Re: [PATCH v3 2/2] kselftest: mm: fix intermittent failure khugepaged test

[Gregory Price]

On Wed, Sep 23, 2026 at 04:29:52PM +0100, Yeoreum Yun wrote:
> There are intermittent failures in collapse_max_ptes_swap() and
> collapse_max_ptes_shared() when using the khugepaged_context:
> 
>   // while running ./khugepaged -s 2
> 
>   # 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.
> 
> For example, in collapse_max_ptes_swap(), after faulting the pages back in
> and paging out up to max_ptes_swap pages, khugepaged may collapse them again
> before c->collapse() is called.
> 
> To prevent this, mark the VMA with MADV_NOHUGEPAGE after it has been
> collapsed by wait_for_scan() for anon. This prevents khugepaged from
> collapsing it again before c->collapse() is called.
> 
> This failure was observed on NVIDIA Spark with 16KB page.
> 
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>

Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>