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

David Hildenbrand (Red Hat)
2025-12-18T08:05:34Z

On 12/16/25 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?

Agreed, and if we have multiple patterns where we want to force_read a
bigger area, maybe we should provide a helper for that?

--
Cheers

David