Re: [PATCH mm-unstable v15 11/13] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[Lorenzo Stoakes (Oracle)]

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 <npache@xxxxxxxxxx>
> >>
> >> With David's concern addressed:
> >>
> >> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@xxxxxxxxxx>
> >>
> >>> ---
> >>>  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