mm: Don't offset memmap for flatmem

mm: Don't offset memmap for flatmem

Srinivas Kandagatla reported bad page messages when trying to remove the
bottom 2MB on an ARM based IFC6410 board

  BUG: Bad page state in process swapper  pfn:fffa8
  page:ef7fb500 count:0 mapcount:0 mapping:  (null) index:0x0
  flags: 0x96640253(locked|error|dirty|active|arch_1|reclaim|mlocked)
  page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
  bad because of flags:
  flags: 0x200041(locked|active|mlocked)
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 3.19.0-rc3-00007-g412f9ba-dirty #816
  Hardware name: Qualcomm (Flattened Device Tree)
    unwind_backtrace
    show_stack
    dump_stack
    bad_page
    free_pages_prepare
    free_hot_cold_page
    __free_pages
    free_highmem_page
    mem_init
    start_kernel
  Disabling lock debugging due to kernel taint

Removing the lower 2MB made the start of the lowmem zone to no longer be
page block aligned.  IFC6410 uses CONFIG_FLATMEM where alloc_node_mem_map
allocates memory for the mem_map.  alloc_node_mem_map will offset for
unaligned nodes with the assumption the pfn/page translation functions
will account for the offset.  The functions for CONFIG_FLATMEM do not
offset however, resulting in overrunning the memmap array.  Just use the
allocated memmap without any offset when running with CONFIG_FLATMEM to
avoid the overrun.

Signed-off-by: Laura Abbott <[email protected]>
Signed-off-by: Laura Abbott <[email protected]>
Reported-by: Srinivas Kandagatla <[email protected]>
Tested-by: Srinivas Kandagatla <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Tested-by: Bjorn Andersson <[email protected]>
Cc: Santosh Shilimkar <[email protected]>
Cc: Russell King <[email protected]>
Cc: Kevin Hilman <[email protected]>
Cc: Arnd Bergman <[email protected]>
Cc: Stephen Boyd <[email protected]>
Cc: Andy Gross <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Dave Hansen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>