Re: [PATCH] x86/mm: Add x86 valid_phys_addr_range() for /dev/mem

[Dan Williams]

On Wed, Feb 17, 2016 at 11:35 AM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> On Wed, 2016-02-17 at 09:58 -0800, Dan Williams wrote:
>> On Tue, Feb 9, 2016 at 6:06 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
>> > x86 does not define ARCH_HAS_VALID_PHYS_ADDR_RANGE, which
>> > leads /dev/mem to use the default valid_phys_addr_range()
>> > and valid_mmap_phys_addr_range() in drivers/char/mem.c.
>> >
>> > The default valid_phys_addr_range() allows any range lower
>> > than __pa(high_memory), which is the end of system RAM, and
>> > disallows any range higher than it.
>> >
>> > Persistent memory may be located at lower and/or higher
>> > address of __pa(high_memory) depending on their memory slots.
>> > When using crash(8) via /dev/mem for analyzing data in
>> > persistent memory, it can only access to the one lower than
>> > __pa(high_memory).
>> >
>> > Add x86 valid_phys_addr_range() and valid_mmap_phys_addr_range()
>> > to provide better checking:
>> >  - Physical address range is valid when it is fully backed by
>> >    IORESOURCE_MEM, regardless of __pa(high_memory).
>> >  - Other ranges, including holes, are invalid.
>> >
>> > This also allows crash(8) to access persistent memory ranges
>> > via /dev/mem (with a minor change to remove high_memory check
>> > from crash itself).
>>
>> If we're modifying crash(8) can't we also teach it to mmap /dev/pmemX
>> directly?  With commit 90a545e98126 "restrict /dev/mem to idle io
>> memory ranges" /dev/mem should not have access to active pmem ranges.
>
> Yes, I am aware of the commit.  Unloading drivers while using crash(8) to
> analyze NVDIMM via /dev/mem makes sense.  /dev/mem does not require any
> other drivers be loaded.

Ah, ok.  I thought this patch was bypassing that safety check.  If it
requires the driver to be unloaded first then I'm fine with this.

> Using /dev/pmemX, on the other hand, requires the driver to be loaded,
> which can be problematic.  For instance, when btt_init() fails due to some
> corruption in arena, it fails to create any pmem device file.  A dev file
> also restricts access range within the dev file.
>
> Thanks,
> -Toshi
>
> ps.
> Looking at iomem_is_exclusive(), it only checks the top-level iomem
> entries.  I think the pmem/btt driver only marks a child entry busy...
>

It looks to me that next_resource(), via r_next(), walks child ranges.