mm, thp: drop do_huge_pmd_wp_zero_page_fallback()

I've realized that there's no need for do_huge_pmd_wp_zero_page_fallback().
We can just split zero page with split_huge_page_pmd() and return
VM_FAULT_FALLBACK.  handle_pte_fault() will handle write-protection
fault for us.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -941,81 +941,6 @@ void huge_pmd_set_accessed(struct mm_struct *mm,
 	spin_unlock(ptl);
 }
 
-static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
-		struct vm_area_struct *vma, unsigned long address,
-		pmd_t *pmd, pmd_t orig_pmd, unsigned long haddr)
-{
-	spinlock_t *ptl;
-	pgtable_t pgtable;
-	pmd_t _pmd;
-	struct page *page;
-	int i, ret = 0;
-	unsigned long mmun_start;	/* For mmu_notifiers */
-	unsigned long mmun_end;		/* For mmu_notifiers */
-
-	page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
-	if (!page) {
-		ret |= VM_FAULT_OOM;
-		goto out;
-	}
-
-	if (mem_cgroup_newpage_charge(page, mm, GFP_KERNEL)) {
-		put_page(page);
-		ret |= VM_FAULT_OOM;
-		goto out;
-	}
-
-	clear_user_highpage(page, address);
-	__SetPageUptodate(page);
-
-	mmun_start = haddr;
-	mmun_end   = haddr + HPAGE_PMD_SIZE;
-	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
-
-	ptl = pmd_lock(mm, pmd);
-	if (unlikely(!pmd_same(*pmd, orig_pmd)))
-		goto out_free_page;
-
-	pmdp_clear_flush(vma, haddr, pmd);
-	/* leave pmd empty until pte is filled */
-
-	pgtable = pgtable_trans_huge_withdraw(mm, pmd);
-	pmd_populate(mm, &_pmd, pgtable);
-
-	for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
-		pte_t *pte, entry;
-		if (haddr == (address & PAGE_MASK)) {
-			entry = mk_pte(page, vma->vm_page_prot);
-			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
-			page_add_new_anon_rmap(page, vma, haddr);
-		} else {
-			entry = pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot);
-			entry = pte_mkspecial(entry);
-		}
-		pte = pte_offset_map(&_pmd, haddr);
-		VM_BUG_ON(!pte_none(*pte));
-		set_pte_at(mm, haddr, pte, entry);
-		pte_unmap(pte);
-	}
-	smp_wmb(); /* make pte visible before pmd */
-	pmd_populate(mm, pmd, pgtable);
-	spin_unlock(ptl);
-	put_huge_zero_page();
-	inc_mm_counter(mm, MM_ANONPAGES);
-
-	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
-
-	ret |= VM_FAULT_WRITE;
-out:
-	return ret;
-out_free_page:
-	spin_unlock(ptl);
-	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
-	mem_cgroup_uncharge_page(page);
-	put_page(page);
-	goto out;
-}
-
 static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
 					struct vm_area_struct *vma,
 					unsigned long address,
@@ -1161,8 +1086,8 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (unlikely(!new_page)) {
 		if (!page) {
-			ret = do_huge_pmd_wp_zero_page_fallback(mm, vma,
-					address, pmd, orig_pmd, haddr);
+			split_huge_page_pmd(vma, address, pmd);
+			ret |= VM_FAULT_FALLBACK;
 		} else {
 			ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
 					pmd, orig_pmd, page, haddr);