[PATCH v3 15/40] mm/vma: add vma[_flags]_is_kernel_owned() predicates

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

Rather than referring to VMA flags with uncertain meaning, add a new
predicate that explicitly describes what possession of the VMA_PFNMAP_BIT
or VMA_MIXEDMAP_BIT flags mean, and then refer to that function for
determining VMA mergeability.

Either flag means the contents of the mapping are owned by the kernel,
usually a driver, rather than by the core mm: the memory may be MMIO,
kernel-allocated pages or even ordinary pages the driver maps itself, but
the core must not populate, reclaim, migrate, copy-on-write or merge the
range on its own initiative.

We initially also include VMA_IO_BIT here, as by implication, these must be
kernel-owned. (mlock() also sets VMA_IO_BIT transiently on ordinary VMAs
while locking them, which is addressed later in this series.)

However the intent is to in future remove this, as no mapping should be
marked as an I/O mapping without also being marked with VMA_PFNMAP_BIT.

This forms the basis of further work intended to improve how we express VMA
properties such as this.

Also update the VMA userland tests to reflect the change.

No functional change intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 include/linux/mm.h              | 56 ++++++++++++++++++++++++++++++++++++++++-
 tools/testing/vma/include/dup.h | 29 ++++++++++++++++++++-
 2 files changed, 83 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2a92193ac6a5..cab29d6e15c1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1612,6 +1612,44 @@ static inline bool vma_is_shared_maywrite(const struct vm_area_struct *vma)
 	return is_shared_maywrite(&vma->flags);
 }
 
+/**
+ * vma_flags_is_kernel_owned() - Do the specified VMA flags indicate that the
+ * contents of the VMA are owned by the kernel rather than the core mm?
+ * @flags: The VMA flags to test.
+ *
+ * A kernel-owned mapping is one whose contents are established and controlled
+ * by the kernel, typically a driver, rather than by the core mm's fault and
+ * rmap machinery.
+ *
+ * The mapping may be memory-mapped I/O, kernel-allocated pages or ordinary
+ * pages the owner has chosen to map itself (shmem via a PFN map, for instance).
+ *
+ * In all cases the core mm must not populate, reclaim, migrate, copy-on-write
+ * or merge it of its own accord.
+ *
+ * Pages mapped this way are not necessarily reference counted or map counted.
+ *
+ * Returns: true if the flags indicate a kernel-owned mapping.
+ */
+static inline bool vma_flags_is_kernel_owned(const vma_flags_t *flags)
+{
+	return vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT,
+				  VMA_IO_BIT);
+}
+
+/**
+ * vma_is_kernel_owned() - Are the contents of @vma owned by the kernel?
+ * @vma: The VMA to test.
+ *
+ * See vma_flags_is_kernel_owned() for a description of this property.
+ *
+ * Returns: true if the VMA is kernel-owned.
+ */
+static inline bool vma_is_kernel_owned(const struct vm_area_struct *vma)
+{
+	return vma_flags_is_kernel_owned(&vma->flags);
+}
+
 /**
  * vma_flags_can_merge() - Do the specified VMA flags permit the VMA to be
  * merged with another?
@@ -1620,7 +1658,23 @@ static inline bool vma_is_shared_maywrite(const struct vm_area_struct *vma)
  */
 static inline bool vma_flags_can_merge(const vma_flags_t *flags)
 {
-	return !vma_flags_test_any_mask(flags, VMA_SPECIAL_FLAGS);
+	/*
+	 * VMA merging assumes that a VMA's flags and fields completely describe
+	 * its state.
+	 *
+	 * However, kernel-owned mappings may have established state upon mapping
+	 * not embodied in any attribute of the VMA.
+	 *
+	 * Additionally, private (CoW) PFN maps encode the source PFN of the
+	 * range in vma->vm_pgoff, which may otherwise cause spurious merges.
+	 */
+	if (vma_flags_is_kernel_owned(flags))
+		return false;
+	/* VMA explicitly marked as being unmergeable. */
+	if (vma_flags_test(flags, VMA_DONTEXPAND_BIT))
+		return false;
+
+	return true;
 }
 
 /**
diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
index 1d5f6b3cbd21..d09148ce2305 100644
--- a/tools/testing/vma/include/dup.h
+++ b/tools/testing/vma/include/dup.h
@@ -1665,7 +1665,34 @@ static inline bool file_is_dev_zero(const struct file *file)
 	return file && file->f_op == &zero_fops;
 }
 
+static inline bool vma_flags_is_kernel_owned(const vma_flags_t *flags)
+{
+	return vma_flags_test_any(flags, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT,
+				  VMA_IO_BIT);
+}
+
+static inline bool vma_is_kernel_owned(const struct vm_area_struct *vma)
+{
+	return vma_flags_is_kernel_owned(&vma->flags);
+}
+
 static inline bool vma_flags_can_merge(const vma_flags_t *flags)
 {
-	return !vma_flags_test_any_mask(flags, VMA_SPECIAL_FLAGS);
+	/*
+	 * VMA merging assumes that a VMA's flags and fields completely describe
+	 * its state.
+	 *
+	 * However, kernel-owned mappings may have established state upon mapping
+	 * not embodied in any attribute of the VMA.
+	 *
+	 * Additionally, private (CoW) PFN maps encode the source PFN of the
+	 * range in vma->vm_pgoff, which may otherwise cause spurious merges.
+	 */
+	if (vma_flags_is_kernel_owned(flags))
+		return false;
+	/* VMA explicitly marked as being unmergeable. */
+	if (vma_flags_test(flags, VMA_DONTEXPAND_BIT))
+		return false;
+
+	return true;
 }

-- 
2.55.0