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

[Lorenzo Stoakes]

On Mon, Nov 17, 2025 at 03:25:05PM +0100, David Hildenbrand (Red Hat) wrote:
> On 14.11.25 18:53, Lorenzo Stoakes 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.
> >
> > Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > ---
>
> Looks reasonable to me. I thought that we had that behavior in the past ...
> but I also remember scenarios where we would have imprecise soft-dirty
> handling. So I assume this was semi-broken for a while (soft-broken :) )

:))

Yeah it's only specific merge scenarios, and only when you e.g. already cleared
refs then mapped a new VMA and it happened to merge.

Nicer thing about this change is we stop treating VM_SOFTDIRTY like an exception
on merge :)

>
> Acked-by: David Hildenbrand (Red Hat) <david@xxxxxxxxxx>

Cheers!

>
> --
> Cheers
>
> David