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

[Lorenzo Stoakes]

On Mon, Nov 17, 2025 at 03:47:51PM +0000, Pedro Falcato wrote:
> On Fri, Nov 14, 2025 at 05:53:18PM +0000, 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.
>
> Does it make sense to backport this to stable? A more minimal version, that is.

No :) This has been subtly broken since forever. I don't think it warrants that
and it'd require significant and risky changes to older kernels to even make it
possible.

It's more a biproduct of features added so let's fix this going forward.

>
> >
> > 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>
>
> Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

Thanks!

>
> --
> Pedro

Cheers, Lorenzo