Re: [PATCH 1/2] selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"

[Zi Yan <ziy@nvidia.com>]

On 17 Jul 2025, at 9:18, wang lian wrote:

> Several mm selftests use the `asm volatile("" : "+r" (variable));`
> construct to force a read of a variable, preventing the compiler from
> optimizing away the memory access. This idiom is cryptic and duplicated
> across multiple test files.
>
> Following a suggestion from David[1], this patch refactors this
> common pattern into a FORCE_READ() macro
>
> [1] https://lore.kernel.org/lkml/4a3e0759-caa1-4cfa-bc3f-402593f1eee3@redhat.com/
>
> Signed-off-by: wang lian <lianux.mm@gmail.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  tools/testing/selftests/mm/cow.c              | 30 +++++++++----------
>  tools/testing/selftests/mm/guard-regions.c    |  7 -----
>  tools/testing/selftests/mm/hugetlb-madvise.c  |  5 +---
>  tools/testing/selftests/mm/migration.c        | 13 ++++----
>  tools/testing/selftests/mm/pagemap_ioctl.c    |  4 +--
>  .../selftests/mm/split_huge_page_test.c       |  4 +--
>  tools/testing/selftests/mm/vm_util.h          |  7 +++++
>  7 files changed, 31 insertions(+), 39 deletions(-)
>

<snip>

> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> index f0d9c035641d..05de1fc0005b 100644
> --- a/tools/testing/selftests/mm/split_huge_page_test.c
> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> @@ -399,7 +399,6 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
>  		char **addr)
>  {
>  	size_t i;
> -	int dummy = 0;
>  	unsigned char buf[1024];
>
>  	srand(time(NULL));
> @@ -441,8 +440,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
>  	madvise(*addr, fd_size, MADV_HUGEPAGE);
>
>  	for (size_t i = 0; i < fd_size; i++)
> -		dummy += *(*addr + i);
> -	asm volatile("" : "+r" (dummy));
> +		FORCE_READ((*addr + i));

I encountered a segfault when running the test on x86_64.
i is 4194297 and fd_size is 4194304.
It seems that FORCE_READ() is reading (*addr + i) in 8 byte size
and i is only 7 bytes away from the end of the memory address.
This led to segfault.

(*(volatile char*)(*addr + i)); works fine.

Both gcc-12 and gcc-14 have the issue.

>
>  	if (!check_huge_file(*addr, fd_size / pmd_pagesize, pmd_pagesize)) {
>  		ksft_print_msg("No large pagecache folio generated, please provide a filesystem supporting large folio\n");
> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> index 2b154c287591..c20298ae98ea  100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -18,6 +18,13 @@
>  #define PM_SWAP                       BIT_ULL(62)
>  #define PM_PRESENT                    BIT_ULL(63)
>
> +/*
> + * Ignore the checkpatch warning, we must read from x but don't want to do
> + * anything with it in order to trigger a read page fault. We therefore must use
> + * volatile to stop the compiler from optimising this away.
> + */
> +#define FORCE_READ(x) (*(volatile typeof(x) *)x)
> +

Also, look at FORCE_READ again, it converts x to a pointer to x and
deferences x as a point. It does not seem right to me.

Best Regards,
Yan, Zi