Re: [PATCH v2 2/8] s390x/mm: Implement arch_remove_memory()

[David Hildenbrand <david@redhat.com>]

On 07.05.19 22:46, Dan Williams wrote:
> On Tue, May 7, 2019 at 11:38 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> Will come in handy when wanting to handle errors after
>> arch_add_memory().
>>
>> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
>> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Vasily Gorbik <gor@linux.ibm.com>
>> Cc: Oscar Salvador <osalvador@suse.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  arch/s390/mm/init.c | 13 +++++++------
>>  1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
>> index 31b1071315d7..1e0cbae69f12 100644
>> --- a/arch/s390/mm/init.c
>> +++ b/arch/s390/mm/init.c
>> @@ -237,12 +237,13 @@ int arch_add_memory(int nid, u64 start, u64 size,
>>  void arch_remove_memory(int nid, u64 start, u64 size,
>>                         struct vmem_altmap *altmap)
>>  {
>> -       /*
>> -        * There is no hardware or firmware interface which could trigger a
>> -        * hot memory remove on s390. So there is nothing that needs to be
>> -        * implemented.
>> -        */
>> -       BUG();
>> +       unsigned long start_pfn = start >> PAGE_SHIFT;
>> +       unsigned long nr_pages = size >> PAGE_SHIFT;
>> +       struct zone *zone;
>> +
>> +       zone = page_zone(pfn_to_page(start_pfn));
> 
> Does s390 actually support passing in an altmap? If 'yes', I think it
> also needs the vmem_altmap_offset() fixup like x86-64:
> 
>         /* With altmap the first mapped page is offset from @start */
>         if (altmap)
>                 page += vmem_altmap_offset(altmap);
> 
> ...but I suspect it does not support altmap since
> arch/s390/mm/vmem.c::vmemmap_populate() does not arrange for 'struct
> page' capacity to be allocated out of an altmap defined page pool.
> 
> I think it would be enough to disallow any arch_add_memory() on s390
> where @altmap is non-NULL. At least until s390 gains ZONE_DEVICE
> support and can enable the pmem use case.
> 

As far as I know, it doesn't yet, however I guess this could change once
virtio-pmem is supported?

Thanks!

-- 

Thanks,

David / dhildenb