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

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Date:   Wed, 13 Nov 2019 22:22:48 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     David Hildenbrand <david@...hat.com>,
        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>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH 2/3] mm: Introduce subsection_dev_map



> Am 13.11.2019 um 22:12 schrieb Dan Williams <dan.j.williams@...el.com>:
> 
> On Wed, Nov 13, 2019 at 12:40 PM David Hildenbrand <david@...hat.com> wrote:
> [..]
>>>>>> 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.
> 
> That's what I was debating with Toshiki [1], whether there is a real
> example of needing to distinguish ZONE_DEVICE from offline memory in a
> pfn walker. The proposed use case in this patch set of being able to
> set hwpoison on ZONE_DEVICE pages does not seem like a good idea to
> me. My suspicion is that this is a common theme and others are looking
> to do these types page manipulations that only make sense for online
> memory. If that is the case then treating ZONE_DEVICE as offline seems
> the right direction.

Right. At least it would be nice to have for zone shrinking - not sure about the other walkers. We would have to special-case ZONE_DEVICE handling there.

But as I said, a subsection map for online memory is a good start, especially to fix pfn_to_online_page(). Also, I think this might be a very good thing to have for Oscars memmap-on-memory work (I have a plan in my head I can discuss with Oscar once he has time to work on that again).


> 
> [1]: https://lore.kernel.org/linux-mm/CAPcyv4joVDwiL21PPyJ7E_mMFR2SL3QTi09VMtfxb_W+-1p8vQ@mail.gmail.com/
>