Re: [PATCH 0/2] make VM_SOFTDIRTY a sticky VMA flag

[Andrei Vagin <avagin@gmail.com>]

On Fri, Nov 14, 2025 at 9:59 AM 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.

Losing VM_SOFTDIRTY is definitely a bug, thank you for fixing it.

A separate concern is whether merging two VMAs should be permitted when
one has the VM_SOFTDIRTY flag set and another does not. I think the
merging operation should be disallowed.The  issue is that
PAGE_IS_SOFT_DIRTY will be reported for every page in the resulting VMA.
Consider a scenario where a large VMA has only a small number of pages
marked SOFT_DIRTY. If we merge it with a smaller VMA that does have
VM_SOFTDIRTY, all pages in the originally large VMA will subsequently be
reported as SOFT_DIRTY. As a result, CRIU will needlessly dump all of
these pages again, even though the vast majority of them were unchanged
since the prior checkpoint iteration.

Thanks,
Andrei

>
>
> 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 | 51 ++++++++++++++++++++++++-
>  tools/testing/vma/vma_internal.h        | 23 ++++++-----
>  3 files changed, 72 insertions(+), 25 deletions(-)
>
> --
> 2.51.0
>