linux-kernel - Re: [PATCH v3 06/10] mm/vma: clean up __vma_enter/exit

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Message-ID: <deb0271a-af67-4971-8e2a-be0f826aee2f@lucifer.local>
Date: Fri, 23 Jan 2026 16:17:54 +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 v3 06/10] mm/vma: clean up __vma_enter/exit_locked()

On Fri, Jan 23, 2026 at 10:16:22AM +0100, Vlastimil Babka wrote:
> On 1/22/26 14:01, Lorenzo Stoakes wrote:
> > These functions are very confusing indeed. 'Entering' a lock could be
> > interpreted as acquiring it, but this is not what these functions are
> > interacting with.
> >
> > Equally they don't indicate at all what kind of lock we are 'entering' or
> > 'exiting'. Finally they are misleading as we invoke these functions when we
> > already hold a write lock to detach a VMA.
> >
> > These functions are explicitly simply 'entering' and 'exiting' a state in
> > which we hold the EXCLUSIVE lock in order that we can either mark the VMA
> > as being write-locked, or mark the VMA detached.
>
> If we hold a write lock (i.e. in vma_mark_detached()), that normally means
> it's also exclusive?
> And if we talk about the state between __vma_enter_exclusive_locked and
> __vma_exit_exclusive_locked() as "holding an EXCLUSIVE lock", it's not
> exactly the same lock as what we call "VMA write lock" right, so what lock
> is it?

Well it's not exclusive because spurious reader refcount increments are
possible (unless already detached of course...)

We are _excluding_ readers, including spurious ones realyl.

>
> Maybe it would help if we stopped calling this internal thing a "lock"?
> Except we use it for lockdep's lock_acquire_exclusive(). Sigh, sorry I don't
> have any great suggestion.
>
> Maybe call those functions __vma_exclude_readers_start() and
> __vma_exclude_readers_end() instead, or something?

Yeah that's probably better actually, will rename accordingly.

Cheers, Lorenzo