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

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Message-ID: <38e58d23-c20b-4e68-5f56-20bba2be2d6c@redhat.com>
Date:   Tue, 17 Sep 2019 09:13:10 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Toshiki Fukasawa <t-fukasawa@...jp.nec.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>
Cc:     "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 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.

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.

-- 

Thanks,

David / dhildenb