On 6/11/26 3:31 PM, David Hildenbrand (Arm) 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 <[email protected]>
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily
> freeable mappings")
> Signed-off-by: David Hildenbrand (Arm) <[email protected]>
> ---
I ran this test before and after applying the patch and recorded the
execution time over 5 runs on an Orion O6 board.
Before the patch, average execution time = 3.87 s
After the patch, average execution time = 0.02 s
Test results remain unaffected after this patch is applied and speedup
is observed.
Tested-by: Sarthak Sharma <[email protected]>
> 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);
> }
> + ksft_test_result_fail("madvise(MADV_PAGEOUT) error:
> %s\n", strerror(errno));
> + exit(KSFT_FAIL);
> }
> +
> + dropped = memchr(alloc, 'A', alloc_size) == NULL;
> +
> + /*
> + * Speculative reference can temporarily prevent some
> + * pages from getting dropped. So sleep and retry.
> + *
> + * If a page is not droppable for 10s, something
> + * is seriously messed up and we want to fail.
> + */
> + if (dropped)
> + break;
> + sleep(1);
> }
> - kill(child, SIGTERM);
>
> - ksft_test_result_pass("MAP_DROPPABLE: PASS\n");
> - exit(KSFT_PASS);
> + ksft_test_result(dropped, "madvise(MADV_PAGEOUT) behavior\n");
> +
> + ksft_finished();
> }
>
> ---
>
> base-commit: d401506a8ee8ac6bc4a7767c17da036e9434a4a3
>
> change-id: 20260611-droppable_test-3737bd791dfb
>
> --
>
> Cheers,
>
> David
>
