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: <20241023193111.GC11151@noisy.programming.kicks-ass.net>
Date: Wed, 23 Oct 2024 21:31:11 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: 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, surenb@...gle.com, akpm@...ux-foundation.org,
	mjguzik@...il.com, brauner@...nel.org, jannh@...gle.com,
	mhocko@...nel.org, vbabka@...e.cz, shakeel.butt@...ux.dev,
	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 Thu, Oct 10, 2024 at 01:56:42PM -0700, 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.
> 
> I didn't use __u64 outright to keep 32-bit architectures unaffected, but
> if it seems important enough, I have nothing against using __u64.

(__uXX are the uapi types)

> 
> Suggested-by: Jann Horn <jannh@...gle.com>

Jann, do you see problems with the normal seqcount being unsigned (int)?
I suppose especially for preemptible seqcounts it might already be
entirely feasible to wrap them?

Doing u64 is tricky but not impossible, it would require something like
we do for GUP_GET_PXX_LOW_HIGH. OTOH, I don't think we really care about
32bit enough to bother.