linux-kernel - Re: [PATCH 2/3] mm: Introduce subsection_dev

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Date:   Wed, 13 Nov 2019 21:40:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Toshiki Fukasawa <t-fukasawa@...jp.nec.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "adobriyan@...il.com" <adobriyan@...il.com>,
        "hch@....de" <hch@....de>,
        "longman@...hat.com" <longman@...hat.com>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "mst@...hat.com" <mst@...hat.com>, "cai@....pw" <cai@....pw>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Junichi Nomura <j-nomura@...jp.nec.com>
Subject: Re: [PATCH 2/3] mm: Introduce subsection_dev_map



> Am 13.11.2019 um 21:23 schrieb David Hildenbrand <david@...hat.com>:
> 
> 
> 
>>> Am 13.11.2019 um 21:10 schrieb Dan Williams <dan.j.williams@...el.com>:
>>> 
>>> On Wed, Nov 13, 2019 at 11:53 AM David Hildenbrand <david@...hat.com> wrote:
>>> 
>>> 
>>> 
>>>>> Am 13.11.2019 um 20:06 schrieb Dan Williams <dan.j.williams@...el.com>:
>>>> 
>>>> On Wed, Nov 13, 2019 at 10:51 AM David Hildenbrand <david@...hat.com> wrote:
>>>>> 
>>>>>> On 08.11.19 20:13, Dan Williams wrote:
>>>>>> On Thu, Nov 7, 2019 at 4:15 PM Toshiki Fukasawa
>>>>>> <t-fukasawa@...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@...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.

Oh, and just to clarify why I proposed pfn_active(): The issue right now is that a PFN that is valid but not online could be offline memory (memmap not initialized) or ZONE_DEVICE. That‘s why I wanted to have a way to detect if a memmap was initialized, independent of the zone. That‘s important for generic PFN walkers.

>> 
>>> 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!
> 
>> 
>