On 6/12/26 03:29, SeongJae Park wrote: > On Thu, 11 Jun 2026 12:01:55 +0200 "David Hildenbrand (Arm)" > <[email protected]> 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") > > 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.
The test will hang forever on very large machines and timeout on large machines, so it's a fix not just an improvement. I think Andrew just automatically CCs stable on any test fixes, so yeah, CC; stable makes sense for this one as well. > >> Signed-off-by: David Hildenbrand (Arm) <[email protected]> > > Reviewed-by: SeongJae Park <[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); > > This check is for a case that this test is running against an old kernel that > doesn't have MADV_PAGEOUT? Yes! > > 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. Yeah, we try to handle older kernels on a best-effort basis. This one is rather simple :) Thanks! -- Cheers, David
