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) <[email protected]>
> >> > Signed-off-by: Yeoreum Yun <[email protected]>
> >> > ---
> >> > 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
