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

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

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?

-- 
Cheers

David