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

[Dan Williams]

On Thu, Jun 21, 2018 at 11:12 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Thu, Jun 21, 2018 at 11:08 PM, Naoya Horiguchi
> <n-horiguchi@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>> ---
>>  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().

I think the kpageflags code needs to lookup the dev_pagemap in the
ZONE_DEVICE case and honor the altmap.