Re: [PATCH] selftests/mm: fix FORCE_READ to read input value correctly.

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Aug 05, 2025 at 01:51:40PM -0400, Zi Yan wrote:
> FORCE_READ() converts input value x to its pointer type then reads from
> address x. This is wrong. If x is a non-pointer, it would be caught it
> easily. But all FORCE_READ() callers are trying to read from a pointer and
> FORCE_READ() basically reads a pointer to a pointer instead of the original
> typed pointer. Almost no access violation was found, except the one from
> split_huge_page_test.

Oops, sorry about that! I had incorrectly assumed typeof() decayed the type
when I wrote the guard-regions test code, and hadn't considered that we
were casting to (t **) and dereffing that.

And as discussed off-list, if you deref a char array like that, and are at
the end of an array, that's err... not brilliant :)

>
> Fix it by implementing a simplified READ_ONCE() instead.

I sort of intended to make this easier for pointers, but the semantics of
this are actually potentially a bit nicer - it's more like READ_ONCE() and
you're passing in the value you're actually reading so this is probably
better.

>
> Fixes: 3f6bfd4789a0 ("selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"")
> Signed-off-by: Zi Yan <ziy@nvidia.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

But see nits below.

> ---
> FORCE_READ() comes from commit 876320d71f51 ("selftests/mm: add self tests for
> guard page feature"). I will a separate patch to stable tree.
>
>
>  tools/testing/selftests/mm/cow.c                  | 4 ++--
>  tools/testing/selftests/mm/guard-regions.c        | 2 +-
>  tools/testing/selftests/mm/hugetlb-madvise.c      | 4 +++-
>  tools/testing/selftests/mm/migration.c            | 2 +-
>  tools/testing/selftests/mm/pagemap_ioctl.c        | 2 +-
>  tools/testing/selftests/mm/split_huge_page_test.c | 7 +++++--
>  tools/testing/selftests/mm/vm_util.h              | 2 +-
>  7 files changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c
> index d30625c18259..c744c603d688 100644
> --- a/tools/testing/selftests/mm/cow.c
> +++ b/tools/testing/selftests/mm/cow.c
> @@ -1554,8 +1554,8 @@ static void run_with_zeropage(non_anon_test_fn fn, const char *desc)
>  	}
>
>  	/* Read from the page to populate the shared zeropage. */
> -	FORCE_READ(mem);
> -	FORCE_READ(smem);
> +	FORCE_READ(*mem);
> +	FORCE_READ(*smem);
>
>  	fn(mem, smem, pagesize);
>  munmap:
> diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
> index b0d42eb04e3a..8dd81c0a4a5a 100644
> --- a/tools/testing/selftests/mm/guard-regions.c
> +++ b/tools/testing/selftests/mm/guard-regions.c
> @@ -145,7 +145,7 @@ static bool try_access_buf(char *ptr, bool write)
>  		if (write)
>  			*ptr = 'x';
>  		else
> -			FORCE_READ(ptr);
> +			FORCE_READ(*ptr);
>  	}
>
>  	signal_jump_set = false;
> diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
> index 1afe14b9dc0c..c5940c0595be 100644
> --- a/tools/testing/selftests/mm/hugetlb-madvise.c
> +++ b/tools/testing/selftests/mm/hugetlb-madvise.c
> @@ -50,8 +50,10 @@ void read_fault_pages(void *addr, unsigned long nr_pages)
>  	unsigned long i;
>
>  	for (i = 0; i < nr_pages; i++) {
> +		unsigned long *addr2 =
> +			((unsigned long *)(addr + (i * huge_page_size)));
>  		/* Prevent the compiler from optimizing out the entire loop: */
> -		FORCE_READ(((unsigned long *)(addr + (i * huge_page_size))));
> +		FORCE_READ(*addr2);
>  	}
>  }
>
> diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
> index c5a73617796a..ea945eebec2f 100644
> --- a/tools/testing/selftests/mm/migration.c
> +++ b/tools/testing/selftests/mm/migration.c
> @@ -110,7 +110,7 @@ void *access_mem(void *ptr)
>  		 * the memory access actually happens and prevents the compiler
>  		 * from optimizing away this entire loop.
>  		 */
> -		FORCE_READ((uint64_t *)ptr);
> +		FORCE_READ(*(uint64_t *)ptr);
>  	}
>
>  	return NULL;
> diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c b/tools/testing/selftests/mm/pagemap_ioctl.c
> index 0d4209eef0c3..e6face7c0166 100644
> --- a/tools/testing/selftests/mm/pagemap_ioctl.c
> +++ b/tools/testing/selftests/mm/pagemap_ioctl.c
> @@ -1525,7 +1525,7 @@ void zeropfn_tests(void)
>
>  	ret = madvise(mem, hpage_size, MADV_HUGEPAGE);
>  	if (!ret) {
> -		FORCE_READ(mem);
> +		FORCE_READ(*mem);
>
>  		ret = pagemap_ioctl(mem, hpage_size, &vec, 1, 0,
>  				    0, PAGE_IS_PFNZERO, 0, 0, PAGE_IS_PFNZERO);
> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> index 718daceb5282..3c761228e451 100644
> --- a/tools/testing/selftests/mm/split_huge_page_test.c
> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> @@ -440,8 +440,11 @@ 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++)
> -		FORCE_READ((*addr + i));
> +	for (size_t i = 0; i < fd_size; i++) {
> +		char *addr2 = *addr + i;
> +
> +		FORCE_READ(*addr2);
> +	}
>
>  	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 c20298ae98ea..b55d1809debc 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -23,7 +23,7 @@
>   * 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)
> +#define FORCE_READ(x) (*(const volatile typeof(x) *)&(x))

NIT: but wonder if const is necessary, and also (as discussed off-list
again :) will this work with a (void) prefixed, just to a. make it clear
we're reading but discarding and b. to avoid any possible compiler warning
on this?

I know for some reason this form doesn't generate one currently (not sure
why), but we may hit that in future.

>
>  extern unsigned int __page_size;
>  extern unsigned int __page_shift;
> --
> 2.47.2
>

Cheers, Lorenzo