thp: skip file huge pmd on copy_huge_pmd()

copy_page_range() has a check for "Don't copy ptes where a page fault
will fill them correctly." It works on VMA level.  We still copy all
page table entries from private mappings, even if they map page cache.

We can simplify copy_huge_pmd() a bit by skipping file PMDs.

We don't map file private pages with PMDs, so they only can map page
cache.  It's safe to skip them as they can be re-faulted later.

Link: http://lkml.kernel.org/r/1466021202-61880-17-git-send-email-kirill.shutemov@linux.intel.comSigned-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -1098,14 +1098,15 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	struct page *src_page;
 	pmd_t pmd;
 	pgtable_t pgtable = NULL;
-	int ret;
+	int ret = -ENOMEM;
+
+	/* Skip if can be re-fill on fault */
+	if (!vma_is_anonymous(vma))
+		return 0;
 
-	if (!vma_is_dax(vma)) {
-		ret = -ENOMEM;
 	pgtable = pte_alloc_one(dst_mm, addr);
 	if (unlikely(!pgtable))
 		goto out;
-	}
 
 	dst_ptl = pmd_lock(dst_mm, dst_pmd);
 	src_ptl = pmd_lockptr(src_mm, src_pmd);
@@ -1113,7 +1114,7 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 
 	ret = -EAGAIN;
 	pmd = *src_pmd;
-	if (unlikely(!pmd_trans_huge(pmd) && !pmd_devmap(pmd))) {
+	if (unlikely(!pmd_trans_huge(pmd))) {
 		pte_free(dst_mm, pgtable);
 		goto out_unlock;
 	}
@@ -1136,8 +1137,6 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		goto out_unlock;
 	}
 
-	if (!vma_is_dax(vma)) {
-		/* thp accounting separate from pmd_devmap accounting */
 	src_page = pmd_page(pmd);
 	VM_BUG_ON_PAGE(!PageHead(src_page), src_page);
 	get_page(src_page);
@@ -1145,7 +1144,6 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
 	atomic_long_inc(&dst_mm->nr_ptes);
 	pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
-	}
 
 	pmdp_set_wrprotect(src_mm, addr, src_pmd);
 	pmd = pmd_mkold(pmd_wrprotect(pmd));