Re: [PATCH 2/3] mm: Introduce subsection_dev_map

[David Hildenbrand <david@redhat.com>]

> Am 13.11.2019 um 21:10 schrieb Dan Williams <dan.j.williams@intel.com>:
> 
> On Wed, Nov 13, 2019 at 11:53 AM David Hildenbrand <david@redhat.com> wrote:
>> 
>> 
>> 
>>>> Am 13.11.2019 um 20:06 schrieb Dan Williams <dan.j.williams@intel.com>:
>>> 
>>> On Wed, Nov 13, 2019 at 10:51 AM David Hildenbrand <david@redhat.com> wrote:
>>>> 
>>>>> On 08.11.19 20:13, Dan Williams wrote:
>>>>> On Thu, Nov 7, 2019 at 4:15 PM Toshiki Fukasawa
>>>>> <t-fukasawa@vx.jp.nec.com> wrote:
>>>>>> 
>>>>>> Currently, there is no way to identify pfn on ZONE_DEVICE.
>>>>>> Identifying pfn on system memory can be done by using a
>>>>>> section-level flag. On the other hand, identifying pfn on
>>>>>> ZONE_DEVICE requires a subsection-level flag since ZONE_DEVICE
>>>>>> can be created in units of subsections.
>>>>>> 
>>>>>> This patch introduces a new bitmap subsection_dev_map so that
>>>>>> we can identify pfn on ZONE_DEVICE.
>>>>>> 
>>>>>> Also, subsection_dev_map is used to prove that struct pages
>>>>>> included in the subsection have been initialized since it is
>>>>>> set after memmap_init_zone_device(). We can avoid accessing
>>>>>> pages currently being initialized by checking subsection_dev_map.
>>>>>> 
>>>>>> Signed-off-by: Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>
>>>>>> ---
>>>>>> include/linux/mmzone.h | 19 +++++++++++++++++++
>>>>>> mm/memremap.c          |  2 ++
>>>>>> mm/sparse.c            | 32 ++++++++++++++++++++++++++++++++
>>>>>> 3 files changed, 53 insertions(+)
>>>>>> 
>>>>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>>>>> index bda2028..11376c4 100644
>>>>>> --- a/include/linux/mmzone.h
>>>>>> +++ b/include/linux/mmzone.h
>>>>>> @@ -1174,11 +1174,17 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
>>>>>> 
>>>>>> struct mem_section_usage {
>>>>>>        DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
>>>>>> +#ifdef CONFIG_ZONE_DEVICE
>>>>>> +       DECLARE_BITMAP(subsection_dev_map, SUBSECTIONS_PER_SECTION);
>>>>>> +#endif
>>>>> 
>>>>> Hi Toshiki,
>>>>> 
>>>>> There is currently an effort to remove the PageReserved() flag as some
>>>>> code is using that to detect ZONE_DEVICE. In reviewing those patches
>>>>> we realized that what many code paths want is to detect online memory.
>>>>> So instead of a subsection_dev_map add a subsection_online_map. That
>>>>> way pfn_to_online_page() can reliably avoid ZONE_DEVICE ranges. I
>>>>> otherwise question the use case for pfn_walkers to return pages for
>>>>> ZONE_DEVICE pages, I think the skip behavior when pfn_to_online_page()
>>>>> == false is the right behavior.
>>>> 
>>>> To be more precise, I recommended an subsection_active_map, to indicate
>>>> which memmaps were fully initialized and can safely be touched (e.g., to
>>>> read the zone/nid). This map would also be set when the devmem memmaps
>>>> were initialized (race between adding memory/growing the section and
>>>> initializing the memmap).
>>>> 
>>>> See
>>>> 
>>>> https://lkml.org/lkml/2019/10/10/87
>>>> 
>>>> and
>>>> 
>>>> https://www.spinics.net/lists/linux-driver-devel/msg130012.html
>>> 
>>> I'm still struggling to understand the motivation of distinguishing
>>> "active" as something distinct from "online". As long as the "online"
>>> granularity is improved from sections down to subsections then most
>>> code paths are good to go. The others can use get_devpagemap() to
>>> check for ZONE_DEVICE in a race free manner as they currently do.
>> 
>> I thought we wanted to unify access if we don’t really care about the zone as in most pfn walkers - E.g., for zone shrinking.
> 
> Agree, when the zone does not matter, which is most cases, then
> pfn_online() and pfn_valid() are sufficient.
> 
>> Anyhow, a subsection online map would be a good start, we can reuse that later for ZONE_DEVICE as well.
> 
> Cool, good to go with me sending a patch to introduce pfn_online() and
> a corresponding subsection_map for the same?

Yeah, let‘s see how this turns out and if we‘re on the same page. Thanks!

>