mm/mmap: don't use __vma_adjust() in shift_arg_pages()

Introduce shrink_vma() which uses the vma_prepare() and vma_complete()
functions to reduce the vma coverage.

Convert shift_arg_pages() to use expand_vma() and the new shrink_vma()
function.  Remove support from __vma_adjust() to reduce a vma size since
shift_arg_pages() is the only user that shrinks a VMA in this way.

Link: https://lkml.kernel.org/r/20230120162650.984577-46-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 43 insertions, 9 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 9599db011b18..07b52acfd565 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -682,6 +682,44 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
 nomem:
 	return -ENOMEM;
 }
+
+/*
+ * vma_shrink() - Reduce an existing VMAs memory area
+ * @vmi: The vma iterator
+ * @vma: The VMA to modify
+ * @start: The new start
+ * @end: The new end
+ *
+ * Returns: 0 on success, -ENOMEM otherwise
+ */
+int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
+	       unsigned long start, unsigned long end, pgoff_t pgoff)
+{
+	struct vma_prepare vp;
+
+	WARN_ON((vma->vm_start != start) && (vma->vm_end != end));
+
+	if (vma_iter_prealloc(vmi))
+		return -ENOMEM;
+
+	init_vma_prep(&vp, vma);
+	vma_adjust_trans_huge(vma, start, end, 0);
+	vma_prepare(&vp);
+
+	if (vma->vm_start < start)
+		vma_iter_clear(vmi, vma->vm_start, start);
+
+	if (vma->vm_end > end)
+		vma_iter_clear(vmi, end, vma->vm_end);
+
+	vma->vm_start = start;
+	vma->vm_end = end;
+	vma->vm_pgoff = pgoff;
+	vma_complete(&vp, vmi, vma->vm_mm);
+	validate_mm(vma->vm_mm);
+	return 0;
+}
+
 /*
  * We cannot adjust vm_start, vm_end, vm_pgoff fields of a vma that
  * is already present in an i_mmap tree without adjusting the tree.
@@ -797,14 +835,7 @@ int __vma_adjust(struct vma_iterator *vmi, struct vm_area_struct *vma,
 
 	vma_prepare(&vma_prep);
 
-	if (vma->vm_start < start)
-		vma_iter_clear(vmi, vma->vm_start, start);
-	else if (start != vma->vm_start)
-		vma_changed = true;
-
-	if (vma->vm_end > end)
-		vma_iter_clear(vmi, end, vma->vm_end);
-	else if (end != vma->vm_end)
+	if (start < vma->vm_start || end > vma->vm_end)
 		vma_changed = true;
 
 	vma->vm_start = start;
@@ -817,7 +848,10 @@ int __vma_adjust(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	if (adjust_next) {
 		next->vm_start += adjust_next;
 		next->vm_pgoff += adjust_next >> PAGE_SHIFT;
-		vma_iter_store(vmi, next);
+		if (adjust_next < 0) {
+			WARN_ON_ONCE(vma_changed);
+			vma_iter_store(vmi, next);
+		}
 	}
 
 	vma_complete(&vma_prep, vmi, mm);