linux-kernel - Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix anon

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Date:   Thu, 27 Jul 2023 17:15:45 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     paulmck@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix
 anon_vma memory ordering

On Thu, Jul 27, 2023 at 5:07 PM Matthew Wilcox <willy@...radead.org> wrote:
> On Thu, Jul 27, 2023 at 04:39:34PM +0200, Jann Horn wrote:
> > The other option is to replace the READ_ONCE() with a
> > smp_load_acquire(), at which point it becomes a lot simpler to show
> > that the code is correct.
>
> Aren't we straining at gnats here?  The context of this is handling a
> page fault, and we used to take an entire rwsem for read.  I'm having
> a hard time caring about "the extra expense" of an unnecessarily broad
> barrier.
>
> Cost of an L3 cacheline miss is in the thousands of cycles.  Cost of a
> barrier is ... tens?

Yeah, fair point. If it's hard to show correctness with READ_ONCE() we
can just use smp_load_acquire() and call it a day.