Re: [PATCH] memremap: Fix NULL pointer BUG in get_zone_device_page()

From: Dan Williams
Date: Wed Aug 24 2016 - 10:56:10 EST

[ adding Konstantin ]

On Wed, Aug 24, 2016 at 7:40 AM, Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote:
> 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@xxxxxxx
>> >
>> > 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...

We could leave the unmapped portions of a section out of the radix,
but I'm also worried about keeping the get_dev_pagemap() lookup cheap.
I saw that Konstantin has some proposed changes to the radix api to
make it easier to fill a range [1]. I might switch to
radix_tree_fill_range() when it becomes available.