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

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 17 Nov 2025 17:33:38 +0000 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:

> 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.
> 

Oh.  This patch messes with the comments which
mm-implement-sticky-vma-flags-fix-2.patch just altered.  Not sure what
you intend here, so I left it as below - please advise.

(also, I fixed a typo: s/most/must/, both files)

--- a/include/linux/mm.h~mm-propagate-vm_softdirty-on-merge
+++ a/include/linux/mm.h
@@ -532,28 +532,28 @@ extern unsigned int kobjsize(const void
  * 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 - When merging VMAs, VMA flags must match, unless they are
- *                'sticky'. If any sticky flags exist in either VMA, we simply
- *                set all of them on the merged VMA.
+ * VM_STICKY - If one VMA has flags which must 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
--- a/tools/testing/vma/vma_internal.h~mm-propagate-vm_softdirty-on-merge
+++ a/tools/testing/vma/vma_internal.h
@@ -122,28 +122,23 @@ extern unsigned long dac_mmap_min_addr;
  * possesses it but the other does not, the merged VMA should nonetheless have
  * applied to it:
  *
- * 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.
+ *   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.
  */
-#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 - When merging VMAs, VMA flags must match, unless they are
- *                'sticky'. If any sticky flags exist in either VMA, we simply
- *                set all of them on the merged 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
_