From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17])
	by smtp.lore.kernel.org (Postfix) with ESMTP id 7BFB1C433EF
	for <linux-mm@archiver.kernel.org>; Sat,  9 Apr 2022 05:06:44 +0000 (UTC)
Received: by kanga.kvack.org (Postfix)
	id 0D3E26B0071; Sat,  9 Apr 2022 01:06:44 -0400 (EDT)
Received: by kanga.kvack.org (Postfix, from userid 40)
	id 084006B0073; Sat,  9 Apr 2022 01:06:44 -0400 (EDT)
X-Delivered-To: int-list-linux-mm@kvack.org
Received: by kanga.kvack.org (Postfix, from userid 63042)
	id E8D716B0074; Sat,  9 Apr 2022 01:06:43 -0400 (EDT)
X-Delivered-To: linux-mm@kvack.org
Received: from forelay.hostedemail.com (smtprelay0180.hostedemail.com [216.40.44.180])
	by kanga.kvack.org (Postfix) with ESMTP id DA7C96B0071
	for <linux-mm@kvack.org>; Sat,  9 Apr 2022 01:06:43 -0400 (EDT)
Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251])
	by forelay04.hostedemail.com (Postfix) with ESMTP id 8F60CA7272
	for <linux-mm@kvack.org>; Sat,  9 Apr 2022 05:06:43 +0000 (UTC)
X-FDA: 79336155486.30.F4380CB
Received: from verein.lst.de (verein.lst.de [213.95.11.211])
	by imf24.hostedemail.com (Postfix) with ESMTP id E149E180002
	for <linux-mm@kvack.org>; Sat,  9 Apr 2022 05:06:42 +0000 (UTC)
Received: by verein.lst.de (Postfix, from userid 2407)
	id AB9CC68AFE; Sat,  9 Apr 2022 07:06:38 +0200 (CEST)
Date: Sat, 9 Apr 2022 07:06:38 +0200
From: Christoph Hellwig <hch@lst.de>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Chuck Lever III <chuck.lever@oracle.com>,
	Mark Hemment <markhemm@googlemail.com>,
	Patrice CHOTARD <patrice.chotard@foss.st.com>,
	Mikulas Patocka <mpatocka@redhat.com>,
	Lukas Czerner <lczerner@redhat.com>, Christoph Hellwig <hch@lst.de>,
	"Darrick J. Wong" <djwong@kernel.org>, linux-mm@kvack.org,
	linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-stm32@st-md-mailman.stormreply.com, viro@zeniv.linux.org.uk
Subject: Re: [PATCH] tmpfs: fix regressions from wider use of ZERO_PAGE
Message-ID: <20220409050638.GB17755@lst.de>
References: <9a978571-8648-e830-5735-1f4748ce2e30@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <9a978571-8648-e830-5735-1f4748ce2e30@google.com>
User-Agent: Mutt/1.5.17 (2007-11-01)
X-Stat-Signature: 1sowrybqe8gb5uiypxcnihiq6bh67531
X-Rspamd-Server: rspam07
X-Rspamd-Queue-Id: E149E180002
Authentication-Results: imf24.hostedemail.com;
	dkim=none;
	spf=none (imf24.hostedemail.com: domain of hch@lst.de has no SPF policy when checking 213.95.11.211) smtp.mailfrom=hch@lst.de;
	dmarc=none
X-Rspam-User: 
X-HE-Tag: 1649480802-755872
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: <linux-mm.kvack.org>

On Fri, Apr 08, 2022 at 01:38:41PM -0700, Hugh Dickins wrote:
> +		} else if (iter_is_iovec(to)) {
> +			/*
> +			 * Copy to user tends to be so well optimized, but
> +			 * clear_user() not so much, that it is noticeably
> +			 * faster to copy the zero page instead of clearing.
> +			 */
> +			ret = copy_page_to_iter(ZERO_PAGE(0), offset, nr, to);

Is the offset and length guaranteed to be less than PAGE_SIZE here?

Either way I'd rather do this optimization in iov_iter_zero rather
than hiding it in tmpfs.