[RFC PATCH 6/8] mm: Avoid modifying vmf.info.address in __collapse_huge_page_swapin()

[Will Deacon <will@xxxxxxxxxx>]

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

Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 mm/khugepaged.c | 41 ++++++++++++++++++++---------------------
 1 file changed, 20 insertions(+), 21 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4494c90075fb..86c51a5d92d2 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -991,40 +991,43 @@ 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;
-	struct vm_fault vmf = {
-		.info = {
-			.vma = vma,
-			.address = address,
-			.pgoff = linear_page_index(vma, address),
-		},
-		.flags = FAULT_FLAG_ALLOW_RETRY,
-		.pmd = pmd,
-	};
-
-	vmf.pte = pte_offset_map(pmd, address);
-	for (; vmf.info.address < address + HPAGE_PMD_NR*PAGE_SIZE;
-			vmf.pte++, vmf.info.address += PAGE_SIZE) {
+	unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
+
+	for (address = haddr; address < end; address += PAGE_SIZE) {
+		struct vm_fault vmf = {
+			.info = {
+				.vma = vma,
+				.address = address,
+				.pgoff = linear_page_index(vma, haddr),
+			},
+			.flags = FAULT_FLAG_ALLOW_RETRY,
+			.pmd = pmd,
+		};
+
+		vmf.pte = pte_offset_map(pmd, address);
 		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.info.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;
 			}
@@ -1033,11 +1036,7 @@ 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.info.address);
 	}
-	vmf.pte--;
-	pte_unmap(vmf.pte);
 
 	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
 	if (swapped_in)
-- 
2.30.0.284.gd98b1dd5eaa7-goog