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

[Zi Yan <ziy@nvidia.com>]

On 5 Aug 2025, at 15:00, Lorenzo Stoakes wrote:

> 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

I just used READ_ONCE() code, but it is not necessary.

> 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?

Adding (void) makes no difference, at least from godbolt.

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

Neither gcc nor clang complains without (void). My guess is that volatile
is doing something there.

Best Regards,
Yan, Zi