On Wed, Mar 18, 2026 at 08:48:30PM +0100, David Hildenbrand (Arm) wrote: > On 3/18/26 19:59, Nico Pache wrote: > > > > > > On 3/17/26 4:35 AM, Lorenzo Stoakes (Oracle) wrote: > >> On Wed, Feb 25, 2026 at 08:26:31PM -0700, Nico Pache wrote: > >>> There are cases where, if an attempted collapse fails, all subsequent > >>> orders are guaranteed to also fail. Avoid these collapse attempts by > >>> bailing out early. > >>> > >>> Signed-off-by: Nico Pache <[email protected]> > >> > >> With David's concern addressed: > >> > >> Reviewed-by: Lorenzo Stoakes (Oracle) <[email protected]> > >> > >>> --- > >>> mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++- > >>> 1 file changed, 34 insertions(+), 1 deletion(-) > >>> > >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >>> index 1c3711ed4513..388d3f2537e2 100644 > >>> --- a/mm/khugepaged.c > >>> +++ b/mm/khugepaged.c > >>> @@ -1492,9 +1492,42 @@ static int mthp_collapse(struct mm_struct *mm, > >>> unsigned long address, > >>> ret = collapse_huge_page(mm, collapse_address, > >>> referenced, > >>> unmapped, cc, mmap_locked, > >>> order); > >>> - if (ret == SCAN_SUCCEED) { > >>> + > >>> + switch (ret) { > >>> + /* Cases were we continue to next collapse candidate */ > >>> + case SCAN_SUCCEED: > >>> collapsed += nr_pte_entries; > >>> + fallthrough; > >>> + case SCAN_PTE_MAPPED_HUGEPAGE: > >>> continue; > >>> + /* Cases were lower orders might still succeed */ > >>> + case SCAN_LACK_REFERENCED_PAGE: > >>> + case SCAN_EXCEED_NONE_PTE: > >>> + case SCAN_EXCEED_SWAP_PTE: > >>> + case SCAN_EXCEED_SHARED_PTE: > >>> + case SCAN_PAGE_LOCK: > >>> + case SCAN_PAGE_COUNT: > >>> + case SCAN_PAGE_LRU: > >>> + case SCAN_PAGE_NULL: > >>> + case SCAN_DEL_PAGE_LRU: > >>> + case SCAN_PTE_NON_PRESENT: > >>> + case SCAN_PTE_UFFD_WP: > >>> + case SCAN_ALLOC_HUGE_PAGE_FAIL: > >>> + goto next_order; > >>> + /* Cases were no further collapse is possible */ > >>> + case SCAN_CGROUP_CHARGE_FAIL: > >>> + case SCAN_COPY_MC: > >>> + case SCAN_ADDRESS_RANGE: > >>> + case SCAN_NO_PTE_TABLE: > >>> + case SCAN_ANY_PROCESS: > >>> + case SCAN_VMA_NULL: > >>> + case SCAN_VMA_CHECK: > >>> + case SCAN_SCAN_ABORT: > >>> + case SCAN_PAGE_ANON: > >>> + case SCAN_PMD_MAPPED: > >>> + case SCAN_FAIL: > >>> + default: > >> > >> Agree with david, let's spell them out please :) > > > > I believe David is arguing for the opposite. To drop all these spelt out > > cases > > and just leave the default case. > > > > @david is that correct or did I misunderstand that. > > Either spell all out (no default) OR add a default. > > I prefer to just ... use the default :)
I mean yup that's fine too I guess, all or nothing, something in between is weird! > > -- > Cheers, > > David Cheers, Lorenzo
