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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 2E209C433E1 for ; Sat, 13 Jun 2020 22:17:08 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C9E3320789 for ; Sat, 13 Jun 2020 22:17:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="p2eUytlx" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C9E3320789 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 3B02C6B0003; Sat, 13 Jun 2020 18:17:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 339136B0005; Sat, 13 Jun 2020 18:17:07 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 200AD6B0006; Sat, 13 Jun 2020 18:17:07 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0211.hostedemail.com [216.40.44.211]) by kanga.kvack.org (Postfix) with ESMTP id 05BFF6B0003 for ; Sat, 13 Jun 2020 18:17:07 -0400 (EDT) Received: from smtpin02.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id B0F14180AD81A for ; Sat, 13 Jun 2020 22:17:06 +0000 (UTC) X-FDA: 76925600052.02.juice59_620395326de8 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin02.hostedemail.com (Postfix) with ESMTP id 82FF27D159 for ; Sat, 13 Jun 2020 22:17:06 +0000 (UTC) X-HE-Tag: juice59_620395326de8 X-Filterd-Recvd-Size: 4211 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) by imf17.hostedemail.com (Postfix) with ESMTP for ; Sat, 13 Jun 2020 22:17:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; 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=i5R8hGtupULp/LcVllMJa3o6cOtVEr3gO4/vRAJX748=; b=p2eUytlxPR3iPhCZ0KngWubLeM zm42uSEM457KwFdzZwuav82XyMjwbtox4oV2A4A7HWqClcOWGfRVHS8sws0VVU4wIwBUMiX3SchMa uFsVAnalBx9WEVFAzcZXqpv9WFHV8b+hgtAOboKUtPFEXg4eG9TUMX49nAhU//h8ikPV8zNFiG0RK 1HrfnY9Rs2dfhZxFE1dxgnu9LHNrRD2fjo1qVZMlVFRfKn7GYIdvLS5+MJevtTuAeMYUZa1jkPciM S9djRDiMdXSS+XfnqqG0iJcgzjHUchMmDWrv//HthfDc+4SyFQHln1Z268xc+L1VCvcM432Vnl2YU /WANM4aQ==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jkESM-000668-Rm; Sat, 13 Jun 2020 22:16:54 +0000 Date: Sat, 13 Jun 2020 15:16:54 -0700 From: Matthew Wilcox To: Arnd Bergmann Cc: afzal mohammed , 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: <20200613221654.GO8681@bombadil.infradead.org> References: <9e1de19f35e2d5e1d115c9ec3b7c3284b4a4e077.1591885760.git.afzal.mohd.ma@gmail.com> <20200612135538.GA13399@afzalpc> <20200613120432.GA5319@afzalpc> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 82FF27D159 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 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 10:45:33PM +0200, Arnd Bergmann wrote: > On Sat, Jun 13, 2020 at 2:04 PM 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). > > Right, this is apparently because tmpfs uses shmem_file_read_iter() to > copy the file pages one at a time. generic_file_buffered_read() seems > similar, to copying between an aligned kernel page and address in > user space that is not page aligned would be an important case to > optimize for. This is kind of the nature of the page cache. The kernel doesn't necessarily have contiguous memory in the page cache, so it's going to be split on page boundaries. This is going to change with my THP series (I haven't actually changed generic_file_buffered_read(), but it'll come later). > > 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. > > 4% boot time increase sounds like a lot, especially if that is only for > copy_from_user/copy_to_user. In the end it really depends on how well > get_user()/put_user() and small copies can be optimized in the end. The write path should also be paid attention to. Look at generic_perform_write() which first calls iov_iter_fault_in_readable() (for the entire length of the write) and then actually does the copy later with iov_iter_copy_from_user_atomic(). So you're going to want to optimise the case where you access the same pages multiple times.