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=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 3C590C48BDF for ; Tue, 22 Jun 2021 22:31:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1EA0F61026 for ; Tue, 22 Jun 2021 22:31:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230185AbhFVWd5 convert rfc822-to-8bit (ORCPT ); Tue, 22 Jun 2021 18:33:57 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:20459 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229955AbhFVWd4 (ORCPT ); Tue, 22 Jun 2021 18:33:56 -0400 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-35-rM-Uk-sYOYOVhFr34Mu4Kw-1; Tue, 22 Jun 2021 23:31:37 +0100 X-MC-Unique: rM-Uk-sYOYOVhFr34Mu4Kw-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.18; Tue, 22 Jun 2021 23:31:36 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.018; Tue, 22 Jun 2021 23:31:36 +0100 From: David Laight To: 'Matthew Wilcox' CC: 'David Howells' , Al Viro , "torvalds@linux-foundation.org" , Ted Ts'o , Dave Hansen , Andrew Morton , "linux-mm@kvack.org" , "linux-ext4@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: RE: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? Thread-Topic: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? Thread-Index: AQHXZ4N03eCp9KNCtEagRX54mD94w6sgkJXg///0tICAABcJIA== Date: Tue, 22 Jun 2021 22:31:36 +0000 Message-ID: <0aac203b65b24776a27468e63114a24b@AcuMS.aculab.com> References: <3221175.1624375240@warthog.procyon.org.uk> <3225322.1624379221@warthog.procyon.org.uk> <7a6d8c55749d46d09f6f6e27a99fde36@AcuMS.aculab.com> In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org From: Matthew Wilcox > Sent: 22 June 2021 23:04 > > On Tue, Jun 22, 2021 at 09:55:09PM +0000, David Laight wrote: > > From: David Howells > > > Sent: 22 June 2021 17:27 > > > > > > Al Viro wrote: > > > > > > > On Tue, Jun 22, 2021 at 04:20:40PM +0100, David Howells wrote: > > > > > > > > > and wondering if the iov_iter_fault_in_readable() is actually effective. > > > > > Yes, it can make sure that the page we're intending to modify is dragged > > > > > into the pagecache and marked uptodate so that it can be read from, but is > > > > > it possible for the page to then get reclaimed before we get to > > > > > iov_iter_copy_from_user_atomic()? a_ops->write_begin() could potentially > > > > > take a long time, say if it has to go and get a lock/lease from a server. > > > > > > > > Yes, it is. So what? We'll just retry. You *can't* take faults while > > > > holding some pages locked; not without shitloads of deadlocks. > > > > > > In that case, can we amend the comment immediately above > > > iov_iter_fault_in_readable()? > > > > > > /* > > > * Bring in the user page that we will copy from _first_. > > > * Otherwise there's a nasty deadlock on copying from the > > > * same page as we're writing to, without it being marked > > > * up-to-date. > > > * > > > * Not only is this an optimisation, but it is also required > > > * to check that the address is actually valid, when atomic > > > * usercopies are used, below. > > > */ > > > if (unlikely(iov_iter_fault_in_readable(i, bytes))) { > > > > > > The first part suggests this is for deadlock avoidance. If that's not true, > > > then this should perhaps be changed. > > > > I'd say something like: > > /* > > * The actual copy_from_user() is done with a lock held > > * so cannot fault in missing pages. > > * So fault in the pages first. > > * If they get paged out the inatomic usercopy will fail > > * and the whole operation is retried. > > * > > * Hopefully there are enough memory pages available to > > * stop this looping forever. > > */ > > > > It is perfectly possible for another application thread to > > invalidate one of the buffer fragments after iov_iter_fault_in_readable() > > return success - so it will then fail on the second pass. > > > > The maximum number of pages required is twice the maximum number > > of iov fragments. > > If the system is crawling along with no available memory pages > > the same physical page could get used for two user pages. > > I would suggest reading the function before you suggest modifications > to it. > > offset = (pos & (PAGE_SIZE - 1)); > bytes = min_t(unsigned long, PAGE_SIZE - offset, > iov_iter_count(i)); Right the transfer size is limited to PAGE_SIZE. But the user supplied iov[] could be a lot of 2 byte buffers all with base addresses (PAGE_SIZE * n - 1). So you might need two pages for each iov[] fragment. And you need not to recycle the earlier ones. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)