linux-kernel - Re: [RFC PATCH v2] mm: initialize struct pages reserved by ZONE

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Message-ID: <3d27953a-88b8-5a7c-de3c-041f8b4436f6@vx.jp.nec.com>
Date:   Tue, 17 Sep 2019 09:32:43 +0000
From:   Toshiki Fukasawa <t-fukasawa@...jp.nec.com>
To:     David Hildenbrand <david@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>
CC:     Toshiki Fukasawa <t-fukasawa@...jp.nec.com>,
        "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>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Junichi Nomura <j-nomura@...jp.nec.com>
Subject: Re: [RFC PATCH v2] mm: initialize struct pages reserved by
 ZONE_DEVICE driver.

On 2019/09/17 16:13, David Hildenbrand wrote:
> On 17.09.19 04:34, Toshiki Fukasawa wrote:
>> On 2019/09/09 16:46, David Hildenbrand wrote:
>>> Let's take a step back here to understand the issues I am aware of. I
>>> think we should solve this for good now:
>>>
>>> A PFN walker takes a look at a random PFN at a random point in time. It
>>> finds a PFN with SECTION_MARKED_PRESENT && !SECTION_IS_ONLINE. The
>>> options are:
>>>
>>> 1. It is buddy memory (add_memory()) that has not been online yet. The
>>> memmap contains garbage. Don't access.
>>>
>>> 2. It is ZONE_DEVICE memory with a valid memmap. Access it.
>>>
>>> 3. It is ZONE_DEVICE memory with an invalid memmap, because the section
>>> is only partially present: E.g., device starts at offset 64MB within a
>>> section or the device ends at offset 64MB within a section. Don't access it.
>>
>> I don't agree with case #3. In the case, struct page area is not allocated on
>> ZONE_DEVICE, but is allocated on system memory. So I think we can access the
>> struct pages. What do you mean "invalid memmap"?
> No, that's not the case. There is no memory, especially not system
> memory. We only allow partially present sections (sub-section memory
> hotplug) for ZONE_DEVICE.

Let me clear my thoughts. If I read correctly, the struct pages for sections
(including partially present sections) on ZONE_DEVICE are allocated by
vmemmap_populate(). And all the struct pages except (altmap->base_pfn) to
(altmap->base_pfn + altmap->reserve) are initialized by memmap_init_zone()
and memmap_init_zone_device().

Do struct pages for partially present sections go through a different process?

Thanks,
Toshiki Fukasawa
> 
> invalid memmap == memmap was not initialized == struct pages contains
> garbage. There is a memmap, but accessing it (e.g., pfn_to_nid()) will
> trigger a BUG.
>