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.

Now we have the concept of making VMA flags 'sticky', that is that they
both don't prevent merge and, importantly, are propagated to merged VMAs,
this seems a sensible alternative to the existing special-casing of
VM_SOFTDIRTY.

We additionally add a self-test that demonstrates that this logic behaves
as expected.

v2:
* Propagated tags (thanks David, Pedro!)
* Corrected comments and added ASCII diagrams as per David.
* Expanded self-test to check for mprotect() case.
* Updated incomplete cover letter.
* Updated commit messages accordingly.

v1:
https://lore.kernel.org/all/cover.1763142412.git.lorenzo.stoakes@oracle.com/

Lorenzo Stoakes (2):
  mm: propagate VM_SOFTDIRTY on merge
  testing/selftests/mm: add soft-dirty merge self-test

 include/linux/mm.h                      |  23 ++---
 tools/testing/selftests/mm/soft-dirty.c | 127 +++++++++++++++++++++++-
 tools/testing/vma/vma_internal.h        |  23 ++---
 3 files changed, 148 insertions(+), 25 deletions(-)

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2.51.0