linux-kernel - Re: [PATCH v3 tip/perf/core 2/4] mm: switch to 64-bit mm_lock_seq/vm_lock

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Message-ID: <20241023190240.GA11151@noisy.programming.kicks-ass.net>
Date: Wed, 23 Oct 2024 21:02:40 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>,
	Andrii Nakryiko <andrii@...nel.org>,
	linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
	oleg@...hat.com, rostedt@...dmis.org, mhiramat@...nel.org,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, jolsa@...nel.org,
	paulmck@...nel.org, willy@...radead.org, akpm@...ux-foundation.org,
	mjguzik@...il.com, brauner@...nel.org, jannh@...gle.com,
	mhocko@...nel.org, vbabka@...e.cz, hannes@...xchg.org,
	Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com
Subject: Re: [PATCH v3 tip/perf/core 2/4] mm: switch to 64-bit
 mm_lock_seq/vm_lock_seq on 64-bit architectures

On Wed, Oct 16, 2024 at 07:01:59PM -0700, Suren Baghdasaryan wrote:
> On Sun, Oct 13, 2024 at 12:56 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >
> > On Thu, Oct 10, 2024 at 01:56:42PM GMT, Andrii Nakryiko wrote:
> > > To increase mm->mm_lock_seq robustness, switch it from int to long, so
> > > that it's a 64-bit counter on 64-bit systems and we can stop worrying
> > > about it wrapping around in just ~4 billion iterations. Same goes for
> > > VMA's matching vm_lock_seq, which is derived from mm_lock_seq.
> 
> vm_lock_seq does not need to be long but for consistency I guess that
> makes sense. While at it, can you please change these seq counters to
> be unsigned?

Yeah, that. Kees is waging war on signed types that 'overflow'. These
sequence counter thingies are designed to wrap and should very much be
unsigned.