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

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Message-ID: <fbf8e6ce-c058-4e87-b023-1544dac857be@arm.com>
Date: Tue, 16 Dec 2025 16:11:58 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Ryan Roberts <ryan.roberts@....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 15:56, Ryan Roberts wrote:
> 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?

It would, thanks! I wanted to use READ_ONCE() from
tools/include/linux/compiler.h but for some reason we don't add
tools/include to the include path in the mm kselftests Makefile and I
didn't want to dive into that rabbit hole.

- Kevin