mm: thp: fix /dev/zero MAP_PRIVATE and vm_flags cleanups

The huge_memory.c THP page fault was allowed to run if vm_ops was null
(which would succeed for /dev/zero MAP_PRIVATE, as the f_op->mmap wouldn't
setup a special vma->vm_ops and it would fallback to regular anonymous
memory) but other THP logics weren't fully activated for vmas with vm_file
not NULL (/dev/zero has a not NULL vma->vm_file).

So this removes the vm_file checks so that /dev/zero also can safely use
THP (the other albeit safer approach to fix this bug would have been to
prevent the THP initial page fault to run if vm_file was set).

After removing the vm_file checks, this also makes huge_memory.c stricter
in khugepaged for the DEBUG_VM=y case. It doesn't replace the vm_file
check with a is_pfn_mapping check (but it keeps checking for VM_PFNMAP
under VM_BUG_ON) because for a is_cow_mapping() mapping VM_PFNMAP should
only be allowed to exist before the first page fault, and in turn when
vma->anon_vma is null (so preventing khugepaged registration). So I tend
to think the previous comment saying if vm_file was set, VM_PFNMAP might
have been set and we could still be registered in khugepaged (despite
anon_vma was not NULL to be registered in khugepaged) was too paranoid.
The is_linear_pfn_mapping check is also I think superfluous (as described
by comment) but under DEBUG_VM it is safe to stay.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=33682

Signed-off-by: Andrea Arcangeli <[e-mail]>
Reported-by: Caspar Zhang <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Acked-by: Rik van Riel <[e-mail]>
Cc: <[e-mail]> [2.6.38.x]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/huge_mm.h

···
 					 unsigned long end,
 					 long adjust_next)
 {
-	if (!vma->anon_vma || vma->vm_ops || vma->vm_file)
+	if (!vma->anon_vma || vma->vm_ops)
 		return;
 	__vma_adjust_trans_huge(vma, start, end, adjust_next);
 }

include/linux/mm.h

···
 #define VM_RandomReadHint(v)		((v)->vm_flags & VM_RAND_READ)
 
 /*
- * special vmas that are non-mergable, non-mlock()able
+ * Special vmas that are non-mergable, non-mlock()able.
+ * Note: mm/huge_memory.c VM_NO_THP depends on this definition.
  */
 #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_RESERVED | VM_PFNMAP)
 

mm/huge_memory.c

···
 	return ret;
 }
 
+#define VM_NO_THP (VM_SPECIAL|VM_INSERTPAGE|VM_MIXEDMAP|VM_SAO| \
+		   VM_HUGETLB|VM_SHARED|VM_MAYSHARE)
+
 int hugepage_madvise(struct vm_area_struct *vma,
 		     unsigned long *vm_flags, int advice)
 {
···
 		/*
 		 * Be somewhat over-protective like KSM for now!
 		 */
-		if (*vm_flags & (VM_HUGEPAGE |
-				 VM_SHARED   | VM_MAYSHARE   |
-				 VM_PFNMAP   | VM_IO      | VM_DONTEXPAND |
-				 VM_RESERVED | VM_HUGETLB | VM_INSERTPAGE |
-				 VM_MIXEDMAP | VM_SAO))
+		if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
 			return -EINVAL;
 		*vm_flags &= ~VM_NOHUGEPAGE;
 		*vm_flags |= VM_HUGEPAGE;
···
 		/*
 		 * Be somewhat over-protective like KSM for now!
 		 */
-		if (*vm_flags & (VM_NOHUGEPAGE |
-				 VM_SHARED   | VM_MAYSHARE   |
-				 VM_PFNMAP   | VM_IO      | VM_DONTEXPAND |
-				 VM_RESERVED | VM_HUGETLB | VM_INSERTPAGE |
-				 VM_MIXEDMAP | VM_SAO))
+		if (*vm_flags & (VM_NOHUGEPAGE | VM_NO_THP))
 			return -EINVAL;
 		*vm_flags &= ~VM_HUGEPAGE;
 		*vm_flags |= VM_NOHUGEPAGE;
···
 		 * page fault if needed.
 		 */
 		return 0;
-	if (vma->vm_file || vma->vm_ops)
+	if (vma->vm_ops)
 		/* khugepaged not yet working on file or special mappings */
 		return 0;
-	VM_BUG_ON(is_linear_pfn_mapping(vma) || is_pfn_mapping(vma));
+	/*
+	 * If is_pfn_mapping() is true is_learn_pfn_mapping() must be
+	 * true too, verify it here.
+	 */
+	VM_BUG_ON(is_linear_pfn_mapping(vma) || vma->vm_flags & VM_NO_THP);
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = vma->vm_end & HPAGE_PMD_MASK;
 	if (hstart < hend)
···
 	    (vma->vm_flags & VM_NOHUGEPAGE))
 		goto out;
 
-	/* VM_PFNMAP vmas may have vm_ops null but vm_file set */
-	if (!vma->anon_vma || vma->vm_ops || vma->vm_file)
+	if (!vma->anon_vma || vma->vm_ops)
 		goto out;
 	if (is_vma_temporary_stack(vma))
 		goto out;
-	VM_BUG_ON(is_linear_pfn_mapping(vma) || is_pfn_mapping(vma));
+	/*
+	 * If is_pfn_mapping() is true is_learn_pfn_mapping() must be
+	 * true too, verify it here.
+	 */
+	VM_BUG_ON(is_linear_pfn_mapping(vma) || vma->vm_flags & VM_NO_THP);
 
 	pgd = pgd_offset(mm, address);
 	if (!pgd_present(*pgd))
···
 			progress++;
 			continue;
 		}
-		/* VM_PFNMAP vmas may have vm_ops null but vm_file set */
-		if (!vma->anon_vma || vma->vm_ops || vma->vm_file)
+		if (!vma->anon_vma || vma->vm_ops)
 			goto skip;
 		if (is_vma_temporary_stack(vma))
 			goto skip;
-
-		VM_BUG_ON(is_linear_pfn_mapping(vma) || is_pfn_mapping(vma));
+		/*
+		 * If is_pfn_mapping() is true is_learn_pfn_mapping()
+		 * must be true too, verify it here.
+		 */
+		VM_BUG_ON(is_linear_pfn_mapping(vma) ||
+			  vma->vm_flags & VM_NO_THP);
 
 		hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 		hend = vma->vm_end & HPAGE_PMD_MASK;