linux-kernel - Re: [PATCH 1/2] mm: propagate VM

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Message-ID: <e16a403c-2194-41d3-acf3-fab09441d7b6@lucifer.local>
Date: Mon, 17 Nov 2025 15:53:36 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...nel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Jann Horn <jannh@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: propagate VM_SOFTDIRTY on merge

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@...e.cz>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Pedro Falcato <pfalcato@...e.de>

Thanks!

>
> --
> Pedro

Cheers, Lorenzo