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

[Toshi Kani]

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:
> > Â- Physical address range is valid when it is fully backed by
> > ÂÂÂIORESOURCE_MEM, regardless of __pa(high_memory).
> > Â- 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?ÂÂWith 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. ÂUnloading drivers while using crash(8) to
analyze NVDIMM via /dev/mem makes sense. Â/dev/mem does not require any
other drivers be loaded.

Using /dev/pmemX, on the other hand, requires the driver to be loaded,
which can be problematic. ÂFor instance, when btt_init() fails due to some
corruption in arena, it fails to create any pmem device file. ÂA 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. ÂI think the pmem/btt driver only marks a child entry busy...