mm: mmap: merge vma after call_mmap() if possible

The vm_flags may be changed after call_mmap() because drivers may set some
flags for their own purpose.  As a result, we failed to merge the adjacent
vma due to the different vm_flags as userspace can't pass in the same one.
Try to merge vma after call_mmap() to fix this issue.

Signed-off-by: Hongxiang Lou <louhongxiang@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1594954065-23733-1-git-send-email-linmiaohe@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: MiaoheLin

mm/mmap.c

@@ -1690,7 +1690,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		struct list_head *uf)
 {
 	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *vma, *prev;
+	struct vm_area_struct *vma, *prev, *merge;
 	int error;
 	struct rb_node **rb_link, *rb_parent;
 	unsigned long charged = 0;
@@ -1774,6 +1774,25 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		if (error)
 			goto unmap_and_free_vma;
 
+		/* If vm_flags changed after call_mmap(), we should try merge vma again
+		 * as we may succeed this time.
+		 */
+		if (unlikely(vm_flags != vma->vm_flags && prev)) {
+			merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
+				NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX);
+			if (merge) {
+				fput(file);
+				vm_area_free(vma);
+				vma = merge;
+				/* Update vm_flags and possible addr to pick up the change. We don't
+				 * warn here if addr changed as the vma is not linked by vma_link().
+				 */
+				addr = vma->vm_start;
+				vm_flags = vma->vm_flags;
+				goto unmap_writable;
+			}
+		}
+
 		/* Can addr have changed??
 		 *
 		 * Answer: Yes, several device drivers can do it in their
@@ -1796,6 +1815,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	vma_link(mm, vma, prev, rb_link, rb_parent);
 	/* Once vma denies write, undo our temporary denial count */
 	if (file) {
+unmap_writable:
 		if (vm_flags & VM_SHARED)
 			mapping_unmap_writable(file->f_mapping);
 		if (vm_flags & VM_DENYWRITE)