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

[Muchun Song <muchun.song@linux.dev>]

> 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.

Thanks,
Muchun

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