linux-kernel - Re: [PATCH RESEND 1/1] selftests/mm: skip soft-dirty tests when CONFIG_MEM_SOFT

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Message-ID: <353bf8f0-d9ad-4800-80d7-4177ae64eb95@redhat.com>
Date: Wed, 17 Sep 2025 12:51:23 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com
Cc: shuah@...nel.org, ioworker0@...il.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RESEND 1/1] selftests/mm: skip soft-dirty tests when
 CONFIG_MEM_SOFT_DIRTY is disabled

On 17.09.25 07:59, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> The madv_populate and soft-dirty kselftests currently fail on systems where
> CONFIG_MEM_SOFT_DIRTY is disabled.
> 
> Introduce a new helper softdirty_is_supported() into vm_util.c/h to ensure
> tests are properly skipped when the feature is not enabled.

I'll note that tools/testing/selftests/mm/config contains

	CONFIG_MEM_SOFT_DIRTY=y

But yes, the current arm64 handling is nasty, because some other archs 
(e.g., riscv) also don't support it yet.

LGTM, some nits below:

> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>   tools/testing/selftests/mm/madv_populate.c | 21 ++--------------
>   tools/testing/selftests/mm/soft-dirty.c    |  5 +++-
>   tools/testing/selftests/mm/vm_util.c       | 28 ++++++++++++++++++++++
>   tools/testing/selftests/mm/vm_util.h       |  1 +
>   4 files changed, 35 insertions(+), 20 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/madv_populate.c b/tools/testing/selftests/mm/madv_populate.c
> index b6fabd5c27ed..43dac7783004 100644
> --- a/tools/testing/selftests/mm/madv_populate.c
> +++ b/tools/testing/selftests/mm/madv_populate.c
> @@ -264,23 +264,6 @@ static void test_softdirty(void)
>   	munmap(addr, SIZE);
>   }
>   
> -static int system_has_softdirty(void)
> -{
> -	/*
> -	 * There is no way to check if the kernel supports soft-dirty, other
> -	 * than by writing to a page and seeing if the bit was set. But the
> -	 * tests are intended to check that the bit gets set when it should, so
> -	 * doing that check would turn a potentially legitimate fail into a
> -	 * skip. Fortunately, we know for sure that arm64 does not support
> -	 * soft-dirty. So for now, let's just use the arch as a corse guide.
> -	 */
> -#if defined(__aarch64__)
> -	return 0;
> -#else
> -	return 1;
> -#endif
> -}
> -
>   int main(int argc, char **argv)
>   {
>   	int nr_tests = 16;
> @@ -288,7 +271,7 @@ int main(int argc, char **argv)
>   
>   	pagesize = getpagesize();
>   
> -	if (system_has_softdirty())
> +	if (softdirty_is_supported())
>   		nr_tests += 5;
>   
>   	ksft_print_header();
> @@ -300,7 +283,7 @@ int main(int argc, char **argv)
>   	test_holes();
>   	test_populate_read();
>   	test_populate_write();
> -	if (system_has_softdirty())
> +	if (softdirty_is_supported())
>   		test_softdirty();
>   
>   	err = ksft_get_fail_cnt();
> diff --git a/tools/testing/selftests/mm/soft-dirty.c b/tools/testing/selftests/mm/soft-dirty.c
> index 8a3f2b4b2186..98e42d2ac32a 100644
> --- a/tools/testing/selftests/mm/soft-dirty.c
> +++ b/tools/testing/selftests/mm/soft-dirty.c
> @@ -200,8 +200,11 @@ int main(int argc, char **argv)
>   	int pagesize;
>   
>   	ksft_print_header();
> -	ksft_set_plan(15);
>   
> +	if (!softdirty_is_supported())
> +		ksft_exit_skip("soft-dirty is not support\n");
> +
> +	ksft_set_plan(15);
>   	pagemap_fd = open(PAGEMAP_FILE_PATH, O_RDONLY);
>   	if (pagemap_fd < 0)
>   		ksft_exit_fail_msg("Failed to open %s\n", PAGEMAP_FILE_PATH);
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index 56e9bd541edd..3173335df775 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -449,6 +449,34 @@ bool check_vmflag_pfnmap(void *addr)
>   	return check_vmflag(addr, "pf");
>   }
>   
> +bool softdirty_is_supported(void)

I'd just call it "softdirty_supported" similar to 
"pagemap_scan_supported()".

> +{
> +	char *addr;
> +	int ret = 0;

bool supported = false;

> +	size_t pagesize;
> +
> +	/* We know for sure that arm64 does not support soft-dirty. */
> +#if defined(__aarch64__)
> +	return ret;
> +#endif

Just drop this arm special case now.

> +	pagesize = getpagesize();

const size_t pagesize = getpagesize();

> +	/*
> +	 * __mmap_complete() always sets VM_SOFTDIRTY for new VMAs, so we
> +	 * just mmap a small region and check its VmFlags in /proc/self/smaps
> +	 * for the "sd" flag.
> +	 */

/* New mappings are expected to be marked with VM_SOFTDIRTY (sd). */

> +	addr = mmap(0, pagesize, PROT_READ | PROT_WRITE,
> +		    MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
> +	if (!addr)
> +		ksft_exit_fail_msg("mmap failed\n");
> +
> +	if (check_vmflag(addr, "sd"))
> +		ret = 1;

supported = true;

> +
> +	munmap(addr, pagesize);
> +	return ret;
> +}
> +

-- 
Cheers

David / dhildenb