linux-kernel - Re: [PATCH] memremap: Fix NULL pointer BUG in get_zone_device

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Date:   Tue, 23 Aug 2016 13:42:16 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Toshi Kani <toshi.kani@....com>
Cc:     dan.j.williams@...el.com, m.abhilash-kumar@....com,
        linux-nvdimm@...1.01.org, linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Brian Starkey <brian.starkey@....com>
Subject: Re: [PATCH] memremap: Fix NULL pointer BUG in
 get_zone_device_page()

On Tue, 23 Aug 2016 12:43:20 -0600 Toshi Kani <toshi.kani@....com> wrote:

> The following BUG was observed while starting up KVM with nvdimm
> device as memory-backend-file to /dev/dax.
> 
>  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
>
> ...
>
> devm_memremap_pages() calls for_each_device_pfn() to walk through
> all pfns in page_map.  pfn_first(), however, returns a wrong pfn
> that leaves page->pgmap uninitialized.
> 
> Since arch_add_memory() has set up direct mappings to the NVDIMM
> range with altmap, pfn_first() should not modify the start pfn.
> Change pfn_first() to simply return pfn of res->start.

Which kernel version(s) do you think need fixing?