[PATCH] x86, PAT: honor CONFIG_STRICT_DEVMEM if pat is disable]

[Michal Hocko]

Hi,
I have just come across a strange behavior of /dev/[k]mem when PAT is
configured while STRICT_DEVMEM is disabled. 
One would expect that /dev/kmem would allow to access also the
system RAM in that configuration but that is not obviously true as pat
code defines range_is_allowed to protect from accessing that memory.

AFAICS this behavior was introduced in 0124cecf (x86, PAT: disable
/dev/mem mmap RAM with PAT) which says that it disables [k]mem with PAT
because it is safer. There is no explanation why it allows to access
that memory if CONFIG_NONPROMISC_DEVMEM (CONFIG_STRICT_DEVMEM now).

The thing is even more complicated by the fact that the access is
allowed when nopat kernel parameter is specified because
range_is_allowed just does't call devmem_is_allowed in that case.

While I do agree that the feature is not safe in general we should honor
STRICT_DEVMEM setting in some way IMO.

What do you think about the following fix? I have tried to preserve
"disabled for PAT" by default behavior.
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