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 8C99BC433DB for ; Fri, 29 Jan 2021 11:53:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 598FC65013 for ; Fri, 29 Jan 2021 11:53:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232934AbhA2LxU (ORCPT ); Fri, 29 Jan 2021 06:53:20 -0500 Received: from hmm.wantstofly.org ([213.239.204.108]:51438 "EHLO mail.wantstofly.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232273AbhA2K0S (ORCPT ); Fri, 29 Jan 2021 05:26:18 -0500 Received: by mail.wantstofly.org (Postfix, from userid 1000) id 90D707F45D; Fri, 29 Jan 2021 12:20:57 +0200 (EET) Date: Fri, 29 Jan 2021 12:20:57 +0200 From: Lennert Buytenhek To: David Laight , Al Viro Cc: Jens Axboe , linux-kernel@vger.kernel.org, io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [RFC PATCH] io_uring: add support for IORING_OP_GETDENTS64 Message-ID: <20210129102057.GD193464@wantstofly.org> References: <20210123114152.GA120281@wantstofly.org> <20210128230710.GA190469@wantstofly.org> <20210129053703.GB190469@wantstofly.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210129053703.GB190469@wantstofly.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 29, 2021 at 07:37:03AM +0200, Lennert Buytenhek wrote: > > > > One open question is whether IORING_OP_GETDENTS64 should be more like > > > > pread(2) and allow passing in a starting offset to read from the > > > > directory from. (This would require some more surgery in fs/readdir.c.) > > > > > > Since directories are seekable this ought to work. > > > Modulo horrid issues with 32bit file offsets. > > > > The incremental patch below does this. (It doesn't apply cleanly on > > top of v1 of the IORING_OP_GETDENTS patch as I have other changes in > > my tree -- I'm including it just to illustrate the changes that would > > make this work.) > > > > This change seems to work, and makes IORING_OP_GETDENTS take an > > explicitly specified directory offset (instead of using the file's > > ->f_pos), making it more like pread(2) [...] > > ...but the fact that this patch avoids taking file->f_pos_lock (as this > proposed version of IORING_OP_GETDENTS avoids using file->f_pos) means > that ->iterate_shared() can then be called concurrently on the same > struct file, which breaks the mutual exclusion guarantees provided here. > > If possible, I'd like to keep the ability to explicitly pass in a > directory offset to start reading from into IORING_OP_GETDENTS, so > perhaps we can simply satisfy the mutual exclusion requirement by > taking ->f_pos_lock by hand -- but then I do need to check that it's OK > for ->iterate{,_shared}() to be called successively with discontinuous > offsets without ->llseek() being called in between. > > (If that's not OK, then we can either have IORING_OP_GETDENTS just > always start reading at ->f_pos like before (which might then require > adding a IORING_OP_GETDENTS2 at some point in the future if we'll > ever want to change that), or we could have IORING_OP_GETDENTS itself > call ->llseek() for now whenever necessary.) Having IORING_OP_GETDENTS seek to sqe->off if needed seems easy enough to implement, and it simplifies the other code as well, so I'll send out a v2 RFC shortly that does this.