Re: [PATCH v2 03/69] powerpc/mm: Fix wrong addr_pfn tracking in compound vmemmap population

[Oscar Salvador <osalvador@suse.de>]

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 <songmuchun@bytedance.com>

Acked-by: Oscar Salvador <osalvador@suse.de>

 

-- 
Oscar Salvador
SUSE Labs