linux-kernel - Re: [PATCH 1/8] mm/rmap: improve anon_vma_clone(), unlink_anon

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Message-ID: <358a6bbd-521d-4d82-aee8-367026ba568f@lucifer.local>
Date: Tue, 6 Jan 2026 13:51:40 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeel.butt@...ux.dev>,
        David Hildenbrand <david@...nel.org>, Rik van Riel <riel@...riel.com>,
        Harry Yoo <harry.yoo@...cle.com>, Jann Horn <jannh@...gle.com>,
        Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Pedro Falcato <pfalcato@...e.de>, Chris Li <chriscli@...gle.com>,
        Barry Song <v-songbaohua@...o.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] mm/rmap: improve anon_vma_clone(),
 unlink_anon_vmas() comments, add asserts

On Fri, Dec 19, 2025 at 01:22:02PM -0500, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [251217 07:27]:
> > Add kdoc comments, describe exactly what these functinos are used for in
> > detail, pointing out importantly that the anon_vma_clone() !dst->anon_vma
> > && src->anon_vma dance is ONLY for fork.
> >
> > Both are confusing functions that will be refactored in a subsequent patch
> > but the first stage is establishing documentation and some invariatns.
> >
> > Add some basic CONFIG_DEBUG_VM asserts that help document expected state,
> > specifically:
> >
> > anon_vma_clone()
> > - mmap write lock held.
> > - We do nothing if src VMA is not faulted.
> > - The destination VMA has no anon_vma_chain yet.
> > - We are always operating on the same active VMA (i.e. vma->anon-vma).
> > - If not forking, must operate on the same mm_struct.
> >
> > unlink_anon_vmas()
> > - mmap lock held (read on unmap downgraded).
> > - That unfaulted VMAs are no-ops.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

Thanks!