Re: [PATCH v2] powerpc/mm: fix uninitialized ZONE_DEVICE struct pages with PMDs

[Mike Rapoport]

On Fri, Oct 30, 2026 at 07:53:25PM +0800, Muchun Song wrote:
> PowerPC radix vmemmap population can reuse an existing PMD mapping for
> a device DAX range marked HVO-enabled. The reused mapping is ordinary:
> its tail struct pages are not remapped to the shared tail page.
> 
> Leaving the section marked HVO-enabled makes memmap_init_zone_device()
> treat the range as optimized and skip initializing real tail struct pages
> that remain in use. This first triggers a zone mismatch warning, then
> faults while initializing the pageblock flags:
> 
>   WARNING: mm/mm_init.c:600 at __init_single_page+0xd0/0xf4
>   Call Trace:
>     memmap_init_zone_device+0x164/0x430
>     memremap_pages+0x43c/0x93c
>     devm_memremap_pages+0x3c/0xa4
>     dev_dax_probe+0x264/0x3d0
>     dax_bus_probe+0x140/0x1c8
> 
>   BUG: Kernel NULL pointer dereference on read at 0x00001400
>     __set_pfnblock_flags_mask.constprop.0+0x34/0x160
>     memmap_init_zone_device+0x358/0x430
> 
> Without these diagnostics, the uninitialized metadata remains latent until
> userspace accesses the device DAX range and triggers a later panic.
> 
> Mark the section non-HVO when reusing a PMD mapping so its state matches
> the actual vmemmap layout and core ZONE_DEVICE code initializes all real
> struct pages.
> 
> The Fixes target predates the section-based vmemmap optimization
> infrastructure, so this patch cannot be applied directly to some older
> kernels. Older kernels instead need to pass the vmemmap bounds to a
> range-aware vmemmap_can_optimize() and reject optimization on PowerPC if
> a PMD leaf overlaps the range. This reuses the existing mapping normally
> and makes core ZONE_DEVICE code initialize all real tail entries.
> 
> Fixes: f2b79c0d7968 ("powerpc/book3s64/radix: add support for vmemmap optimization for radix")
> Cc: <stable@xxxxxxxxxxxxxxx> # see patch description, needs a different fix for <= 7.3
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Reported-by: Pavithra <pavrampu@xxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/r/97f85592bb78a273be8519e6897e7f9b9bd31469.1793070429.git.ritesh.list@xxxxxxxxx
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> Tested-by: Pavithra Prakash <pavrampu@xxxxxxxxxxxxx>

Acked-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

> ---
> v2:
> - Rewrite the commit message.
> - Describe the range-aware vmemmap_can_optimize() fix for older kernels
> - Note that kernels through v7.3 require a different stable fix
>   (suggested by Ritesh Harjani)
> - Collect Tested-by from Pavithra Prakash
> ---
>  arch/powerpc/mm/book3s64/radix_pgtable.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
> index 44868b62b14e..4f75f10ed68f 100644
> --- a/arch/powerpc/mm/book3s64/radix_pgtable.c
> +++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
> @@ -1261,7 +1261,16 @@ int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
>  			return -ENOMEM;
>  
>  		if (pmd_leaf(READ_ONCE(*pmd))) {
> -			/* existing huge mapping. Skip the range */
> +			unsigned long addr_pfn = page_to_pfn((struct page *)addr);
> +			struct mem_section *ms = __pfn_to_section(addr_pfn);
> +
> +			/*
> +			 * Existing PMD mappings can cover the edges of a valid
> +			 * HVO populate range. They are ordinary vmemmap mappings,
> +			 * so the current section must not be treated as HVO-backed
> +			 * when its struct pages are initialized later.
> +			 */
> +			section_set_compound_order(ms, 0);
>  			next = pmd_addr_end(addr, end);
>  			continue;
>  		}
> 
> base-commit: d865f2677c44cb2171b833d6775cf692756fa8be
> -- 
> 2.54.0
> 

-- 
Sincerely yours,
Mike.