[PATCH v2 0/2] fix a kernel oops when reading sysfs valid_zones

[Toshi Kani <toshi.kani@hpe.com>]

A sysfs memory file is created for each 2GiB memory block on x86-64
when the system has 64GiB or more memory. [1]  When the start address
of a memory block is not backed by struct page, i.e. a memory range is
not aligned by 2GiB, reading its 'valid_zones' attribute file leads to
a kernel oops.  This issue was observed on multiple x86-64 systems
with more than 64GiB of memory.  This patch-set fixes this issue.

Patch 1 first fixes an issue in test_pages_in_a_zone(), which does
not test the start section.

Patch 2 then fixes the kernel oops by extending test_pages_in_a_zone()
to return valid [start, end).

Note for stable kernels: The memory block size change was made by commit
bdee237c034, which was accepted to 3.9.  However, this patch-set depends
on (and fixes) the change to test_pages_in_a_zone() made by commit
5f0f2887f4, which was accepted to 4.4.  So, I recommend that we backport
it up to 4.4.

[1] 'Commit bdee237c0343 ("x86: mm: Use 2GB memory block size on
    large-memory x86-64 systems")'

v2:
 - Rebase to the -mm tree. (Andrew Morton)
 - Add more descriptions about the issue. (Andrew Morton)
 - Add cc to stable kernels. (Greg Kroah-Hartman, Andrew Morton)

---
Toshi Kani (2):
 1/2 mm/memory_hotplug.c: check start_pfn in test_pages_in_a_zone() 
 2/2 base/memory, hotplug: fix a kernel oops in show_valid_zones()

---
 drivers/base/memory.c          | 12 ++++++------
 include/linux/memory_hotplug.h |  3 ++-
 mm/memory_hotplug.c            | 28 +++++++++++++++++++++-------