mm: fix incorrect pfn passed to untrack_pfn() in remap_pfn_range()

We use generic hooks in remap_pfn_range() to help archs to track pfnmap
regions.  The code is something like:

  int remap_pfn_range()
  {
	...
	track_pfn_remap(vma, &prot, pfn, addr, PAGE_ALIGN(size));
	...
	pfn -= addr >> PAGE_SHIFT;
	...
	untrack_pfn(vma, pfn, PAGE_ALIGN(size));
	...
  }

Here we can easily find the pfn is changed but not recovered before
untrack_pfn() is called.  That's incorrect.

There are no known runtime effects - this is from inspection.
Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -1744,6 +1744,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 	unsigned long next;
 	unsigned long end = addr + PAGE_ALIGN(size);
 	struct mm_struct *mm = vma->vm_mm;
+	unsigned long remap_pfn = pfn;
 	int err;
 
 	/*
@@ -1770,7 +1771,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 		vma->vm_pgoff = pfn;
 	}
 
-	err = track_pfn_remap(vma, &prot, pfn, addr, PAGE_ALIGN(size));
+	err = track_pfn_remap(vma, &prot, remap_pfn, addr, PAGE_ALIGN(size));
 	if (err)
 		return -EINVAL;
 
@@ -1789,7 +1790,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 	} while (pgd++, addr = next, addr != end);
 
 	if (err)
-		untrack_pfn(vma, pfn, PAGE_ALIGN(size));
+		untrack_pfn(vma, remap_pfn, PAGE_ALIGN(size));
 
 	return err;
 }