linux-kernel - Re: [PATCH v2 6/8] selftests/mm: fix faulting-in code in pagemap

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Message-ID: <a3a8784b-4a4d-457c-8fba-1f27a71d5a0e@arm.com>
Date: Thu, 22 Jan 2026 11:46:01 +0530
From: Dev Jain <dev.jain@....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>, Ryan Roberts <ryan.roberts@....com>,
 Shuah Khan <shuah@...nel.org>, Usama Anjum <Usama.Anjum@....com>
Subject: Re: [PATCH v2 6/8] selftests/mm: fix faulting-in code in
 pagemap_ioctl test


On 07/01/26 10:18 pm, 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.
>
> Cc: Usama Anjum <Usama.Anjum@....com>
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>

Reviewed-by: Dev Jain <dev.jain@....com>

> ---
>  tools/testing/selftests/mm/pagemap_ioctl.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c b/tools/testing/selftests/mm/pagemap_ioctl.c
> index 2cb5441f29c7..80d7c391f8f5 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 +1166,7 @@ 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);
> +	force_read_pages_in_range(fmem, sbuf.st_size);
>  
>  	ret = pagemap_ioctl(fmem, sbuf.st_size, vec, vec_size, 0, 0,
>  			    0, PAGEMAP_NON_WRITTEN_BITS, 0, PAGEMAP_NON_WRITTEN_BITS);