Re: [PATCH mm-unstable v18 11/14] mm/khugepaged: Introduce mTHP collapse support

[Lorenzo Stoakes]

On Wed, Jun 03, 2026 at 10:05:08AM +0200, David Hildenbrand (Arm) wrote:
> On 6/2/26 17:44, Lance Yang wrote:
> >
> >
> > On 2026/6/2 18:58, Nico Pache wrote:
> >> On Sun, May 31, 2026 at 1:19 AM Lance Yang <lance.yang@xxxxxxxxx> wrote:
> >>>
> >>>
> >>> [...]
> >>>
> >>> Hmm ... don't we lose the allocation-failure result here?
> >>>
> >>> Previously collapse_scan_pmd() propagated SCAN_ALLOC_HUGE_PAGE_FAIL from
> >>> collapse_huge_page(), so khugepaged would call khugepaged_alloc_sleep()
> >>> in khugepaged_do_scan().
> >>>
> >>> Now if allocation fails and nr_collapsed stays 0, we just return
> >>> SCAN_FAIL. So we won't back off via khugepaged_alloc_sleep() anymore?
> >>
> >> Ok I did the error propagation! I think I handled both of these cases
> >> you brought up pretty easily.
> >
> > Thanks.
> >
> >> However I don't know what to do in the following case: We successfully
> >> collapsed some portion of the PMD, but during that process, we also
> >> hit an allocation failure. Is it best to back off entirely? or can we
> >> treat some forward progress as a sign we can continue trying collapses
> >> without sleeping.
> >>
> >> Basically, do we prioritize SCAN_ALLOC_HUGE_PAGE_FAIL or the
> >> successful collapses as the returned value?
> >
> > Thinking out loud, forward progress should win here, the allocation
> > failure only matter if we made no progress at all?
>
> Agreed, in the first approach, forward progress makes sense.

Sounds sensible to me.

>
> --
> Cheers,
>
> David

Thanks, Lorenzo