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

On Tue, Oct 06, 2026 at 03:32:43PM +0200, Muchun Song wrote:

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On Oct 6, 2026, at 12:52, Mike Rapoport [off-list ref] wrote:

On Mon, Oct 05, 2026 at 06:27:52PM +0200, Muchun Song wrote:

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On Oct 2, 2026, at 20:12, Mike Rapoport [off-list ref] wrote:

On Fri, Oct 02, 2026 at 05:56:40PM +0800, Muchun Song wrote:

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On Oct 2, 2026, at 16:22, Mike Rapoport [off-list ref] wrote:

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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.