Re: [PATCH] selftests: mm: fix and speedup "droppable" test

[SeongJae Park <sj@kernel.org>]

On Thu, 11 Jun 2026 12:01:55 +0200 "David Hildenbrand (Arm)" <david@kernel.org> wrote:

> The droppable test currently relies on creating memory pressure in a
> child process to trigger dropping the droppable pages.
> 
> That not only takes a long time on some machines (allocating and filling
> all that memory), on large machines this will not work as we hardcode the
> area size to 134217728 bytes.
> 
> ... further, we rely on timeouts to detect that memory was not dropped,
> which is really suboptimal.
> 
> Instead, let's just use MADV_PAGEOUT on a 2 MiB region. MADV_PAGEOUT works
> with droppable memory even without swap.
> 
> There is the low chance of MADV_PAGEOUT failing to drop a page because
> of speculative references. We'll wait 1s and retry 10 times to
> rule that unlikely case out as best as we can.
> 
> On a machine without swap:
> 
> 	$ ./droppable
> 	TAP version 13
> 	1..1
> 	ok 1 madvise(MADV_PAGEOUT) behavior
> 	# Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
> Reported-by: Aishwarya TCV <Aishwarya.TCV@arm.com>
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")

Because this is a fix for a test, I think not Cc-ing stable@ is ok.  Further,
arguably this is not a fix but an improvement?  No strong opinion, just
thinking loud.

> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---
>  tools/testing/selftests/mm/droppable.c | 46 +++++++++++++++++++---------------
>  1 file changed, 26 insertions(+), 20 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/droppable.c b/tools/testing/selftests/mm/droppable.c
> index 30c8be37fcb9..57e1b6fc5569 100644
> --- a/tools/testing/selftests/mm/droppable.c
> +++ b/tools/testing/selftests/mm/droppable.c
> @@ -17,10 +17,10 @@
>  
>  int main(int argc, char *argv[])
>  {
> -	size_t alloc_size = 134217728;
> -	size_t page_size = getpagesize();
> +	const size_t alloc_size = 2 * 1024 * 1024;
> +	int retry_count = 10;
> +	bool dropped;
>  	void *alloc;
> -	pid_t child;
>  
>  	ksft_print_header();
>  	ksft_set_plan(1);
> @@ -35,26 +35,32 @@ int main(int argc, char *argv[])
>  		exit(KSFT_FAIL);
>  	}
>  	memset(alloc, 'A', alloc_size);
> -	for (size_t i = 0; i < alloc_size; i += page_size)
> -		assert(*(uint8_t *)(alloc + i));
> -
> -	child = fork();
> -	assert(child >= 0);
> -	if (!child) {
> -		for (;;)
> -			*(char *)malloc(page_size) = 'B';
> -	}
>  
> -	for (bool done = false; !done;) {
> -		for (size_t i = 0; i < alloc_size; i += page_size) {
> -			if (!*(uint8_t *)(alloc + i)) {
> -				done = true;
> -				break;
> +	while (retry_count--) {
> +		if (madvise(alloc, alloc_size, MADV_PAGEOUT)) {
> +			if (errno == EINVAL) {
> +				ksft_test_result_skip("madvise(MADV_PAGEOUT) not supported\n");
> +				exit(KSFT_SKIP);

This check is for a case that this test is running against an old kernel that
doesn't have MADV_PAGEOUT?

Assuming so, I was first thinking this check might not really needed, assuming
the test runner would pick the kselftest code from the running kernel's source
code.  But I recalled some people do get kselftest code from random place.  So
this check seems nice to me.

Just thinking loud.


Thanks,
SJ

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