linux-kernel - Re: uninitialized pmem struct pages

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Date:   Mon, 4 Jan 2021 17:30:48 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: uninitialized pmem struct pages

>> Let's assume this is indeed a reserved pfn in the altmap. What's the
>> actual address of the memmap?
> 
> Not sure what exactly you are asking for but crash says
> crash> kmem -p 6060000
>       PAGE          PHYSICAL      MAPPING       INDEX CNT FLAGS
> fffff8c600181800     6060000                0        0  0 fffffc0000000

^ this looks like it was somewhat initialized. All flags zero, nid/zone
set to -1 (wild guess) and thus the crash? weird

>  
>> 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?
> 
> I am not really familiar with the pmem so I would need more assistance
> here. I've tried this (shot into the dark):
> crash> struct page.pgmap fffff8c600181800
>       pgmap = 0xfffff8c600181808

That's weird. If the memmap is at fffff8c600181800, why should the pgmap
be at an offset of 8 bytes from there. The "pgmap" field is actually at
an offset of 8 bytes within the memmap ...

Assuming the memmap is not actually ZONE_DEVICE, fffff8c600181800 really
only contains garbage, including the pgmap pointer :(

> crash> struct -x dev_pagemap 0xfffff8c600181808
> struct dev_pagemap {
>   altmap = {
>     base_pfn = 0xfffff8c600181808, 
>     end_pfn = 0xfffff8c600181808,

^ because this is very weird

>     reserve = 0x0, 

^ and this indicates nothing was actually reserved.


-- 
Thanks,

David / dhildenb