Subject: mm: Fix alloc_node_mem_map with ARCH_PFN_OFFSET calculation
From: Tobias Wolf <[email-masked]>

An rt288x (ralink) based router (Belkin F5D8235 v1) does not boot with any
kernel beyond version 4.3 resulting in:

BUG: Bad page state in process swapper  pfn:086ac

bisect resulted in:

a1c34a3bf00af2cede839879502e12dc68491ad5 is the first bad commit
commit a1c34a3bf00af2cede839879502e12dc68491ad5
Author: Laura Abbott <[email-masked]>
Date:   Thu Nov 5 18:48:46 2015 -0800

    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
    [...]
:040000 040000 2de013c372345fd471cd58f0553c9b38b0ef1cc4
0a8156f848733dfa21e16c196dfb6c0a76290709 M      mm

This fix for ARM does not account ARCH_PFN_OFFSET for mem_map as later used by
page_to_pfn anymore.

The following output was generated with two hacked in printk statements:

printk("before %p vs. %p or %p\n", mem_map, mem_map - offset, mem_map -
(pgdat->node_start_pfn - ARCH_PFN_OFFSET));
		if (page_to_pfn(mem_map) != pgdat->node_start_pfn)
			mem_map -= offset + (pgdat->node_start_pfn - ARCH_PFN_OFFSET);
printk("after %p\n", mem_map);

Output:

[    0.000000] before 8861b280 vs. 8861b280 or 8851b280
[    0.000000] after 8851b280

As seen in the first line mem_map with subtraction of offset does not equal the
mem_map after subtraction of ARCH_PFN_OFFSET.

After adding the offset of ARCH_PFN_OFFSET as well to mem_map as the
previously calculated offset is zero for the named platform it is able to boot
4.4 and 4.9-rc7 again.

Signed-off-by: Tobias Wolf <[email-masked]>
---

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6951,7 +6951,7 @@ static void __ref alloc_node_mem_map(str
 	if (pgdat == NODE_DATA(0)) {
 		mem_map = NODE_DATA(0)->node_mem_map;
 		if (page_to_pfn(mem_map) != pgdat->node_start_pfn)
-			mem_map -= offset;
+			mem_map -= offset + (pgdat->node_start_pfn - ARCH_PFN_OFFSET);
 	}
 #endif
 }