> On Oct 30, 2026, at 19:53, Muchun Song <[email protected]> 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: <[email protected]> # see patch description, needs a different fix
> for <= 7.3
> Signed-off-by: Muchun Song <[email protected]>
> Reported-by: Pavithra <[email protected]>
> Closes:
> https://lore.kernel.org/r/97f85592bb78a273be8519e6897e7f9b9bd31469.1793070429.git.ritesh.l...@gmail.com
> Reviewed-by: Ritesh Harjani (IBM) <[email protected]>
> Tested-by: Pavithra Prakash <[email protected]>
Hi Andrew,
Could you pick up this?
Thanks,
Muchun
