Re: [PATCH v2 03/69] powerpc/mm: Fix wrong addr_pfn tracking in compound vmemmap population
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From: Oscar Salvador <osalvador@suse.de>
Date: 2026-05-14 08:00:21
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On Wed, May 13, 2026 at 09:04:31PM +0800, Muchun Song wrote:
vmemmap_populate_compound_pages() uses addr_pfn to determine the PFN
offset within a compound page and to decide whether the current
vmemmap slot should be populated as a head page mapping or should reuse
a tail page mapping.
However, addr_pfn is advanced manually in parallel with addr. The loop
itself progresses in vmemmap address space, so each PAGE_SIZE step in
addr covers PAGE_SIZE / sizeof(struct page) struct page slots. Since
addr_pfn is compared against nr_pages in data-PFN units, it should
advance by the same number of PFNs. The existing manual increments do
not match that and therefore do not reliably track the PFN
corresponding to the current addr.
As a result, pfn_offset can be computed from the wrong PFN and the code
can make the head/tail decision for the wrong compound-page position.
Fix this by deriving addr_pfn directly from the current vmemmap address
instead of carrying it as loop state.
Fixes: f2b79c0d7968 ("powerpc/book3s64/radix: add support for vmemmap optimization for radix")
Signed-off-by: Muchun Song <redacted>Acked-by: Oscar Salvador <osalvador@suse.de> -- Oscar Salvador SUSE Labs