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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 1C83DC83000 for ; Tue, 28 Apr 2020 14:06:35 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D1DF1206D9 for ; Tue, 28 Apr 2020 14:06:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="TRuRDNIw" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D1DF1206D9 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=9bTFKgbxCaxOT5nq5+RQNEmVPiLkjW3G2mUTE4DrS98=; b=TRuRDNIwoo/GJ9 sqaNWGGSuX1AWohxxu+8O2ojJ34sOWLU2jJGkSzXVpZIDSS6c+Qzht/VOs7PXcQrZ4UNcfgVwjCpl EqAnZTa6Rq/1zW1b2WppNd0tSxqzSgGmp4UGTz4v0oojJbNIjJIVabaB7lWFnZ8linzYtbqH8vM/M bVf5omvRziAdFN0a2pTHznce+zeC+ZvRZVL8GQSViEpNvbqI+lKIsQBfKc1PB3TMMyddzzfACxQOA e2QVPkmMWz2FxQ7uUS/cK6vTzzRqdBQoZIJWheJIichOPOjo30iG7QdO7R7aAW6Sx8FNZohjAq51L OqSVeSS8pVLpNYcWqzag==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jTQsc-0001Z1-FI; Tue, 28 Apr 2020 14:06:34 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jTQsa-0001YC-00 for linux-arm-kernel@lists.infradead.org; Tue, 28 Apr 2020 14:06:33 +0000 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 D9E2731B; Tue, 28 Apr 2020 07:06:30 -0700 (PDT) Received: from gaia (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 30FA73F68F; Tue, 28 Apr 2020 07:06:29 -0700 (PDT) Date: Tue, 28 Apr 2020 15:06:27 +0100 From: Catalin Marinas To: Dave Martin Subject: Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass Message-ID: <20200428140626.GJ3868@gaia> References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-21-catalin.marinas@arm.com> <20200427165641.GC15808@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200427165641.GC15808@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200428_070632_079848_B72F5ADA X-CRM114-Status: GOOD ( 20.64 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Richard Earnshaw , Will Deacon , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , linux-mm@kvack.org, Alexander Viro , Vincenzo Frascino , Peter Collingbourne , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Apr 27, 2020 at 05:56:42PM +0100, Dave P Martin wrote: > 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? Our copy routines already read 16 bytes at a time, so that's the tag granule. With current copy_mount_options() we have the issue that it assumes a fault in the first page is fatal. Even if we change it to a loop of smaller uaccess, we still have the issue of unaligned accesses which can fail without reading all that's possible (i.e. the access goes across a tag granule boundary). The previous copy_mount_options() implementation (from couple of months ago I think) had a fallback to byte-by-byte, didn't have this issue. > 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. strncpy_from_user() has a fallback to byte by byte, so we don't have an issue here. The above is only for synchronous accesses. For async, in v3 I disabled such checks for the uaccess routines. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel