Re: [PATCH 3/4] selftests/mm: fix faulting-in code in pagemap_ioctl test

[Kevin Brodsky <kevin.brodsky@arm.com>]

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@arm.com>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.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