[PATCH 3/4] mm: abstract merge for new VMAs into vma_merge_new_vma()

[Lorenzo Stoakes]

Only in mmap_region() and copy_vma() do we add VMAs which occupy entirely
new regions of virtual memory.

We can share the logic between these invocations and make it absolutely
explici to reduce confusion around the rather inscrutible parameters
possessed by vma_merge().

This also paves the way for a simplification of the core vma_merge()
implementation, as we seek to make the function entirely an implementation
detail.

Note that on mmap_region(), vma fields are initialised to zero, so we can
simply reference these rather than explicitly specifying NULL.

Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
---
 mm/mmap.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 58d71f84e917..51be864b876b 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2530,6 +2530,22 @@ struct vm_area_struct *vma_modify_uffd(struct vma_iterator *vmi,
 			  vma_policy(vma), new_ctx, anon_vma_name(vma));
 }
 
+/*
+ * Attempt to merge a newly mapped VMA with those adjacent to it. The caller
+ * must ensure that [start, end) does not overlap any existing VMA.
+ */
+static struct vm_area_struct *vma_merge_new_vma(struct vma_iterator *vmi,
+						struct vm_area_struct *prev,
+						struct vm_area_struct *vma,
+						unsigned long start,
+						unsigned long end,
+						pgoff_t pgoff)
+{
+	return vma_merge(vmi, vma->vm_mm, prev, start, end, vma->vm_flags,
+			 vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
+			 vma->vm_userfaultfd_ctx, anon_vma_name(vma));
+}
+
 /*
  * do_vmi_align_munmap() - munmap the aligned region from @start to @end.
  * @vmi: The vma iterator
@@ -2885,10 +2901,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		 * vma again as we may succeed this time.
 		 */
 		if (unlikely(vm_flags != vma->vm_flags && prev)) {
-			merge = vma_merge(&vmi, mm, prev, vma->vm_start,
-				    vma->vm_end, vma->vm_flags, NULL,
-				    vma->vm_file, vma->vm_pgoff, NULL,
-				    NULL_VM_UFFD_CTX, NULL);
+			merge = vma_merge_new_vma(&vmi, prev, vma,
+						  vma->vm_start, vma->vm_end,
+						  pgoff);
 			if (merge) {
 				/*
 				 * ->mmap() can change vma->vm_file and fput
@@ -3430,9 +3445,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 	if (new_vma && new_vma->vm_start < addr + len)
 		return NULL;	/* should never get here */
 
-	new_vma = vma_merge(&vmi, mm, prev, addr, addr + len, vma->vm_flags,
-			    vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
-			    vma->vm_userfaultfd_ctx, anon_vma_name(vma));
+	new_vma = vma_merge_new_vma(&vmi, prev, vma, addr, addr + len, pgoff);
 	if (new_vma) {
 		/*
 		 * Source vma may have been merged into new_vma
-- 
2.42.0