mm: abstract VMA merge and extend into vma_merge_extend() helper

mremap uses vma_merge() in the case where a VMA needs to be extended. This
can be significantly simplified and abstracted.

This makes it far easier to understand what the actual function is doing,
avoids future mistakes in use of the confusing vma_merge() function and
importantly allows us to make future changes to how vma_merge() is
implemented by knowing explicitly which merge cases each invocation uses.

Note that in the mremap() extend case, we perform this merge only when
old_len == vma->vm_end - addr. The extension_start, i.e. the start of the
extended portion of the VMA is equal to addr + old_len, i.e. vma->vm_end.

With this refactoring, vma_merge() is no longer required anywhere except
mm/mmap.c, so mark it static.

Link: https://lkml.kernel.org/r/f16cbdc2e72d37a1a097c39dc7d1fee8919a1c93.1697043508.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 24 insertions, 7 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index e5e50e547ebf..3ea52451623b 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -860,13 +860,13 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
  * **** is not represented - it will be merged and the vma containing the
  *      area is returned, or the function will return NULL
  */
-struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
-			struct vm_area_struct *prev, unsigned long addr,
-			unsigned long end, unsigned long vm_flags,
-			struct anon_vma *anon_vma, struct file *file,
-			pgoff_t pgoff, struct mempolicy *policy,
-			struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
-			struct anon_vma_name *anon_name)
+static struct vm_area_struct
+*vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
+	   struct vm_area_struct *prev, unsigned long addr, unsigned long end,
+	   unsigned long vm_flags, struct anon_vma *anon_vma, struct file *file,
+	   pgoff_t pgoff, struct mempolicy *policy,
+	   struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
+	   struct anon_vma_name *anon_name)
 {
 	struct vm_area_struct *curr, *next, *res;
 	struct vm_area_struct *vma, *adjust, *remove, *remove2;
@@ -2500,6 +2500,23 @@ static struct vm_area_struct
 }
 
 /*
+ * Expand vma by delta bytes, potentially merging with an immediately adjacent
+ * VMA with identical properties.
+ */
+struct vm_area_struct *vma_merge_extend(struct vma_iterator *vmi,
+					struct vm_area_struct *vma,
+					unsigned long delta)
+{
+	pgoff_t pgoff = vma->vm_pgoff + vma_pages(vma);
+
+	/* vma is specified as prev, so case 1 or 2 will apply. */
+	return vma_merge(vmi, vma->vm_mm, vma, vma->vm_end, vma->vm_end + delta,
+			 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
  * @vma: The starting vm_area_struct