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=-3.8 required=3.0 tests=BAYES_00, 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 33504C4320A for ; Tue, 10 Aug 2021 08:47:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1900560F25 for ; Tue, 10 Aug 2021 08:47:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237141AbhHJIrs convert rfc822-to-8bit (ORCPT ); Tue, 10 Aug 2021 04:47:48 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:33061 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233034AbhHJIri (ORCPT ); Tue, 10 Aug 2021 04:47:38 -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-275-lqUBwIiNOQKJbW6sqSZqNw-1; Tue, 10 Aug 2021 09:47:14 +0100 X-MC-Unique: lqUBwIiNOQKJbW6sqSZqNw-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.23; Tue, 10 Aug 2021 09:47:13 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.023; Tue, 10 Aug 2021 09:47:13 +0100 From: David Laight To: 'Al Viro' , Pavel Begunkov CC: "linux-fsdevel@vger.kernel.org" , "Jens Axboe" , "io-uring@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH 0/2] iter revert problems Thread-Topic: [PATCH 0/2] iter revert problems Thread-Index: AQHXjTbk1nsSHvSB3kSUkMDvGdtDEqtsbGoA Date: Tue, 10 Aug 2021 08:47:13 +0000 Message-ID: <1fc5348f7598404088d4ecda3bbb397a@AcuMS.aculab.com> References: 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-kernel@vger.kernel.org From: Al Viro > Sent: 09 August 2021 16:53 > > On Mon, Aug 09, 2021 at 12:52:35PM +0100, Pavel Begunkov wrote: > > For the bug description see 2/2. As mentioned there the current problems > > is because of generic_write_checks(), but there was also a similar case > > fixed in 5.12, which should have been triggerable by normal > > write(2)/read(2) and others. > > > > It may be better to enforce reexpands as a long term solution, but for > > now this patchset is quickier and easier to backport. > > Umm... Won't that screw the cases where we *are* doing proper > reexpands? AFAICS, with your patches that flag doesn't go away once > it had been set... >From what I remember the pointer into the iov[] gets incremented as it is processed - which makes 'backing up' hard. The caller also has to remember the original pointer because it might point to kmalloced memory. So if the 'iter' always contained a pointer to the base of the iov[] then various bits of code could be simplified. Another useful change would be to embed the short iov_cache[8] inside 'iter'. Almost all the callers allocate both together (usually on stack) so the stack use won't change. I have local patches for most of this (somewhere) but the io_uring changes start being non-trivial. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)