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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 75F27C433E0 for ; Sat, 13 Jun 2020 12:56:21 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 420C92074D for ; Sat, 13 Jun 2020 12:56:21 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 420C92074D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id CBF8F6B0003; Sat, 13 Jun 2020 08:56:20 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C49356B0005; Sat, 13 Jun 2020 08:56:20 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B37F66B0006; Sat, 13 Jun 2020 08:56:20 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0186.hostedemail.com [216.40.44.186]) by kanga.kvack.org (Postfix) with ESMTP id 988876B0003 for ; Sat, 13 Jun 2020 08:56:20 -0400 (EDT) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 5D6C01EF2 for ; Sat, 13 Jun 2020 12:56:20 +0000 (UTC) X-FDA: 76924186920.15.fang09_3c0941726de5 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin15.hostedemail.com (Postfix) with ESMTP id 380421814B0C1 for ; Sat, 13 Jun 2020 12:56:20 +0000 (UTC) X-HE-Tag: fang09_3c0941726de5 X-Filterd-Recvd-Size: 3129 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [195.92.253.2]) by imf38.hostedemail.com (Postfix) with ESMTP for ; Sat, 13 Jun 2020 12:56:19 +0000 (UTC) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.93 #3 (Red Hat Linux)) id 1jk5hn-008AgX-Br; Sat, 13 Jun 2020 12:56:15 +0000 Date: Sat, 13 Jun 2020 13:56:15 +0100 From: Al Viro To: afzal mohammed Cc: Arnd Bergmann , Russell King - ARM Linux admin , Linus Walleij , "linux-kernel@vger.kernel.org" , Linux-MM , Linux ARM , Nicolas Pitre , Catalin Marinas , Will Deacon Subject: Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic() Message-ID: <20200613125615.GF23230@ZenIV.linux.org.uk> References: <9e1de19f35e2d5e1d115c9ec3b7c3284b4a4e077.1591885760.git.afzal.mohd.ma@gmail.com> <20200612135538.GA13399@afzalpc> <20200613120432.GA5319@afzalpc> <20200613125126.GE23230@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200613125126.GE23230@ZenIV.linux.org.uk> X-Rspamd-Queue-Id: 380421814B0C1 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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 Sat, Jun 13, 2020 at 01:51:26PM +0100, Al Viro wrote: > On Sat, Jun 13, 2020 at 05:34:32PM +0530, afzal mohammed wrote: > > > Observation is that max. pages reaching copy_{from,to}_user() is 2, > > observed maximum of n (number of bytes) being 1 page size. i think C > > library cuts any size read, write to page size (if it exceeds) & > > invokes the system call. Max. pages reaching 2, happens when 'n' > > crosses page boundary, this has been observed w/ small size request > > as well w/ ones of exact page size (but not page aligned). > > > > Even w/ dd of various size >4K, never is the number of pages required > > to be mapped going greater than 2 (even w/ 'dd' 'bs=1M') > > > > i have a worry (don't know whether it is an unnecessary one): even > > if we improve performance w/ large copy sizes, it might end up in a > > sluggishness w.r.t user experience due to most (hence a high amount) > > of user copy calls being few bytes & there the penalty being higher. > > And benchmark would not be able to detect anything abnormal since > > usercopy are being tested on large sizes. > > > > Quickly comparing boot-time on Beagle Bone White, boot time increases > > by only 4%, perhaps this worry is irrelevant, but just thought will > > put it across. > > Do stat(2) of the same tmpfs file in a loop (on tmpfs, to eliminate > the filesystem playing silly buggers). And I wouldn't expect anything > good there... Incidentally, what about get_user()/put_user()? _That_ is where it's going to really hurt...