mm: Avoid modifying vmf.address in __collapse_huge_page_swapin()

In preparation for const-ifying the anonymous struct field of
'struct vm_fault', rework __collapse_huge_page_swapin() to avoid
continuously updating vmf.address and instead populate a new
'struct vm_fault' on the stack for each page being processed.

Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will@kernel.org>

mm/khugepaged.c

@@ -991,38 +991,41 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 
 static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 					struct vm_area_struct *vma,
-					unsigned long address, pmd_t *pmd,
+					unsigned long haddr, pmd_t *pmd,
 					int referenced)
 {
 	int swapped_in = 0;
 	vm_fault_t ret = 0;
+	unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
+
+	for (address = haddr; address < end; address += PAGE_SIZE) {
 		struct vm_fault vmf = {
 			.vma = vma,
 			.address = address,
+			.pgoff = linear_page_index(vma, haddr),
 			.flags = FAULT_FLAG_ALLOW_RETRY,
 			.pmd = pmd,
-		.pgoff = linear_page_index(vma, address),
 		};
 
 		vmf.pte = pte_offset_map(pmd, address);
-	for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE;
-			vmf.pte++, vmf.address += PAGE_SIZE) {
 		vmf.orig_pte = *vmf.pte;
-		if (!is_swap_pte(vmf.orig_pte))
+		if (!is_swap_pte(vmf.orig_pte)) {
+			pte_unmap(vmf.pte);
 			continue;
+		}
 		swapped_in++;
 		ret = do_swap_page(&vmf);
 
 		/* do_swap_page returns VM_FAULT_RETRY with released mmap_lock */
 		if (ret & VM_FAULT_RETRY) {
 			mmap_read_lock(mm);
-			if (hugepage_vma_revalidate(mm, address, &vmf.vma)) {
+			if (hugepage_vma_revalidate(mm, haddr, &vma)) {
 				/* vma is no longer available, don't continue to swapin */
 				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
 				return false;
 			}
 			/* check if the pmd is still valid */
-			if (mm_find_pmd(mm, address) != pmd) {
+			if (mm_find_pmd(mm, haddr) != pmd) {
 				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
 				return false;
 			}
@@ -1031,11 +1034,7 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 			trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
 			return false;
 		}
-		/* pte is unmapped now, we need to map it */
-		vmf.pte = pte_offset_map(pmd, vmf.address);
 	}
-	vmf.pte--;
-	pte_unmap(vmf.pte);
 
 	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
 	if (swapped_in)