linux-kernel - Re: [PATCH RESEND v3 04/10] mm/vma: add+use vma lockdep acquire/release defines

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Message-ID: <1fda7a07-beb7-4591-b7d2-d791c67e808a@lucifer.local>
Date: Fri, 23 Jan 2026 15:10:35 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...nel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeel.butt@...ux.dev>, Jann Horn <jannh@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-rt-devel@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Clark Williams <clrkwllms@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RESEND v3 04/10] mm/vma: add+use vma lockdep
 acquire/release defines

On Fri, Jan 23, 2026 at 09:48:28AM +0100, Vlastimil Babka wrote:
> On 1/22/26 14:01, Lorenzo Stoakes wrote:
> > The code is littered with inscrutable and duplicative lockdep incantations,
> > replace these with defines which explain what is going on and add
> > commentary to explain what we're doing.
> >
> > If lockdep is disabled these become no-ops. We must use defines so _RET_IP_
> > remains meaningful.
> >
> > These are self-documenting and aid readability of the code.
> >
> > Additionally, instead of using the confusing rwsem_*() form for something
> > that is emphatically not an rwsem, we instead explicitly use
> > lock_[acquired, release]_shared/exclusive() lockdep invocations since we
> > are doing something rather custom here and these make more sense to use.
> >
> > No functional change intended.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

>
> Nits:
>
> > ---
> >  include/linux/mmap_lock.h | 35 ++++++++++++++++++++++++++++++++---
> >  mm/mmap_lock.c            | 10 +++++-----
> >  2 files changed, 37 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > index 0b3614aadbb4..da63b1be6ec0 100644
> > --- a/include/linux/mmap_lock.h
> > +++ b/include/linux/mmap_lock.h
> > @@ -78,6 +78,36 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> >
> >  #ifdef CONFIG_PER_VMA_LOCK
> >
> > +/*
> > + * VMA locks do not behave like most ordinary locks found in the kernel, so we
> > + * cannot quite have full lockdep tracking in the way we would ideally prefer.
> > + *
> > + * Read locks act as shared locks which exclude an exclusive lock being
> > + * taken. We therefore mark these accordingly on read lock acquire/release.
> > + *
> > + * Write locks are acquired exclusively per-VMA, but released in a shared
> > + * fashion, that is upon vma_end_write_all(), we update the mmap's seqcount such
> > + * that write lock is de-acquired.
>
> de-acquired -> released?

Yeah don't know why I said it that way :) Fixed.

>
> > + * We therefore cannot track write locks per-VMA, nor do we try. Mitigating this
> > + * is the fact that, of course, we do lockdep-track the mmap lock rwsem.
>
> "... which has to be held in order to take a VMA write lock" ?

Slightly edited to:

 * We therefore cannot track write locks per-VMA, nor do we try. Mitigating this
 * is the fact that, of course, we do lockdep-track the mmap lock rwsem which
 * must be held when taking a VMA write lock.

Cheers, Lorenzo