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 <[email protected]>
> Signed-off-by: Kevin Brodsky <[email protected]>
> ---
> 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);
