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

[Lance Yang <lance.yang@linux.dev>]

On Thu, Jun 11, 2026 at 01:26:57PM +0200, David Hildenbrand (Arm) wrote:
>On 6/11/26 13:07, Sarthak Sharma wrote:
>> 
>> 
>> 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 <Aishwarya.TCV@arm.com>
>>> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
>>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>>> ---
>> 
>> I ran this test before and after applying the patch and recorded the
>> execution time over 5 runs on an Orion O6 board.
>> 
>
>Thanks!
>
>> Before the patch, average execution time = 3.87 s
>
>Out of interest, how much memory did your machine have?
>
>I ran into this myself on a 200gig machine, and there was essentially no
>progress ...

Same here ... With the old test, I waited a quite a while and didn't see
any progress ...  on a 512 GiB machine.