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 <aarcange@redhat.com>
Reported-by: Caspar Zhang <bugs@casparzhang.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: <stable@kernel.org>		[2.6.38.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 24 insertions, 19 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 470dcda10ad..83326ad66d9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1408,6 +1408,9 @@ out:
         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)
 {
@@ -1416,11 +1419,7 @@ int hugepage_madvise(struct vm_area_struct *vma,
                 /*
                  * 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;
@@ -1436,11 +1435,7 @@ int hugepage_madvise(struct vm_area_struct *vma,
                 /*
                  * 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;
@@ -1574,10 +1569,14 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma)
                  * 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)
@@ -1828,12 +1827,15 @@ static void collapse_huge_page(struct mm_struct *mm,
             (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))
@@ -2066,13 +2068,16 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
                         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;