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 7541BC433EF for ; Fri, 10 Sep 2021 03:27:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4FB4861074 for ; Fri, 10 Sep 2021 03:27:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230023AbhIJD3G (ORCPT ); Thu, 9 Sep 2021 23:29:06 -0400 Received: from zeniv-ca.linux.org.uk ([142.44.231.140]:59322 "EHLO zeniv-ca.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229461AbhIJD3G (ORCPT ); Thu, 9 Sep 2021 23:29:06 -0400 Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mOXCk-002n9d-In; Fri, 10 Sep 2021 03:27:54 +0000 Date: Fri, 10 Sep 2021 03:27:54 +0000 From: Al Viro To: Jens Axboe Cc: Linus Torvalds , Pavel Begunkov , Linux Kernel Mailing List , linux-fsdevel Subject: Re: [git pull] iov_iter fixes Message-ID: References: <5971af96-78b7-8304-3e25-00dc2da3c538@kernel.dk> <9ae5f07f-f4c5-69eb-bcb1-8bcbc15cbd09@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9ae5f07f-f4c5-69eb-bcb1-8bcbc15cbd09@kernel.dk> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Sep 09, 2021 at 09:22:30PM -0600, Jens Axboe wrote: > On 9/9/21 9:11 PM, Al Viro wrote: > > On Thu, Sep 09, 2021 at 09:05:13PM -0600, Jens Axboe wrote: > >> On 9/9/21 8:57 PM, Al Viro wrote: > >>> On Thu, Sep 09, 2021 at 03:19:56PM -0600, Jens Axboe wrote: > >>> > >>>> Not sure how we'd do that, outside of stupid tricks like copy the > >>>> iov_iter before we pass it down. But that's obviously not going to be > >>>> very efficient. Hence we're left with having some way to reset/reexpand, > >>>> even in the presence of someone having done truncate on it. > >>> > >>> "Obviously" why, exactly? It's not that large a structure; it's not > >>> the optimal variant, but I'd like to see profiling data before assuming > >>> that it'll cause noticable slowdowns. > >> > >> It's 48 bytes, and we have to do it upfront. That means we'd be doing it > >> for _all_ requests, not just when we need to retry. As an example, current > >> benchmarks are at ~4M read requests per core. That'd add ~200MB/sec of > >> memory traffic just doing this copy. > > > > Umm... How much of that will be handled by cache? > > Depends? And what if the iovec itself has been modified in the middle? > We'd need to copy that whole thing too. It's just not workable as a > solution. Huh? Why the hell would we need to copy iovecs themselves? They are never modified by ->read_iter()/->write_iter(). That's the whole fucking point of iov_iter - the iovec itself is made constant, with all movable parts taken to iov_iter. Again, we should never, ever modify the iovec (or bvec, etc.) array in ->read_iter()/->write_iter()/->sendmsg()/etc. instances. If you see such behaviour anywhere, report it immediately. Any such is a blatant bug.