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

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

> On Oct 30, 2026, at 19:53, Muchun Song <songmuchun@bytedance.com> 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@vger.kernel.org> # see patch description, needs a different fix for <= 7.3
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Reported-by: Pavithra <pavrampu@linux.ibm.com>
> Closes: https://lore.kernel.org/r/97f85592bb78a273be8519e6897e7f9b9bd31469.1793070429.git.ritesh.list@gmail.com
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Tested-by: Pavithra Prakash <pavrampu@linux.ibm.com>

Hi Andrew,

Could you pick up this?

Thanks,
Muchun