Re: [PATCH v2 1/2] mm: propagate VM_SOFTDIRTY on merge

[Vlastimil Babka <vbabka@suse.cz>]

On 11/17/25 18:33, 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.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
> Reviewed-by: Pedro Falcato <pfalcato@suse.de>

Seems it's been like that since 34228d473efe ("mm: ignore VM_SOFTDIRTY on
VMA merging") (unless it was in the meanwhile fixed and broken again) but as
was discussed, not urgent for stable.

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>