Re: [PATCH v7 1/3] kho: make kho_scratch_overlap usable outside debugging

[Pasha Tatashin]

On Tue, Apr 7, 2026 at 12:09 PM Pratyush Yadav <pratyush@xxxxxxxxxx> wrote:
>
> On Tue, Apr 07 2026, Pasha Tatashin wrote:
>
> > On Tue, Apr 7, 2026 at 6:55 AM Pratyush Yadav <pratyush@xxxxxxxxxx> wrote:
> >>
> >> On Wed, Mar 18 2026, Mike Rapoport wrote:
> >>
> >> > Hi Michal,
> >> >
> >> > On Tue, Mar 17, 2026 at 03:15:32PM +0100, Michal Clapinski wrote:
> >> >> Also return false if kho_scratch is NULL.
> >> >>
> >> [...]
> >> >> diff --git a/kernel/liveupdate/Makefile b/kernel/liveupdate/Makefile
> >> >> index d2f779cbe279..dc352839ccf0 100644
> >> >> --- a/kernel/liveupdate/Makefile
> >> >> +++ b/kernel/liveupdate/Makefile
> >> >> @@ -7,7 +7,6 @@ luo-y :=                                                             \
> >> >>              luo_session.o
> >> >>
> >> >>  obj-$(CONFIG_KEXEC_HANDOVER)                += kexec_handover.o
> >> >> -obj-$(CONFIG_KEXEC_HANDOVER_DEBUG)  += kexec_handover_debug.o
> >> >>  obj-$(CONFIG_KEXEC_HANDOVER_DEBUGFS)        += kexec_handover_debugfs.o
> >> >>
> >> >>  obj-$(CONFIG_LIVEUPDATE)            += luo.o
> >> >> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> >> >> index 532f455c5d4f..c9b982372d6e 100644
> >> >> --- a/kernel/liveupdate/kexec_handover.c
> >> >> +++ b/kernel/liveupdate/kexec_handover.c
> >> >> @@ -820,7 +820,8 @@ int kho_preserve_folio(struct folio *folio)
> >> >>      const unsigned long pfn = folio_pfn(folio);
> >> >>      const unsigned int order = folio_order(folio);
> >> >>
> >> >> -    if (WARN_ON(kho_scratch_overlap(pfn << PAGE_SHIFT, PAGE_SIZE << order)))
> >> >> +    if (WARN_ON(kho_scratch_overlap_debug(pfn << PAGE_SHIFT,
> >> >> +                                          PAGE_SIZE << order)))
> >> >
> >> > Can't say I'm fond of kho_scratch_overlap_debug(). How about we make it
> >> >
> >> >       if (IS_ENABLED(CONFIG_KEXEC_HANDOVER_DEBUG) &&
> >> >           WARN_ON(kho_scratch_overlap(...))
> >>
> >> +1. And we can get rid of kexec_handover_debug.c, for now at least. We
> >> can add it back when we have something else to put in there.
> >
> > Are you proposing moving kho_scratch_overlap() into kexec_handover.c?
> > That would make it uglier to have #ifdefs in the C file. If you mean
> > removing this function entirely, I think that is too dangerous because
> > we have already had a memory corruption issue [1] that was challenging
> > to root cause, and having this simple check prevents this from
> > occurring going forward. The problem is that changes to defences such
> > as kfence, kasan, and asi are happening outside of the core KHO code,
> > and it is very easy to miss when something unexpectedly causes a
> > preservation from the scratch area, as we have seen this with kfence.
> > Worst of all, some of those mitigations use randomized or sampling
> > approaches and might not be reproducible on every try, so having a
> > CONFIG that tests it every time in a debug build is the only solid
> > defense against that.
>
> I think you miss the context here. This patchset uses
> kho_scratch_overlap() during MM init to set the migrate type of
> pageblocks. So it will no longer be gated by
> CONFIG_KEXEC_HANDOVER_DEBUG, but by CONFIG_KEXEC_HANDOVER instead. So
> there is no need for any #ifdefs. All we need to change is to have the
> debug checks gated with a IS_ENABLED(CONFIG_KEXEC_HANDOVER_DEBUG).

Ah, this makes sense, IS_ENABLED(CONFIG_KEXEC_HANDOVER_DEBUG) Mike
proposed was just for optimization for debug checks while making the
patch is  making the function generic.

Thank you for the clarifications. Need to review this series.

Pasha

> So the function stays around, and so do the debug checks. Since core KHO
> now uses this function, we just move it out to the main file. And since
> kexec_handover_debug.c has nothing else, we can delete it for now.
>
> Anyway, based on the discussion it looks like people want to ask
> memblock directly and not use kho_scratch_overlap(), so the next version
> might not have this patch at all.



>
> >
> > Pasha
> >
> > [1] https://lore.kernel.org/all/20251021000852.2924827-1-pasha.tatashin@xxxxxxxxxx
> >
> >>
> >> >
> >> >>              return -EINVAL;
> >> >>
> >> >>      return kho_radix_add_page(tree, pfn, order);
> >> [...]
> >>
> >> --
> >> Regards,
> >> Pratyush Yadav
>
> --
> Regards,
> Pratyush Yadav