 0.

> 	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
> 

-- 
Sincerely,
Yeoreum Yun