Re: [PATCH v3 6/6] mm/mm_init: add zone mismatch warning during page init

[Mike Rapoport <rppt@kernel.org>]

On Tue, Oct 06, 2026 at 03:32:43PM +0200, Muchun Song wrote:
> > On Oct 6, 2026, at 12:52, Mike Rapoport <rppt@kernel.org> wrote:
> > 
> > On Mon, Oct 05, 2026 at 06:27:52PM +0200, Muchun Song wrote:
> >>> On Oct 2, 2026, at 20:12, Mike Rapoport <rppt@kernel.org> wrote:
> >>> 
> >>> On Fri, Oct 02, 2026 at 05:56:40PM +0800, Muchun Song wrote:
> >>>>> On Oct 2, 2026, at 16:22, Mike Rapoport <rppt@kernel.org> wrote:
> >>>> 
> >>>>>>>> diff --git a/mm/mm_init.c b/mm/mm_init.c
> >>>>>>>> index 1650d6bc1211..bd02e8d06965 100644
> >>>>>>>> --- a/mm/mm_init.c
> >>>>>>>> +++ b/mm/mm_init.c
> >>>>>>>> @@ -609,6 +609,9 @@ void __meminit __init_single_page(struct page *page, unsigned long pfn,
> >>>>>>>> if (!is_highmem_idx(zone))
> >>>>>>>> set_page_address(page, __va(pfn << PAGE_SHIFT));
> >>>>>>>> #endif
> >>>>>>>> + VM_WARN_ON_ONCE(vmemmap_optimizable_order(pfn_to_section_compound_order(pfn)) &&
> >>>>>>>> + page_zone_id(page + VMEMMAP_OPTIMIZATION_NR_STRUCT_PAGES) !=
> >>>>>>>> + page_zone_id(page));
> >>>>>>> 
> >>>>>>> Hmm, page + VMEMMAP_OPTIMIZATION_NR_STRUCT_PAGES is initialized a tad later
> >>>>>>> than page so it'll have stale data in the page->flags, won't it?
> >>>>>> 
> >>>>>> Lance is right. The shared tail struct pages are already initialized by
> >>>>>> vmemmap_shared_tail_page() during vmemmap population, so they're not stale.
> >>>>>> The head 64 struct pages are initialized later — right here, after vmemmap
> >>>>>> population.
> >>>>> 
> >>>>> Still it looks out of place here, can this check be done in sparse-vmemmap
> >>>>> somehow?
> >>>> 
> >>>> The struct page entries of a vmemmap-optimizable compound page
> >>>> are currently initialized in two stages. During vmemmap
> >>>> population, the shared tail entries are initialized first. The
> >>>> retained head area—normally 64—is initialized later through
> >>>> __init_single_page().
> >>>> 
> >>>> This warning connects the two stages: while initializing the
> >>>> retained entries in the second stage, it verifies that their zone
> >>>> information is consistent with the shared entries initialized in
> >>>> the first stage. Therefore, the same check cannot be performed
> >>>> during vmemmap population.
> >>>> 
> >>>> I am planning to first unify the HugeTLB and Device DAX
> >>>> compound-page initialization through a common helper [1]. Once that
> >>>> work is complete, maybe it will be easy to move the initialization
> >>>> of the retained head area into vmemmap population. With both the
> >>>> retained and shared entries initialized in the same stage, there
> >>>> will be no cross-stage inconsistency to check, and this warning
> >>>> can be removed.
> >>>> 
> >>>> Would keeping the check here for now and removing it as part of
> >>>> that follow-up sound reasonable to you?
> >>> 
> >>> While it feels really out of place in __init_single_page(), but having it
> >>> memmap_init_range() close to the if that skips shared tail pages makes
> >>> sense.
> >>> 
> >>> What do you say?
> >> 
> >> Sorry for the late reply because of traveling to LPC.
> >> 
> >> Putting the check in memmap_init_range() would validate the
> >> invariant for the shared-tail range skipped there. It would
> >> not, however, cover all vmemmap-optimized initialization
> >> paths: optimized Device DAX bypasses memmap_init_range()
> >> when there is no altmap, and deferred boot-time HugeTLB
> >> initialization may also initialize retained entries
> >> elsewhere.
> >> 
> >> If the intention is to validate only the skip in
> >> memmap_init_range(), moving it there makes sense. If we want
> >> the warning to cover both HugeTLB and Device DAX generally,
> >> it still needs to remain in __init_single_page() or be
> >> called from the individual initialization paths.
> > 
> > Yeah, putting this into all the callers sucks too :(
> > 
> > With all the callers,__init_single_page() is the right place for this
> > check, but I think we can add a static inline in sparse.h, like e.g.
> > 
> > vmemmap_optimization_verify_zone(page, pfn)
> > 
> > to make it a bit nicer.
> 
> Yeah, factoring this into a helper makes sense.
> 
> Since it currently has only one caller, how about keeping it as a local
> static inline in mm_init.c instead of putting it in sparse.h?

Presuming we don't expect new callers any time soon let's put it just above
__init_single_page().

-- 
Sincerely yours,
Mike.