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 04:28:01PM +0800, Baolin Wang wrote:
> 
> 
> 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.

Okay. thanks!

> >   {
> > -	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.
> 
> [root@]./khugepaged -c 4 mthp_khugepaged:anon
> TAP version 13
> # Save THP and khugepaged settings... OK
> 1..4
> # Allocate huge page on fault... OK
> # Split huge PMD on MADV_DONTNEED... OK
> ok 1 allocate on fault and split
> #
> # Run test: collapse_full (mthp_khugepaged:anon)
> # Collapse multiple fully populated PTE table.... OK
> ok 2 collapse_full
> #
> # Run test: collapse_empty (mthp_khugepaged:anon)
> # Do not collapse empty PTE table.... OK
> ok 3 collapse_empty
> #
> # Run test: collapse_single_mthp (mthp_khugepaged:anon)
> # Collapse PTE table with half PTE entries present.... Fail
> not ok 4 collapse_single_mthp
> # Totals: pass:3 fail:1 xfail:0 xpass:0 skip:0 error:0

Oh sorry. I've forgotten to this case to test.

Thanks!

 

-- 
Sincerely,
Yeoreum Yun