[PATCH v1 01/10] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes

[David Hildenbrand]

On 05.11.19 02:30, Dan Williams wrote:
> On Thu, Oct 24, 2019 at 5:10 AM David Hildenbrand <david at redhat.com> wrote:
>>
>> Our onlining/offlining code is unnecessarily complicated. Only memory
>> blocks added during boot can have holes (a range that is not
>> IORESOURCE_SYSTEM_RAM). Hotplugged memory never has holes (e.g., see
>> add_memory_resource()). All boot memory is alread online.
> 
> s/alread/already/
> 

Thanks.

> ...also perhaps clarify "already online" by what point in time and why
> that is relevant. For example a description of the difference between
> the SetPageReserved() in the bootmem path and the one in the hotplug
> path.

Will add.

> 
>> Therefore, when we stop allowing to offline memory blocks with holes, we
>> implicitly no longer have to deal with onlining memory blocks with holes.
> 
> Maybe an explicit reference of the code areas that deal with holes
> would help to back up that assertion. Certainly it would have saved me
> some time for the review.

I can add a reference to the onlining code that will only online pages 
that don't fall into memory holes.

> 
>> This allows to simplify the code. For example, we no longer have to
>> worry about marking pages that fall into memory holes PG_reserved when
>> onlining memory. We can stop setting pages PG_reserved.
> 
> ...but not for bootmem, right?

Yes, bootmem is not changed. (especially, early allocations and memory 
holes are marked PG_reserved - basically everything not given to the 
buddy after boot)

> 
>>
>> Offlining memory blocks added during boot is usually not guranteed to work
> 
> s/guranteed/guaranteed/

Thanks.

> 
>> either way (unmovable data might have easily ended up on that memory during
>> boot). So stopping to do that should not really hurt (+ people are not
>> even aware of a setup where that used to work
> 
> Maybe put a "Link: https://lkml.kernel.org/r/$msg_id" to that discussion?
> 
>> and that the existing code
>> still works correctly with memory holes). For the use case of offlining
>> memory to unplug DIMMs, we should see no change. (holes on DIMMs would be
>> weird).
> 
> However, less memory can be offlined than was theoretically allowed
> previously, so I don't understand the "we should see no change"
> comment. I still agree that's a price worth paying to get the code
> cleanups and if someone screams we can look at adding it back, but the
> fact that it was already fragile seems decent enough protection.

Hotplugged DIMMs (add_memory()) have no holes and will therefore see no 
change.

>>
>> Please note that hardware errors (PG_hwpoison) are not memory holes and
>> not affected by this change when offlining.
>>
>> Cc: Andrew Morton <akpm at linux-foundation.org>
>> Cc: Michal Hocko <mhocko at suse.com>
>> Cc: Oscar Salvador <osalvador at suse.de>
>> Cc: Pavel Tatashin <pasha.tatashin at soleen.com>
>> Cc: Dan Williams <dan.j.williams at intel.com>
>> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
>> Signed-off-by: David Hildenbrand <david at redhat.com>
>> ---
>>   mm/memory_hotplug.c | 26 ++++++++++++++++++++++++--
>>   1 file changed, 24 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 561371ead39a..8d81730cf036 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1447,10 +1447,19 @@ static void node_states_clear_node(int node, struct memory_notify *arg)
>>                  node_clear_state(node, N_MEMORY);
>>   }
>>
>> +static int count_system_ram_pages_cb(unsigned long start_pfn,
>> +                                    unsigned long nr_pages, void *data)
>> +{
>> +       unsigned long *nr_system_ram_pages = data;
>> +
>> +       *nr_system_ram_pages += nr_pages;
>> +       return 0;
>> +}
>> +
>>   static int __ref __offline_pages(unsigned long start_pfn,
>>                    unsigned long end_pfn)
>>   {
>> -       unsigned long pfn, nr_pages;
>> +       unsigned long pfn, nr_pages = 0;
>>          unsigned long offlined_pages = 0;
>>          int ret, node, nr_isolate_pageblock;
>>          unsigned long flags;
>> @@ -1461,6 +1470,20 @@ static int __ref __offline_pages(unsigned long start_pfn,
>>
>>          mem_hotplug_begin();
>>
>> +       /*
>> +        * Don't allow to offline memory blocks that contain holes.
>> +        * Consecuently, memory blocks with holes can never get onlined
> 
> s/Consecuently/Consequently/

Thanks.

> 
>> +        * (hotplugged memory has no holes and all boot memory is online).
>> +        * This allows to simplify the onlining/offlining code quite a lot.
>> +        */
> 
> The last sentence of this comment makes sense in the context of this
> patch, but I don't think it stands by itself in the code base after
> the fact. The person reading the comment can't see the simplifications
> because the code is already gone. I'd clarify it to talk about why it
> is safe to not mess around with PG_Reserved in the hotplug path
> because of this check.

I'll think of something. It's not just the PG_reserved handling but the 
whole pfn_valid()/walk_system_ram_range() handling that can be simplified.

> 
> After those clarifications you can add:
> 
> Reviewed-by: Dan Williams <dan.j.williams at intel.com>
> 

Thanks!

-- 

Thanks,

David / dhildenb