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

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Message-ID: <1472049570.1532.10.camel@hpe.com>
Date:   Wed, 24 Aug 2016 14:40:07 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     "dan.j.williams@...el.com" <dan.j.williams@...el.com>
CC:     "Mulumudi, Abhilash Kumar" <m.abhilash-kumar@....com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "brian.starkey@....com" <brian.starkey@....com>
Subject: Re: [PATCH] memremap: Fix NULL pointer BUG in get_zone_device_page()

On Tue, 2016-08-23 at 21:48 -0700, Dan Williams wrote:
> On Tue, Aug 23, 2016 at 9:28 PM, Kani, Toshimitsu <toshi.kani@....com
> > 
> > BTW, why does devm_memremap_pages() put a whole range to
> > pgmap_radix as device memory, but only initialize page->pgmap for
> > its data range?  Is there particular reason for this inconsistency?
>
> The radix tree is indexed by section number, but we don't always
> initialize a full section.  The cases when we don't use a full
> section is when it overlaps device metadata, or if a platform
> multiplexes the device memory range with another resource within the
> same section.

I see, but I still feel odd about making get_dev_pagemap() to work for
metadata, but get_page() -> get_zone_device_page() to crash like this.
 follow_devmap_pmd() assumes get_page() to work when get_dev_pagemap()
returns a valid pgmap...

Thanks,
-Toshi