From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 291C3C54FD0 for ; Mon, 27 Apr 2020 16:56:48 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id DB493206D4 for ; Mon, 27 Apr 2020 16:56:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DB493206D4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 811498E0005; Mon, 27 Apr 2020 12:56:47 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7C0D18E0001; Mon, 27 Apr 2020 12:56:47 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6D6498E0005; Mon, 27 Apr 2020 12:56:47 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0045.hostedemail.com [216.40.44.45]) by kanga.kvack.org (Postfix) with ESMTP id 5380D8E0001 for ; Mon, 27 Apr 2020 12:56:47 -0400 (EDT) Received: from smtpin24.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 0DF6B181AEF00 for ; Mon, 27 Apr 2020 16:56:47 +0000 (UTC) X-FDA: 76754239254.24.trees20_5848b78735121 X-HE-Tag: trees20_5848b78735121 X-Filterd-Recvd-Size: 3137 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf21.hostedemail.com (Postfix) with ESMTP for ; Mon, 27 Apr 2020 16:56:46 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id ADB1531B; Mon, 27 Apr 2020 09:56:45 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 297AA3F68F; Mon, 27 Apr 2020 09:56:44 -0700 (PDT) Date: Mon, 27 Apr 2020 17:56:42 +0100 From: Dave Martin To: Catalin Marinas Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, Richard Earnshaw , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , Peter Collingbourne , linux-mm@kvack.org, Alexander Viro , Vincenzo Frascino , Will Deacon Subject: Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass Message-ID: <20200427165641.GC15808@arm.com> References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-21-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200421142603.3894-21-catalin.marinas@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) 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: On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote: > The copy_mount_options() function takes a user pointer argument but not > a size. It tries to read up to a PAGE_SIZE. However, copy_from_user() is > not guaranteed to return all the accessible bytes if, for example, the > access crosses a page boundary and gets a fault on the second page. To > work around this, the current copy_mount_options() implementations > performs to copy_from_user() passes, first to the end of the current > page and the second to what's left in the subsequent page. > > Some architectures like arm64 can guarantee an exact copy_from_user() > depending on the size (since the arch function performs some alignment > on the source register). Introduce an arch_has_exact_copy_from_user() > function and allow copy_mount_options() to perform the user access in a > single pass. > > While this function is not on a critical path, the single-pass behaviour > is required for arm64 MTE (memory tagging) support where a uaccess can > trigger intra-page faults (tag not matching). With the current > implementation, if this happens during the first page, the function will > return -EFAULT. Do you know how much extra overhead we'd incur if we read at must one tag granule at a time, instead of PAGE_SIZE? I'm guessing that in practice strcpy_from_user() type operations copy much less than a page most of the time, so what we lose in uaccess overheads we _might_ regain in less redundant copying. Would need behchmarking though. [...] Cheers ---Dave