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

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Message-ID: <f5291639-042c-4ecb-8e7d-8d0fbc8b80ba@lucifer.local>
Date: Tue, 6 Jan 2026 13:01:15 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 Tue, Jan 06, 2026 at 12:54:17PM +0000, Lorenzo Stoakes wrote:
> > > > +     /* Unfaulted is a no-op. */
> > > > +     VM_WARN_ON_ONCE(!vma->anon_vma && !list_empty(&vma->anon_vma_chain));
> >
> > Hmm. anon_vma_clone() calls unlink_anon_vmas() after setting
> > dst->anon_vma=NULL in the enomem_failure path. This warning would
> > imply that in such case dst->anon_vma_chain is always non-empty. But I
> > don't think we can always expect that... What if the very first call
> > to anon_vma_chain_alloc() in anon_vma_clone()'s loop failed, I think
> > this would result in dst->anon_vma_chain being empty, no?
>
> OK well that's a good spot, though this is never going to actually happen in
> reality as an allocation failure here would really be 'too small to fail'.
>
> It's a pity we have to give up a completely sensible invariant because of
> terribly written code for an event that will never happen.
>
> But sure will drop this then, that's awful to have to do though :/

Actually let me just update the stupid hack exit path code to increment
anon_vma->num_active_vmas in this case, and do it that way (so
unlink_anon_vmas() drops it again).

That actually makes the whole thing _less_ of a hack as it really makes zero
sense for the anon_vma not to be specified but to be working through
vma->anon_vma_chain, and that's a very important invariant.