[PATCH 02/39] mm/vma: introduce and use vma_[flags_]can_merge()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Replace the open-coded VMA_SPECIAL_FLAGS check in the VMA merge logic with
two new functions vma_flags_can_merge() and vma_can_merge() and update the
merge logic to use the former.

This abstracts the check and expresses it in terms of the desired behaviour
rather than an arbitrary and confusing VMA flag.

This also lays the groundwork for making further improvements in VMA flag
usage.

Also update the userland VMA tests to reflect the change.

No functional change intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 include/linux/mm.h              | 21 +++++++++++++++++++++
 mm/vma.c                        | 19 +++++++++++--------
 tools/testing/vma/include/dup.h |  5 +++++
 3 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c49ef99b4413..83979ec28f5c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1612,6 +1612,27 @@ static inline bool vma_is_shared_maywrite(const struct vm_area_struct *vma)
 	return is_shared_maywrite(&vma->flags);
 }
 
+/**
+ * vma_flags_can_merge() - Do the specified VMA flags permit the VMA to be
+ * merged with another?
+ * @flags: The VMA flags to test.
+ * Returns: true if the flags permit merging, false otherwise.
+ */
+static inline bool vma_flags_can_merge(const vma_flags_t *flags)
+{
+	return !vma_flags_test_any_mask(flags, VMA_SPECIAL_FLAGS);
+}
+
+/**
+ * vma_can_merge() - Do @vma's flags permit it to be merged with another VMA?
+ * @vma: The VMA to test.
+ * Returns: true if the flags permit merging, otherwise false.
+ */
+static inline bool vma_can_merge(const struct vm_area_struct *vma)
+{
+	return vma_flags_can_merge(&vma->flags);
+}
+
 /**
  * vma_kernel_pagesize - Default page size granularity for this VMA.
  * @vma: The user mapping.
diff --git a/mm/vma.c b/mm/vma.c
index 2a90c509bb31..0db2fc306993 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -924,13 +924,14 @@ static __must_check struct vm_area_struct *vma_merge_existing_range(
 
 	vmg->state = VMA_MERGE_NOMERGE;
 
+	if (!vma_flags_can_merge(&vmg->vma_flags))
+		return NULL;
 	/*
-	 * If a special mapping or if the range being modified is neither at the
-	 * furthermost left or right side of the VMA, then we have no chance of
-	 * merging and should abort.
+	 * If the range being modified is neither at the furthermost left or
+	 * right side of the VMA, then we have no chance of merging and should
+	 * abort.
 	 */
-	if (vma_flags_test_any_mask(&vmg->vma_flags, VMA_SPECIAL_FLAGS) ||
-	    (!left_side && !right_side))
+	if (!left_side && !right_side)
 		return NULL;
 
 	if (left_side)
@@ -1152,9 +1153,11 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
 
 	vmg->state = VMA_MERGE_NOMERGE;
 
-	/* Special VMAs are unmergeable, also if no prev/next. */
-	if (vma_flags_test_any_mask(&vmg->vma_flags, VMA_SPECIAL_FLAGS) ||
-	    (!prev && !next))
+	if (!vma_flags_can_merge(&vmg->vma_flags))
+		return NULL;
+
+	/* VMAs with no prev/next are unmergeable. */
+	if (!prev && !next)
 		return NULL;
 
 	can_merge_left = can_vma_merge_left(vmg);
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index 16c09dac59d9..2fd422789717 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1647,3 +1647,8 @@ static inline bool file_is_dev_zero(const struct file *file)
 {
 	return file && file->f_op == &zero_fops;
 }
+
+static inline bool vma_flags_can_merge(const vma_flags_t *flags)
+{
+	return !vma_flags_test_any_mask(flags, VMA_SPECIAL_FLAGS);
+}

-- 
2.55.0