linux-kernel - Re: [PATCH v3 6/9] selftests/mm: introduce helper to read every page

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Message-ID: <29fc7fb9-ec07-48f1-9883-82a44e637338@arm.com>
Date: Thu, 22 Jan 2026 17:39:27 +0000
From: Muhammad Usama Anjum <usama.anjum@....com>
To: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...nel.org>, Dev Jain <dev.jain@....com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Mark Brown
 <broonie@...nel.org>, Ryan Roberts <ryan.roberts@....com>,
 Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v3 6/9] selftests/mm: introduce helper to read every page

On 22/01/2026 5:02 pm, 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 a range of
> pages, ensuring that every page is faulted in, if it wasn't already.
> 
> Introduce a new helper force_read_pages() that does exactly that and
> replace existing loops with a call to it.
> 
> The step size (regular/huge page size) is preserved for all loops,
> except in split_huge_page_test. Reading every byte is unnecessary;
> we now read every huge page, matching the following call to
> check_huge_file().
> 
> Reviewed-by: Dev Jain <dev.jain@....com>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>   tools/testing/selftests/mm/hugetlb-madvise.c      | 9 +--------
>   tools/testing/selftests/mm/pfnmap.c               | 9 +++------
>   tools/testing/selftests/mm/split_huge_page_test.c | 6 +-----
>   tools/testing/selftests/mm/vm_util.h              | 7 +++++++
>   4 files changed, 12 insertions(+), 19 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
> index 05d9d2805ae4..5b12041fa310 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(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..45b5f1cf6019 100644
> --- a/tools/testing/selftests/mm/pfnmap.c
> +++ b/tools/testing/selftests/mm/pfnmap.c
> @@ -35,18 +35,15 @@ static void signal_handler(int sig)
>   
>   static int test_read_access(char *addr, size_t size, size_t pagesize)
>   {
> -	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(addr, size/pagesize, pagesize);
> +
>   	if (signal(SIGSEGV, SIG_DFL) == SIG_ERR)
>   		return -EINVAL;
>   
> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> index 40799f3f0213..e0167111bdd1 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(*addr, fd_size / pmd_pagesize, pmd_pagesize);
>   
>   	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..522f7f9050f5 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -54,6 +54,13 @@ static inline unsigned int pshift(void)
>   	return __page_shift;
>   }
>   
> +static inline void force_read_pages(char *addr, unsigned int nr_pages,
> +				    size_t pagesize)
> +{
> +	for (unsigned int i = 0; i < nr_pages; i++)
> +		FORCE_READ(addr[i * pagesize]);
> +}
Reviewed-by: Muhammad Usama Anjum <usama.anjum@....com>

> +
>   bool detect_huge_zeropage(void);
>   
>   /*