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SubjectRe: [PATCH 2/3] mm: Introduce subsection_dev_map
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> 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. Anyhow, a subsection online map would be a good start, we can reuse that later for ZONE_DEVICE as well.

>
>> I dislike a map that is specific to ZONE_DEVICE or (currently)
>> !ZONE_DEVICE. I rather want an indication "this memmap is safe to
>> touch". As discussed along the mentioned threads, we can combine this
>> later with RCU to handle some races that are currently possible.
>
> The rcu protection is independent of the pfn_active vs pfn_online
> distinction afaics.

It’s one part of the bigger picture IMHO.

>

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