From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5DCC32FE593; Tue, 17 Feb 2026 09:17:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771319854; cv=none; b=CIoTXeMHixMCcbnbijNLsfHiFSXEAEZRhFdba7McmIGVoGtKW40mQ2uHVUqUGrWP5HLIPuvC5i8nRV+zvImWTchmdwF14yZfkEJ/eK2dXGM3JksuWDyaRyh7/RbHc4EtcgRQY/IE5hraFf2bdWLahRF+7nPgOuKwf2pF7NFbTCc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771319854; c=relaxed/simple; bh=FK8HEp1MtP9smfJ/su3Qtt0DpWL4giBXluxVWOSq4g4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=JFCbPyl/IUS/8dsbr0tqCbufMM90HndIVt1Ebt4yu104j5IsNhksBrcTaXWbKYlJWJ2M2dm/5on2qOkSndi4jC+Kpdqq8K4dkwkV10i8nEAbPXr3f6Dx7r341AxF17MOLk+K+8EzPmBnJlf+65/ccKjx1UcatLh03eAxdiJRUDw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=QZlDrP5Y; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="QZlDrP5Y" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=NMcaCvJ+VsP6giBf7ZN1YSDtg3HQ/fOah5gzjE9s5r8=; b=QZlDrP5Y0JpwgCqf29i4FnN/Nu Iko1dF7HxW+ARiccGLYmXJN4amHanoHJai1FlCcBNgV8bZyS1klk8UZ+N8f5TBumj2ZEsJTPZd0Wi jtfcFU1AnXOOo1bGYjaIDGWRAsa0RmO7y2FY1qvHeMk4W33YFQLt8/S6O4aeApNth5zqEj6QjFVmu EVe90V71dcXClO75FXC4DyHGhKSFwS3oTLcn1jBvGZmEF/RnPKrC5UqVejsEDxJkW6C1UasvQxvuP o+zZTCYXTAnga9lWEf6E9rrJZaXTLL4C6d2P2BG+m3dNUqUm1Zb+L96JExEZd1OuCJM4UJAAd70eR ZLFszwEg==; Received: from 2001-1c00-8d85-5700-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:5700:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vsHCu-0000000Fe2o-2CHV; Tue, 17 Feb 2026 09:17:24 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id CA152300CDE; Tue, 17 Feb 2026 10:17:23 +0100 (CET) Date: Tue, 17 Feb 2026 10:17:23 +0100 From: Peter Zijlstra To: Boqun Feng Cc: Greg KH , Andreas Hindborg , Alice Ryhl , Lorenzo Stoakes , "Liam R. Howlett" , Miguel Ojeda , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Trevor Gross , Danilo Krummrich , Will Deacon , Mark Rutland , linux-mm@kvack.org, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] rust: page: add byte-wise atomic memory copy methods Message-ID: <20260217091723.GU1395266@noisy.programming.kicks-ass.net> References: <20260213095557.GS2995752@noisy.programming.kicks-ass.net> <40xUh92AU5E9oFxQrdej-AXVg76jmaWGKXZMLoOHXe35Lw9x_eNEoLup9bB60LyGZ_0USPmoxr-9hE3ujA67cQ==@protonmail.internalid> <2026021343-germicide-baritone-efe8@gregkh> <877bsgu7fb.fsf@kernel.org> <2026021313-embody-deprive-9da5@gregkh> <873434u3yq.fsf@kernel.org> <20260213142608.GV2995752@noisy.programming.kicks-ass.net> <2026021311-shorten-veal-532c@gregkh> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Feb 13, 2026 at 07:45:19AM -0800, Boqun Feng wrote: > > > Suppose the memory was 'AAAA' and while you're reading it, it is written > > > to be 'BBBB'. The resulting copy can be any combination of > > > '[AB][AB][AB][AB]'. Not one of them is better than the other. > > > > > The idea is if using Rust's own `core::ptr::copy()` or > `core::ptr::copy_nonoverlapping()`, you may get `CCCC`, because they are > not semantically guaranteed atomic per byte (i.e. tearing can happen at > bit level, because they are not designed for using in case of data > races, and there is no defined asm implementation of them, compilers can > do anything). How the heck would they do out-of-thin-air? Any memcpy() implementation that can cause that is insane and broken. Compilers are broken crap if they do this.