On Tue, Sep 29, 2026 at 11:33:19AM +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: Gregory Price (Meta) <[email protected]> > Reviewed-by: Baolin Wang <[email protected]> > Tested-by: Baolin Wang <[email protected]> > Signed-off-by: Yeoreum Yun <[email protected]>
Looks sensible to me so: Acked-by: Lorenzo Stoakes (ARM) <[email protected]> > --- > tools/testing/selftests/mm/khugepaged.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/tools/testing/selftests/mm/khugepaged.c > b/tools/testing/selftests/mm/khugepaged.c > index 2aa7c9197158..4e888b7bf310 100644 > --- a/tools/testing/selftests/mm/khugepaged.c > +++ b/tools/testing/selftests/mm/khugepaged.c > @@ -618,6 +618,16 @@ static bool wait_for_scan(const char *msg, char *p, > size_t len, > usleep(TICK); > } > > + /* > + * The file and shmem tests rely on refaults to install PMD mappings > + * after collapse. MADV_NOHUGEPAGE would prevent those mappings. > + * > + * Apply MADV_NOHUGEPAGE only to anonymous VMAs to prevent khugepaged > + * from unexpectedly collapsing pages during the test. > + */ > + if (is_anon(ops)) > + madvise(p, len, MADV_NOHUGEPAGE); > + > return timeout == -1; > } > > > -- > 2.43.0 > -- Cheers, Lorenzo
