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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98 #2 (Red Hat Linux)) id 1t3h4Z-00000008XFB-3X7B; Wed, 23 Oct 2024 19:31:12 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 766BF30073F; Wed, 23 Oct 2024 21:31:11 +0200 (CEST) Date: Wed, 23 Oct 2024 21:31:11 +0200 From: Peter Zijlstra To: Andrii Nakryiko Cc: linux-trace-kernel@vger.kernel.org, linux-mm@kvack.org, oleg@redhat.com, rostedt@goodmis.org, mhiramat@kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, jolsa@kernel.org, paulmck@kernel.org, willy@infradead.org, surenb@google.com, akpm@linux-foundation.org, mjguzik@gmail.com, brauner@kernel.org, jannh@google.com, mhocko@kernel.org, vbabka@suse.cz, shakeel.butt@linux.dev, hannes@cmpxchg.org, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.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 Message-ID: <20241023193111.GC11151@noisy.programming.kicks-ass.net> References: <20241010205644.3831427-1-andrii@kernel.org> <20241010205644.3831427-3-andrii@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241010205644.3831427-3-andrii@kernel.org> X-Rspam-User: X-Stat-Signature: ro9e69qr4mxytbg3oe57okow63ku3qxj X-Rspamd-Queue-Id: A80414001F X-Rspamd-Server: rspam02 X-HE-Tag: 1729711862-724368 X-HE-Meta: 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 W5nmA8x6 ZNUdRjgsYeemqJeLQH+7iQ04Ly2kX9PEG23ZkdWDX4jQanzsu0A0T4QGSJr8QkFFfsl+GFg/xVswXMaw3AqwaxVdiytDgHOlAE5FvVnYiAGhOV6OY5qK3AsmuXzQtXqhTkaYAn6rReMOK4XQE0bD0h64YQmzLwMxDj0W6XVWz2gMVM0l52lSJtBOjoKer0cs9fF41D9aWw79GJag7z5/IiTKnK/HMFO48kkB/VCFF1GGzH2ooZUNfGaNvloFgGPJUe2yjeD+6AKeoL7k= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 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.