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

[David Hildenbrand (Red Hat)]

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 :) )

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

--
Cheers

David