Re: [PATCH v1] mm: initialize struct page for reserved pages in ZONE_DEVICE

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Jun 21, 2018 at 11:08 PM, Naoya Horiguchi
<n-horiguchi@ah.jp.nec.com> wrote:
> Reading /proc/kpageflags for pfns allocated by pmem namespace triggers
> kernel panic with a message like "BUG: unable to handle kernel paging
> request at fffffffffffffffe".
>
> The first few pages (controlled by altmap passed to memmap_init_zone())
> in the ZONE_DEVICE can skip struct page initialization, which causes
> the reported issue.
>
> This patch simply adds some initialization code for them.
>
> Fixes: 4b94ffdc4163 ("x86, mm: introduce vmem_altmap to augment vmemmap_populate()")
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  mm/page_alloc.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git v4.17-mmotm-2018-06-07-16-59/mm/page_alloc.c v4.17-mmotm-2018-06-07-16-59_patched/mm/page_alloc.c
> index 1772513..0b36afe 100644
> --- v4.17-mmotm-2018-06-07-16-59/mm/page_alloc.c
> +++ v4.17-mmotm-2018-06-07-16-59_patched/mm/page_alloc.c
> @@ -5574,8 +5574,16 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>          * Honor reservation requested by the driver for this ZONE_DEVICE
>          * memory
>          */
> -       if (altmap && start_pfn == altmap->base_pfn)
> +       if (altmap && start_pfn == altmap->base_pfn) {
> +               unsigned long i;
> +
> +               for (i = 0; i < altmap->reserve; i++) {
> +                       page = pfn_to_page(start_pfn + i);
> +                       __init_single_page(page, start_pfn + i, zone, nid);
> +                       SetPageReserved(page);
> +               }
>                 start_pfn += altmap->reserve;
> +       }

No, unfortunately this will clobber metadata that lives in that
reserved area, see __nvdimm_setup_pfn().