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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 92500C47257 for ; Wed, 6 May 2020 12:59:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7AB37206DB for ; Wed, 6 May 2020 12:59:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728510AbgEFM7f (ORCPT ); Wed, 6 May 2020 08:59:35 -0400 Received: from mx2.suse.de ([195.135.220.15]:56334 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728081AbgEFM7e (ORCPT ); Wed, 6 May 2020 08:59:34 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id D694CAC90; Wed, 6 May 2020 12:59:34 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id CEA541E12A8; Wed, 6 May 2020 14:59:30 +0200 (CEST) Date: Wed, 6 May 2020 14:59:30 +0200 From: Jan Kara To: Souptick Joarder Cc: Jan Kara , John Hubbard , Tony Luck , fenghua.yu@intel.com, Rob Springer , Todd Poynor , benchan@chromium.org, Greg KH , Jens Wiklander , Andrew Morton , santosh.shilimkar@oracle.com, "David S. Miller" , kuba@kernel.org, Ira Weiny , =?iso-8859-1?B?Suly9G1l?= Glisse , inux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, "open list:ANDROID DRIVERS" , tee-dev@lists.linaro.org, Linux-MM , netdev@vger.kernel.org, linux-rdma@vger.kernel.org, rds-devel@oss.oracle.com Subject: Re: [RFC] mm/gup.c: Updated return value of {get|pin}_user_pages_fast() Message-ID: <20200506125930.GJ17863@quack2.suse.cz> References: <1588706059-4208-1-git-send-email-jrdr.linux@gmail.com> <0bfe4a8a-0d91-ef9b-066f-2ea7c68571b3@nvidia.com> <20200506100649.GI17863@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed 06-05-20 17:51:39, Souptick Joarder wrote: > On Wed, May 6, 2020 at 3:36 PM Jan Kara wrote: > > > > On Wed 06-05-20 02:06:56, Souptick Joarder wrote: > > > On Wed, May 6, 2020 at 1:08 AM John Hubbard wrote: > > > > > > > > On 2020-05-05 12:14, Souptick Joarder wrote: > > > > > Currently {get|pin}_user_pages_fast() have 3 return value 0, -errno > > > > > and no of pinned pages. The only case where these two functions will > > > > > return 0, is for nr_pages <= 0, which doesn't find a valid use case. > > > > > But if at all any, then a -ERRNO will be returned instead of 0, which > > > > > means {get|pin}_user_pages_fast() will have 2 return values -errno & > > > > > no of pinned pages. > > > > > > > > > > Update all the callers which deals with return value 0 accordingly. > > > > > > > > Hmmm, seems a little shaky. In order to do this safely, I'd recommend > > > > first changing gup_fast/pup_fast so so that they return -EINVAL if > > > > the caller specified nr_pages==0, and of course auditing all callers, > > > > to ensure that this won't cause problems. > > > > > > While auditing it was figured out, there are 5 callers which cares for > > > return value > > > 0 of gup_fast/pup_fast. What problem it might cause if we change > > > gup_fast/pup_fast > > > to return -EINVAL and update all the callers in a single commit ? > > > > Well, first I'd ask a different question: Why do you want to change the > > current behavior? It's not like the current behavior is confusing. Callers > > that pass >0 pages can happily rely on the simple behavior of < 0 return on > > error or > 0 return if we mapped some pages. Callers that can possibly ask > > to map 0 pages can get 0 pages back - kind of expected - and I don't see > > any benefit in trying to rewrite these callers to handle -EINVAL instead... > > Callers with a request to map 0 pages doesn't have a valid use case. But if any > caller end up doing it mistakenly, -errno should be returned to caller > rather than 0 > which will indicate more precisely that map 0 pages is not a valid > request from caller. Well, I believe this depends on the point of view. Similarly as reading 0 bytes is successful, we could consider mapping 0 pages successful as well. And there can be valid cases where number of pages to map is computed from some input and when 0 pages should be mapped, it is not a problem and your change would force such callers to special case this with explicitely checking for 0 pages to map and not calling GUP in that case at all. I'm not saying what you propose is necessarily bad, I just say I don't find it any better than the current behavior and so IMO it's not worth the churn. Now if you can come up with some examples of current in-kernel users who indeed do get the handling of the return value wrong, I could be convinced otherwise. Honza -- Jan Kara SUSE Labs, CR