[PATCH v2 1/2] mm: propagate VM_SOFTDIRTY on merge

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Currently we set VM_SOFTDIRTY when a new mapping is set up (whether by
establishing a new VMA, or via merge) as implemented in __mmap_complete()
and do_brk_flags().

However, when performing a merge of existing mappings such as when
performing mprotect(), we may lose the VM_SOFTDIRTY flag.

This is because currently we simply ignore VM_SOFTDIRTY for the purposes
of merge, so one VMA may possess the flag and another not, and whichever
happens to be the target VMA will be the one upon which the merge is
performed which may or may not have VM_SOFTDIRTY set.

Now we have the concept of 'sticky' VMA flags, let's make VM_SOFTDIRTY one
which solves this issue.

Additionally update VMA userland tests to propagate changes.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
---
 include/linux/mm.h               | 23 +++++++++++------------
 tools/testing/vma/vma_internal.h | 23 +++++++++++------------
 2 files changed, 22 insertions(+), 24 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 73fc90cbad94..057649b948f2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -532,29 +532,28 @@ extern unsigned int kobjsize(const void *objp);
  * possesses it but the other does not, the merged VMA should nonetheless have
  * applied to it:
  *
+ *   VM_SOFTDIRTY - if a VMA is marked soft-dirty, that is has not had its
+ *                  references cleared via /proc/$pid/clear_refs, any merged VMA
+ *                  should be considered soft-dirty also as it operates at a VMA
+ *                  granularity.
+ *
  * VM_MAYBE_GUARD - If a VMA may have guard regions in place it implies that
  *                  mapped page tables may contain metadata not described by the
  *                  VMA and thus any merged VMA may also contain this metadata,
  *                  and thus we must make this flag sticky.
  */
-#define VM_STICKY VM_MAYBE_GUARD
+#define VM_STICKY (VM_SOFTDIRTY | VM_MAYBE_GUARD)

 /*
  * VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
  * of these flags and the other not does not preclude a merge.
  *
- * VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
- *                dirty bit -- the caller should mark merged VMA as dirty. If
- *                dirty bit won't be excluded from comparison, we increase
- *                pressure on the memory system forcing the kernel to generate
- *                new VMAs when old one could be extended instead.
- *
- *    VM_STICKY - If one VMA has flags which most be 'sticky', that is ones
- *                which should propagate to all VMAs, but the other does not,
- *                the merge should still proceed with the merge logic applying
- *                sticky flags to the final VMA.
+ * VM_STICKY - If one VMA has flags which most be 'sticky', that is ones
+ *             which should propagate to all VMAs, but the other does not,
+ *             the merge should still proceed with the merge logic applying
+ *             sticky flags to the final VMA.
  */
-#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
+#define VM_IGNORE_MERGE VM_STICKY

 /*
  * Flags which should result in page tables being copied on fork. These are
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index bd6352a5f24d..10f46a95a73a 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -122,29 +122,28 @@ extern unsigned long dac_mmap_min_addr;
  * possesses it but the other does not, the merged VMA should nonetheless have
  * applied to it:
  *
+ *   VM_SOFTDIRTY - if a VMA is marked soft-dirty, that is has not had its
+ *                  references cleared via /proc/$pid/clear_refs, any merged VMA
+ *                  should be considered soft-dirty also as it operates at a VMA
+ *                  granularity.
+ *
  * VM_MAYBE_GUARD - If a VMA may have guard regions in place it implies that
  *                  mapped page tables may contain metadata not described by the
  *                  VMA and thus any merged VMA may also contain this metadata,
  *                  and thus we must make this flag sticky.
  */
-#define VM_STICKY VM_MAYBE_GUARD
+#define VM_STICKY (VM_SOFTDIRTY | VM_MAYBE_GUARD)

 /*
  * VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
  * of these flags and the other not does not preclude a merge.
  *
- * VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
- *                dirty bit -- the caller should mark merged VMA as dirty. If
- *                dirty bit won't be excluded from comparison, we increase
- *                pressure on the memory system forcing the kernel to generate
- *                new VMAs when old one could be extended instead.
- *
- *    VM_STICKY - If one VMA has flags which most be 'sticky', that is ones
- *                which should propagate to all VMAs, but the other does not,
- *                the merge should still proceed with the merge logic applying
- *                sticky flags to the final VMA.
+ * VM_STICKY - If one VMA has flags which most be 'sticky', that is ones
+ *             which should propagate to all VMAs, but the other does not,
+ *             the merge should still proceed with the merge logic applying
+ *             sticky flags to the final VMA.
  */
-#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
+#define VM_IGNORE_MERGE VM_STICKY

 /*
  * Flags which should result in page tables being copied on fork. These are
--
2.51.0