On Tue, Sep 29, 2026 at 10:46:50AM +0200, David Hildenbrand (Arm) wrote: > On 9/29/26 10:43, Baolin Wang wrote: > > > > > > On 9/29/26 4:38 PM, David Hildenbrand (Arm) wrote: > >> On 9/23/26 17:29, 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 <[email protected]> > >>> Tested-by: Baolin Wang <[email protected]> > >>> Signed-off-by: Yeoreum Yun <[email protected]> > >>> --- > >>> tools/testing/selftests/mm/khugepaged.c | 3 +++ > >>> 1 file changed, 3 insertions(+) > >>> > >>> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/ > >>> selftests/mm/khugepaged.c > >>> index 2aa7c9197158..b0cb02bf1a73 100644 > >>> --- a/tools/testing/selftests/mm/khugepaged.c > >>> +++ b/tools/testing/selftests/mm/khugepaged.c > >>> @@ -618,6 +618,9 @@ static bool wait_for_scan(const char *msg, char *p, > >>> size_t len, > >>> usleep(TICK); > >>> } > >>> + if (is_anon(ops)) > >>> + madvise(p, len, MADV_NOHUGEPAGE); > >>> + > >> > >> Any reason we just do that unconditionally? > > > > Although it's a bit messy, as I mentioned before [1], unconditionally > > setting > > MADV_NOHUGEPAGE will break shmem testing. Maybe add some comments. > > Ah, thanks for clarifying. The problem really is that we cannot undo a > MADV_HUGEPAGE (give me hugepages) cleanly. We can only go to the other extreme > (no huge pages). > > Yes, let's please add a comment describing why we limit it to anon.
Okay. -- Sincerely, Yeoreum Yun
