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

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Date:   Tue, 17 Sep 2019 13:04:28 -0400
From:   Waiman Long <longman@...hat.com>
To:     Qian Cai <cai@....pw>, David Hildenbrand <david@...hat.com>,
        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>,
        "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 9/17/19 12:21 PM, Qian Cai wrote:
> On Tue, 2019-09-17 at 11:49 -0400, Waiman Long wrote:
>> On 9/17/19 3:13 AM, 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.
>>>
>>> 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.
>>>
>> As long as the page structures exist, they should be initialized to some
>> known state. We could set PagePoison for those invalid memmap. It is the
> Sounds like you want to run page_init_poison() by default.

Yes for those pages that are not initialized otherwise. I don't want to
run page_init_poison() for the whole ZONE_DEVICE memory range as it can
take a while if we are talking about TBs of persistent memory. Also most
of the pages will be reinitialized anyway in the init process. So it is
mostly a wasted effort. However, for those reserved pages that are not
being exported to the memory management layer, having them initialized
to a known state will cause less problem down the road.

Cheers,
Longman