Re: [PATCH 0/2] kselftest: mm: fix intermittent failure khugepaged test

[Andrew Morton]

On Tue, 15 Sep 2026 10:21:35 +0100 Yeoreum Yun <yeoreum....@arm.com> wrote:

> There are intermittent failures in collapse_max_ptes_swap() and
> collapse_max_ptes_shared() when using the khugepaged_context:
> 
>   # Run test: collapse_max_ptes_shared (khugepaged:anon)
>   # Allocate huge page... OK
>   # Share huge page over fork()... OK
>   # Trigger CoW on page 1023 of 2048... OK
>   # Maybe collapse with max_ptes_shared exceeded.... OK
>   # Trigger CoW on page 1024 of 2048... Fail
>   Bail out! Unexpected huge page
>   # Planned tests != run tests (26 != 23)
>   # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
>   # Run test: collapse_max_ptes_swap (khugepaged:anon)
>   # Swapout 257 of 2048 pages... OK
>   # Maybe collapse with max_ptes_swap exceeded.... OK
>   # Swapout 256 of 2048 pages... OK
>   Bail out! Unexpected huge page
>   # Planned tests != run tests (26 != 17)
>   # Totals: pass:17 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
> This happens because khugepaged may collapse the pages before wait_for_scan()
> is called, causing a sanity check that expects uncollapsed pages to fail.

Is this something we should be testing for?  If I'm understanding
correctly, success/failure is dependent on timing vargaries inside the
kernel and that isn't something we can reliably test for.

IOW, should we say "bad idea" and just delete this test case?