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

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Date:   Tue, 17 Sep 2019 02:34:43 +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 2019/09/09 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"?

> 
> 4. It is ZONE_DEVICE memory with an invalid memmap, because the memmap
> was not initialized yet. memmap_init_zone_device() did not yet succeed
> after dropping the mem_hotplug lock in mm/memremap.c. Don't access it.
> 
> 5. It is reserved ZONE_DEVICE memory ("pages mapped, but reserved for
> driver") with an invalid memmap. Don't access it.
> 
> I can see that your patch tries to make #5 vanish by initializing the
> memmap, fair enough. #3 and #4 can't be detected. The PFN walker could
> still stumble over uninitialized memmaps.
>