Re: [PATCH v2 5/8] selftests/mm: introduce helper to read every page in range

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 19/01/2026 11:55, David Hildenbrand (Red Hat) wrote:
> On 1/7/26 17:48, Kevin Brodsky wrote:
>> FORCE_READ(*addr) ensures that the compiler will emit a load from
>> addr. Several tests need to trigger such a load for every page in
>> the range [addr, addr + len), ensuring that every page is faulted
>> in, if it wasn't already.
>>
>> Introduce a new helper force_read_pages_in_range() that does exactly
>> that and replace existing loops with a call to it. Some of those
>> loops have a different step size, but reading from every page is
>> appropriate in all cases.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
>> ---
>>   tools/testing/selftests/mm/hugetlb-madvise.c     |  9 +--------
>>   tools/testing/selftests/mm/pfnmap.c              | 16 ++++++----------
>>   .../testing/selftests/mm/split_huge_page_test.c  |  6 +-----
>>   tools/testing/selftests/mm/vm_util.h             |  6 ++++++
>>   4 files changed, 14 insertions(+), 23 deletions(-)
>>
>> diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c
>> b/tools/testing/selftests/mm/hugetlb-madvise.c
>> index 05d9d2805ae4..1f82568ae262 100644
>> --- a/tools/testing/selftests/mm/hugetlb-madvise.c
>> +++ b/tools/testing/selftests/mm/hugetlb-madvise.c
>> @@ -47,14 +47,7 @@ void write_fault_pages(void *addr, unsigned long
>> nr_pages)
>>     void read_fault_pages(void *addr, unsigned long nr_pages)
>>   {
>> -    unsigned long i;
>> -
>> -    for (i = 0; i < nr_pages; i++) {
>> -        unsigned long *addr2 =
>> -            ((unsigned long *)(addr + (i * huge_page_size)));
>> -        /* Prevent the compiler from optimizing out the entire loop: */
>> -        FORCE_READ(*addr2);
>> -    }
>> +    force_read_pages_in_range(addr, nr_pages * huge_page_size);
>>   }
>>     int main(int argc, char **argv)
>> diff --git a/tools/testing/selftests/mm/pfnmap.c
>> b/tools/testing/selftests/mm/pfnmap.c
>> index f546dfb10cae..35b0e3ed54cd 100644
>> --- a/tools/testing/selftests/mm/pfnmap.c
>> +++ b/tools/testing/selftests/mm/pfnmap.c
>> @@ -33,20 +33,17 @@ static void signal_handler(int sig)
>>       siglongjmp(sigjmp_buf_env, -EFAULT);
>>   }
>>   -static int test_read_access(char *addr, size_t size, size_t pagesize)
>> +static int test_read_access(char *addr, size_t size)
>>   {
>> -    size_t offs;
>>       int ret;
>>         if (signal(SIGSEGV, signal_handler) == SIG_ERR)
>>           return -EINVAL;
>>         ret = sigsetjmp(sigjmp_buf_env, 1);
>> -    if (!ret) {
>> -        for (offs = 0; offs < size; offs += pagesize)
>> -            /* Force a read that the compiler cannot optimize out. */
>> -            *((volatile char *)(addr + offs));
>> -    }
>> +    if (!ret)
>> +        force_read_pages_in_range(addr, size);
>> +
>>       if (signal(SIGSEGV, SIG_DFL) == SIG_ERR)
>>           return -EINVAL;
>>   @@ -138,7 +135,7 @@ FIXTURE_SETUP(pfnmap)
>>           SKIP(return, "Invalid file: '%s'. Not pfnmap'ed\n", file);
>>         /* ... and want to be able to read from them. */
>> -    if (test_read_access(self->addr1, self->size1, self->pagesize))
>> +    if (test_read_access(self->addr1, self->size1))
>>           SKIP(return, "Cannot read-access mmap'ed '%s'\n", file);
>>         self->size2 = 0;
>> @@ -243,8 +240,7 @@ TEST_F(pfnmap, fork)
>>       ASSERT_GE(pid, 0);
>>         if (!pid) {
>> -        EXPECT_EQ(test_read_access(self->addr1, self->size1,
>> -                       self->pagesize), 0);
>> +        EXPECT_EQ(test_read_access(self->addr1, self->size1), 0);
>>           exit(0);
>>       }
>>   diff --git a/tools/testing/selftests/mm/split_huge_page_test.c
>> b/tools/testing/selftests/mm/split_huge_page_test.c
>> index 40799f3f0213..65a89ceca4a5 100644
>> --- a/tools/testing/selftests/mm/split_huge_page_test.c
>> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
>> @@ -652,11 +652,7 @@ static int create_pagecache_thp_and_fd(const
>> char *testfile, size_t fd_size,
>>       }
>>       madvise(*addr, fd_size, MADV_HUGEPAGE);
>>   -    for (size_t i = 0; i < fd_size; i++) {
>> -        char *addr2 = *addr + i;
>> -
>> -        FORCE_READ(*addr2);
>> -    }
>> +    force_read_pages_in_range(*addr, fd_size);
>>         if (!check_huge_file(*addr, fd_size / pmd_pagesize,
>> pmd_pagesize)) {
>>           ksft_print_msg("No large pagecache folio generated, please
>> provide a filesystem supporting large folio\n");
>> diff --git a/tools/testing/selftests/mm/vm_util.h
>> b/tools/testing/selftests/mm/vm_util.h
>> index 6ad32b1830f1..74bdf96161d7 100644
>> --- a/tools/testing/selftests/mm/vm_util.h
>> +++ b/tools/testing/selftests/mm/vm_util.h
>> @@ -54,6 +54,12 @@ static inline unsigned int pshift(void)
>>       return __page_shift;
>>   }
>>   +static inline void force_read_pages_in_range(char *addr, size_t len)
>> +{
>> +    for (size_t i = 0; i < len; i += psize())
>> +        FORCE_READ(addr[i]);
>> +}
>> +
>
> Of course, we could also just pass the pagesize
>
> static inline void force_read_pages_in_range(char *addr, size_t len,
>     size_t pagesize)
> {
>     for (size_t i = 0; i < len; i += pagesize)
>         FORCE_READ(addr[i]);
> }
>
>
> Or alternatively even better:
>
> static inline void force_read_pages(char *addr, unsigned int nr_pages,
>     size_t pagesize)
> {
>     for (size_t i = 0; i < nr_pages; i++)
>         FORCE_READ(addr[i * pagesize]);
> }
>
> Then there is no change at all and we avoid the repeated psize()
> naturally.
>
> Thoughts?

Sure we can do that. I concluded that all tests fetch the pagesize in a
different way and some aren't using psize() sparingly either (e.g.
gup_test) but I suppose that avoiding some ugliness doesn't hurt :)

Of course this especially makes sense for the hugepage tests (then we
can still read hugepage by hugepage).

- Kevin