exec: use VMA iterator instead of linked list

Remove a use of the vm_next list by doing the initial lookup with the VMA
iterator and then using it to find the next entry.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Liam R. Howlett <[email protected]>
Tested-by: Yu Zhao <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Howells <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

fs/exec.c

@@ -683,6 +683,8 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
 	unsigned long length = old_end - old_start;
 	unsigned long new_start = old_start - shift;
 	unsigned long new_end = old_end - shift;
+	VMA_ITERATOR(vmi, mm, new_start);
+	struct vm_area_struct *next;
 	struct mmu_gather tlb;
 
 	BUG_ON(new_start > new_end);
@@ -691,7 +693,7 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
 	 * ensure there are no vmas between where we want to go
 	 * and where we are
 	 */
-	if (vma != find_vma(mm, new_start))
+	if (vma != vma_next(&vmi))
 		return -EFAULT;
 
 	/*
@@ -710,12 +712,13 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
 
 	lru_add_drain();
 	tlb_gather_mmu(&tlb, mm);
+	next = vma_next(&vmi);
 	if (new_end > old_start) {
 		/*
 		 * when the old and new regions overlap clear from new_end.
 		 */
 		free_pgd_range(&tlb, new_end, old_end, new_end,
-			vma->vm_next ? vma->vm_next->vm_start : USER_PGTABLES_CEILING);
+			next ? next->vm_start : USER_PGTABLES_CEILING);
 	} else {
 		/*
 		 * otherwise, clean from old_start; this is done to not touch
@@ -724,7 +727,7 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
 		 * for the others its just a little faster.
 		 */
 		free_pgd_range(&tlb, old_start, old_end, new_end,
-			vma->vm_next ? vma->vm_next->vm_start : USER_PGTABLES_CEILING);
+			next ? next->vm_start : USER_PGTABLES_CEILING);
 	}
 	tlb_finish_mmu(&tlb);