mm: fix kernel crash in khugepaged thread

This crash is caused by NULL pointer deference, in page_to_pfn() marco,
when page == NULL :

  Unable to handle kernel NULL pointer dereference at virtual address 00000000
  Internal error: Oops: 94000006 [#1] SMP
  Modules linked in:
  CPU: 1 PID: 26 Comm: khugepaged Tainted: G        W       4.3.0-rc6-next-20151022ajb-00001-g32f3386-dirty #3
  PC is at khugepaged+0x378/0x1af8
  LR is at khugepaged+0x418/0x1af8
  Process khugepaged (pid: 26, stack limit = 0xffffffc079638020)
  Call trace:
    khugepaged+0x378/0x1af8
    kthread+0xdc/0xf4
    ret_from_fork+0xc/0x40
  Code: 35001700 f0002c60 aa0703e3 f9009fa0 (f94000e0)
  ---[ end trace 637503d8e28ae69e  ]---
  Kernel panic - not syncing: Fatal exception
  CPU2: stopping
  CPU: 2 PID: 0 Comm: swapper/2 Tainted: G      D W       4.3.0-rc6-next-20151022ajb-00001-g32f3386-dirty #3
  Hardware name: linux,dummy-virt (DT)

[akpm@linux-foundation.org: fix fat-fingered merge resolution]
Signed-off-by: yalin wang <yalin.wang2010@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/trace/events/huge_memory.h

@@ -46,10 +46,10 @@ SCAN_STATUS
 
 TRACE_EVENT(mm_khugepaged_scan_pmd,
 
-	TP_PROTO(struct mm_struct *mm, unsigned long pfn, bool writable,
+	TP_PROTO(struct mm_struct *mm, struct page *page, bool writable,
 		 bool referenced, int none_or_zero, int status),
 
-	TP_ARGS(mm, pfn, writable, referenced, none_or_zero, status),
+	TP_ARGS(mm, page, writable, referenced, none_or_zero, status),
 
 	TP_STRUCT__entry(
 		__field(struct mm_struct *, mm)
@@ -62,7 +62,7 @@ TRACE_EVENT(mm_khugepaged_scan_pmd,
 
 	TP_fast_assign(
 		__entry->mm = mm;
-		__entry->pfn = pfn;
+		__entry->pfn = page ? page_to_pfn(page) : -1;
 		__entry->writable = writable;
 		__entry->referenced = referenced;
 		__entry->none_or_zero = none_or_zero;
@@ -104,10 +104,10 @@ TRACE_EVENT(mm_collapse_huge_page,
 
 TRACE_EVENT(mm_collapse_huge_page_isolate,
 
-	TP_PROTO(unsigned long pfn, int none_or_zero,
+	TP_PROTO(struct page *page, int none_or_zero,
 		 bool referenced, bool  writable, int status),
 
-	TP_ARGS(pfn, none_or_zero, referenced, writable, status),
+	TP_ARGS(page, none_or_zero, referenced, writable, status),
 
 	TP_STRUCT__entry(
 		__field(unsigned long, pfn)
@@ -118,7 +118,7 @@ TRACE_EVENT(mm_collapse_huge_page_isolate,
 	),
 
 	TP_fast_assign(
-		__entry->pfn = pfn;
+		__entry->pfn = page ? page_to_pfn(page) : -1;
 		__entry->none_or_zero = none_or_zero;
 		__entry->referenced = referenced;
 		__entry->writable = writable;

mm/huge_memory.c

@@ -2072,7 +2072,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	if (likely(writable)) {
 		if (likely(referenced)) {
 			result = SCAN_SUCCEED;
-			trace_mm_collapse_huge_page_isolate(page_to_pfn(page), none_or_zero,
+			trace_mm_collapse_huge_page_isolate(page, none_or_zero,
 							    referenced, writable, result);
 			return 1;
 		}
@@ -2082,7 +2082,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 
 out:
 	release_pte_pages(pte, _pte);
-	trace_mm_collapse_huge_page_isolate(page_to_pfn(page), none_or_zero,
+	trace_mm_collapse_huge_page_isolate(page, none_or_zero,
 					    referenced, writable, result);
 	return 0;
 }
@@ -2580,7 +2580,7 @@ out_unmap:
 		collapse_huge_page(mm, address, hpage, vma, node);
 	}
 out:
-	trace_mm_khugepaged_scan_pmd(mm, page_to_pfn(page), writable, referenced,
+	trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
 				     none_or_zero, result);
 	return ret;
 }