mm: introduce commit_merge(), abstracting final commit of merge

Pull the part of vma_expand() which actually commits the merge operation,
that is inserts it into the maple tree and sets the VMA's vma->vm_start
and vma->vm_end parameters, into its own function.

We implement only the parts needed for vma_expand() which now as a result
of previous work is also the means by which new VMA ranges are merged.

The next commit in the series will implement merging of existing ranges
which will extend commit_merge() to accommodate this case and result in
all merges using this common code.

Link: https://lkml.kernel.org/r/7b985a20dfa549e3c370cd274d732b64c44f6dbd.1725040657.git.lorenzo.stoakes@oracle.comSigned-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Bert Karwatzki <spasswolf@web.de>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Jiri Olsa <olsajiri@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vma.c

@@ -585,6 +585,31 @@ void validate_mm(struct mm_struct *mm)
 }
 #endif /* CONFIG_DEBUG_VM_MAPLE_TREE */
 
+/* Actually perform the VMA merge operation. */
+static int commit_merge(struct vma_merge_struct *vmg,
+			struct vm_area_struct *remove)
+{
+	struct vma_prepare vp;
+
+	init_multi_vma_prep(&vp, vmg->vma, NULL, remove, NULL);
+
+	/* Note: vma iterator must be pointing to 'start'. */
+	vma_iter_config(vmg->vmi, vmg->start, vmg->end);
+
+	if (vma_iter_prealloc(vmg->vmi, vmg->vma))
+		return -ENOMEM;
+
+	vma_prepare(&vp);
+	vma_adjust_trans_huge(vmg->vma, vmg->start, vmg->end, 0);
+	vma_set_range(vmg->vma, vmg->start, vmg->end, vmg->pgoff);
+
+	vma_iter_store(vmg->vmi, vmg->vma);
+
+	vma_complete(&vp, vmg->vmi, vmg->vma->vm_mm);
+
+	return 0;
+}
+
 /*
  * vma_merge_new_range - Attempt to merge a new VMA into address space
  *
@@ -712,7 +737,6 @@ int vma_expand(struct vma_merge_struct *vmg)
 	bool remove_next = false;
 	struct vm_area_struct *vma = vmg->vma;
 	struct vm_area_struct *next = vmg->next;
-	struct vma_prepare vp;
 
 	mmap_assert_write_locked(vmg->mm);
 
@@ -727,24 +751,15 @@ int vma_expand(struct vma_merge_struct *vmg)
 			return ret;
 	}
 
-	init_multi_vma_prep(&vp, vma, NULL, remove_next ? next : NULL, NULL);
 	/* Not merging but overwriting any part of next is not handled. */
-	VM_WARN_ON(next && !vp.remove &&
+	VM_WARN_ON(next && !remove_next &&
 		  next != vma && vmg->end > next->vm_start);
 	/* Only handles expanding */
 	VM_WARN_ON(vma->vm_start < vmg->start || vma->vm_end > vmg->end);
 
-	/* Note: vma iterator must be pointing to 'start' */
-	vma_iter_config(vmg->vmi, vmg->start, vmg->end);
-	if (vma_iter_prealloc(vmg->vmi, vma))
+	if (commit_merge(vmg, remove_next ? next : NULL))
 		goto nomem;
 
-	vma_prepare(&vp);
-	vma_adjust_trans_huge(vma, vmg->start, vmg->end, 0);
-	vma_set_range(vma, vmg->start, vmg->end, vmg->pgoff);
-	vma_iter_store(vmg->vmi, vma);
-
-	vma_complete(&vp, vmg->vmi, vma->vm_mm);
 	return 0;
 
 nomem: