mm: thp: register mm for khugepaged when merging vma for shmem

When merging anonymous page vma, if the size of the vma can fit in at
least one hugepage, the mm will be registered for khugepaged for
collapsing THP in the future.

But it skips shmem vmas.  Do so for shmem also, but not for file-private
mappings when merging a vma in order to increase the odds of collapsing a
hugepage via khugepaged.

hugepage_vma_check() sounds like a good fit to do the check.  And move the
definition of it before khugepaged_enter_vma_merge() to avoid a build
error.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Shi <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/khugepaged.c

@@ -397,6 +397,25 @@ static inline int khugepaged_test_exit(struct mm_struct *mm)
 	return atomic_read(&mm->mm_users) == 0;
 }
 
+static bool hugepage_vma_check(struct vm_area_struct *vma)
+{
+	if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
+	    (vma->vm_flags & VM_NOHUGEPAGE) ||
+	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
+		return false;
+	if (shmem_file(vma->vm_file)) {
+		if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
+			return false;
+		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
+				HPAGE_PMD_NR);
+	}
+	if (!vma->anon_vma || vma->vm_ops)
+		return false;
+	if (is_vma_temporary_stack(vma))
+		return false;
+	return !(vma->vm_flags & VM_NO_KHUGEPAGED);
+}
+
 int __khugepaged_enter(struct mm_struct *mm)
 {
 	struct mm_slot *mm_slot;
@@ -434,15 +453,14 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
 			       unsigned long vm_flags)
 {
 	unsigned long hstart, hend;
-	if (!vma->anon_vma)
-		/*
-		 * Not yet faulted in so we will register later in the
-		 * page fault if needed.
-		 */
-		return 0;
-	if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
-		/* khugepaged not yet working on file or special mappings */
+
+	/*
+	 * khugepaged does not yet work on non-shmem files or special
+	 * mappings. And file-private shmem THP is not supported.
+	 */
+	if (!hugepage_vma_check(vma))
 		return 0;
+
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = vma->vm_end & HPAGE_PMD_MASK;
 	if (hstart < hend)
@@ -819,25 +837,6 @@ khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
 }
 #endif
 
-static bool hugepage_vma_check(struct vm_area_struct *vma)
-{
-	if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
-	    (vma->vm_flags & VM_NOHUGEPAGE) ||
-	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
-		return false;
-	if (shmem_file(vma->vm_file)) {
-		if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
-			return false;
-		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
-				HPAGE_PMD_NR);
-	}
-	if (!vma->anon_vma || vma->vm_ops)
-		return false;
-	if (is_vma_temporary_stack(vma))
-		return false;
-	return !(vma->vm_flags & VM_NO_KHUGEPAGED);
-}
-
 /*
  * If mmap_sem temporarily dropped, revalidate vma
  * before taking mmap_sem.