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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 17/11/25 6:23 AM, Andrei Vagin wrote:
> 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

If merging VM_SOFTDIRTY and non-VM_SOFTDIRTY VMAs would not be allowed then
what is the point for moving VM_SOFTDIRTY as VM_STICKY ?
> 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
>>
>