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Subject: Re: uninitialized pmem struct pages
From:   David Hildenbrand <david@redhat.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Dan Williams <dan.j.williams@intel.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Oscar Salvador <osalvador@suse.de>
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Organization: Red Hat GmbH
Message-ID: <6106ca7f-3247-0916-3e1e-ad6af17272ea@redhat.com>
Date:   Mon, 4 Jan 2021 16:44:52 +0100
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On 04.01.21 16:43, David Hildenbrand wrote:
> On 04.01.21 16:33, Michal Hocko wrote:
>> On Mon 04-01-21 16:15:23, David Hildenbrand wrote:
>>> On 04.01.21 16:10, Michal Hocko wrote:
>> [...]
>>> Do the physical addresses you see fall into the same section as boot
>>> memory? Or what's around these addresses?
>>
>> Yes I am getting a garbage for the first struct page belonging to the
>> pmem section [1]
>> [    0.020161] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x603fffffff]
>> [    0.020163] ACPI: SRAT: Node 4 PXM 4 [mem 0x6060000000-0x11d5fffffff] non-volatile
>>
>> The pfn without the initialized struct page is 0x6060000. This is a
>> first pfn in a section.
> 
> Okay, so we're not dealing with the "early section" mess I described,
> different story.
> 
> Due to [1], is_mem_section_removable() called
> pfn_to_page(PHYS_PFN(0x6060000)). page_zone(page) made it crash, as not
> initialized.
> 
> Let's assume this is indeed a reserved pfn in the altmap. What's the
> actual address of the memmap?
> 
> I do wonder what hosts pfn_to_page(PHYS_PFN(0x6060000)) - is it actually
> part of the actual altmap (i.e. > 0x6060000) or maybe even self-hosted?
> 
> If it's not self-hosted, initializing the relevant memmaps should work
> just fine I guess. Otherwise things get more complicated.

Oh, I forgot: pfn_to_online_page() should at least in your example make
sure other pfn walkers are safe. It was just an issue of
is_mem_section_removable().


-- 
Thanks,

David / dhildenb