kernel/resource: clean up and optimize iomem_is_exclusive()

kernel/resource: clean up and optimize iomem_is_exclusive()

Patch series "virtio-mem: disallow mapping virtio-mem memory via /dev/mem", v5.

Let's add the basic infrastructure to exclude some physical memory regions
marked as "IORESOURCE_SYSTEM_RAM" completely from /dev/mem access, even
though they are not marked IORESOURCE_BUSY and even though "iomem=relaxed"
is set.  Resource IORESOURCE_EXCLUSIVE for that purpose instead of adding
new flags to express something similar to "soft-busy" or "not busy yet,
but already prepared by a driver and not to be mapped by user space".

Use it for virtio-mem, to disallow mapping any virtio-mem memory via
/dev/mem to user space after the virtio-mem driver was loaded.

This patch (of 3):

We end up traversing subtrees of ranges we are not interested in; let's
optimize this case, skipping such subtrees, cleaning up the function a
bit.

For example, in the following configuration (/proc/iomem):

  00000000-00000fff : Reserved
  00001000-00057fff : System RAM
  00058000-00058fff : Reserved
  00059000-0009cfff : System RAM
  0009d000-000fffff : Reserved
     000a0000-000bffff : PCI Bus 0000:00
     000c0000-000c3fff : PCI Bus 0000:00
     000c4000-000c7fff : PCI Bus 0000:00
     000c8000-000cbfff : PCI Bus 0000:00
     000cc000-000cffff : PCI Bus 0000:00
     000d0000-000d3fff : PCI Bus 0000:00
     000d4000-000d7fff : PCI Bus 0000:00
     000d8000-000dbfff : PCI Bus 0000:00
     000dc000-000dffff : PCI Bus 0000:00
     000e0000-000e3fff : PCI Bus 0000:00
     000e4000-000e7fff : PCI Bus 0000:00
     000e8000-000ebfff : PCI Bus 0000:00
     000ec000-000effff : PCI Bus 0000:00
     000f0000-000fffff : PCI Bus 0000:00
       000f0000-000fffff : System ROM
  00100000-3fffffff : System RAM
  40000000-403fffff : Reserved
     40000000-403fffff : pnp 00:00
  40400000-80a79fff : System RAM
  ...

We don't have to look at any children of "0009d000-000fffff : Reserved"
if we can just skip these 15 items directly because the parent range is
not of interest.

Link: https://lkml.kernel.org/r/20210920142856.17758-1-david@redhat.com
Link: https://lkml.kernel.org/r/20210920142856.17758-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>