[PATCH 30/39] mm/vma: introduce vma[_flags]_is_persistent()

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

Introduce vma[_flags]_is_persistent() for the purposes of identifying
mappings that are persistent in the sense that bytes to the mapping stay
there, and bytes read from the mapping are the same unless changed by
actions taken by userland.

Kernel-owned mappings do not fall into this category, as their owner may
change the contents without the user having initiated it, and nor of course
does memory-mapped I/O.

We exclude fixed mappings as these are singled out as being unmergeable and
so cannot be guaranteed to persist user data.

hugetlb mappings are fixed mappings, but their contents are entirely the
user's, so they are explicitly carved out as persistent, as the MADV_DODUMP
check already does.

It excludes droppable mappings, which by their nature are ephemeral.

Use this functionality to update the madvise MADV_DODUMP check to test for
persistence rather than open-coding this.

This replaces the VM_SPECIAL check which means it no longer checks for
VMA_IO_BIT, however this is safe as we have established the invariant that
only kernel-owned mappings may set VMA_IO_BIT, so we implicitly include
these.

No functional change intended.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 include/linux/mm.h | 44 ++++++++++++++++++++++++++++++++++++++++++++
 mm/madvise.c       |  4 ++--
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d8ee0ca63ccf..e6c3ebb09c8d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1741,6 +1741,50 @@ static inline bool vma_can_merge(const struct vm_area_struct *vma)
 	return vma_flags_can_merge(&vma->flags);
 }
 
+/**
+ * vma_flags_is_persistent() - Do the specified VMA flags imply that the VMA
+ * contains persistent data?
+ * @flags: The VMA flags to test.
+ *
+ * Persistent in the sense that - if you write bytes to the mapping - do they
+ * stay written?
+ *
+ * If the kernel or a device could write to the memory independently of
+ * userland, or the kernel could arbitrarily discard it, then it is not
+ * persistent.
+ *
+ * Returns: true if the flags imply this VMA is persistent, otherwise false.
+ */
+static inline bool vma_flags_is_persistent(const vma_flags_t *flags)
+{
+	/* hugetlb is a fixed mapping, but its contents are the user's own. */
+	if (vma_flags_is_hugetlb(flags))
+		return true;
+	/*
+	 * MMIO mappings may not store what is written and may be changed by the
+	 * device. Kernel-owned and fixed mappings may be changed by their owner
+	 * without the user having initiated it.
+	 */
+	if (vma_flags_is_kernel_owned(flags) ||
+	    vma_flags_is_fixed_mapping(flags))
+		return false;
+	/* Droppable memory is discardable by definition. */
+	return !vma_flags_test_single_mask(flags, VMA_DROPPABLE);
+}
+
+/**
+ * vma_is_persistent() - Does the VMA contain persistent data?
+ * @vma: The VMA to test.
+ *
+ * See vma_flags_is_persistent() for details.
+ *
+ * Returns: true if the VMA is persistent, otherwise false.
+ */
+static inline bool vma_is_persistent(const struct vm_area_struct *vma)
+{
+	return vma_flags_is_persistent(&vma->flags);
+}
+
 /**
  * vma_kernel_pagesize - Default page size granularity for this VMA.
  * @vma: The user mapping.
diff --git a/mm/madvise.c b/mm/madvise.c
index d0b14cfe38a1..0fc31835fee1 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1419,8 +1419,8 @@ static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)
 		new_flags |= VM_DONTDUMP;
 		break;
 	case MADV_DODUMP:
-		if ((!vma_is_hugetlb(vma) && (new_flags & VM_SPECIAL)) ||
-		    (new_flags & VM_DROPPABLE))
+		/* Non-persistent memory cannot be dumped. */
+		if (!vma_is_persistent(vma))
 			return -EINVAL;
 		new_flags &= ~VM_DONTDUMP;
 		break;

-- 
2.55.0