linux-kernel - Re: [PATCH 3/4] selftests/mm: fix faulting-in code in pagemap

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Message-ID: <37210500-6f6e-46ac-ac2f-ac996308590d@arm.com>
Date: Tue, 16 Dec 2025 14:56:58 +0000
From: Ryan Roberts <ryan.roberts@....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>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Mark Brown
 <broonie@...nel.org>, Shuah Khan <shuah@...nel.org>,
 Usama Anjum <Usama.Anjum@....com>
Subject: Re: [PATCH 3/4] selftests/mm: fix faulting-in code in pagemap_ioctl
 test

On 16/12/2025 14:26, Kevin Brodsky wrote:
> One of the pagemap_ioctl tests attempts to fault in pages by
> memcpy()'ing them to an unused buffer. This probably worked
> originally, but since commit 46036188ea1f ("selftests/mm: build with
> -O2") the compiler is free to optimise away that unused buffer and
> the memcpy() with it. As a result there might not be any resident
> page in the mapping and the test may fail.
> 
> We don't need to copy all that memory anyway. Just fault in every
> page by forcing the compiler to read the first byte.
> 
> Cc: Usama Anjum <Usama.Anjum@....com>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>  tools/testing/selftests/mm/pagemap_ioctl.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c b/tools/testing/selftests/mm/pagemap_ioctl.c
> index 2cb5441f29c7..67a7a3705604 100644
> --- a/tools/testing/selftests/mm/pagemap_ioctl.c
> +++ b/tools/testing/selftests/mm/pagemap_ioctl.c
> @@ -1056,7 +1056,6 @@ int sanity_tests(void)
>  	struct page_region *vec;
>  	char *mem, *fmem;
>  	struct stat sbuf;
> -	char *tmp_buf;
>  
>  	/* 1. wrong operation */
>  	mem_size = 10 * page_size;
> @@ -1167,8 +1166,9 @@ int sanity_tests(void)
>  	if (fmem == MAP_FAILED)
>  		ksft_exit_fail_msg("error nomem %d %s\n", errno, strerror(errno));
>  
> -	tmp_buf = malloc(sbuf.st_size);
> -	memcpy(tmp_buf, fmem, sbuf.st_size);
> +	/* Fault in every page by reading the first byte */
> +	for (i = 0; i < sbuf.st_size; i += page_size)
> +		(void)*(volatile char *)(fmem + i);

We have FORCE_READ() in vm_util.h for this. Perhaps that would be better?

>  
>  	ret = pagemap_ioctl(fmem, sbuf.st_size, vec, vec_size, 0, 0,
>  			    0, PAGEMAP_NON_WRITTEN_BITS, 0, PAGEMAP_NON_WRITTEN_BITS);