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

[Andrei Vagin <avagin@gmail.com>]

On Mon, Nov 17, 2025 at 3:33 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> On Sun, Nov 16, 2025 at 04:53:36PM -0800, 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
>
>
> This patch doesn't change anything in terms of merging, it only _correctly_
> marks VMAs as soft-dirty where certain, very specific, circumstances might
> result in a merged VMA being incorrectly indicated to not be soft-dirty
> when it in fact contains pages which are.

As I mentioned in the previous message, this patch is correct, and I
appreciate your effort to solve this issue. My comment was about whether
we should allow merging VMAs if one has VM_SOFTDIRTY and the other does
not. You are right, this is a separate question unrelated to this patch.

I recall correctly that initially, merging vma-s with different
VM_SORTDIRTY bit values was not allowed. It was a bit surprising that
this behavior was changed by Cyrill in 34228d473efe.  Cyrill was an
active CRIU contributor at the time, so we can't even blame anyone for
breaking CRIU :).

Thanks,
Andrei