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

[Mike Rapoport <rppt@kernel.org>]

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?
 
> [1] https://lore.kernel.org/20260513132044.41690-18-songmuchun@bytedance.com/
> 
> Thanks,
> Muchun

-- 
Sincerely yours,
Mike.