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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/27/26 10:28, 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.
> 
>>   {
>> -    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

FWIW, the CI flags this as well:

https://github.com/linux-mm/linux-mm/actions/runs/33028595020/job/98375618491

-- 
Cheers,

David