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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 A8D3AC54FD3 for ; Mon, 20 Apr 2020 11:54:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7F2FB20857 for ; Mon, 20 Apr 2020 11:54:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="key not found in DNS" (0-bit key) header.d=szeredi.hu header.i=@szeredi.hu header.b="eMGJ6i/T" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726852AbgDTLyz (ORCPT ); Mon, 20 Apr 2020 07:54:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53648 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1726845AbgDTLyy (ORCPT ); Mon, 20 Apr 2020 07:54:54 -0400 Received: from mail-ej1-x643.google.com (mail-ej1-x643.google.com [IPv6:2a00:1450:4864:20::643]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2C843C061A10 for ; Mon, 20 Apr 2020 04:54:54 -0700 (PDT) Received: by mail-ej1-x643.google.com with SMTP id n17so7664618ejh.7 for ; Mon, 20 Apr 2020 04:54:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=szeredi.hu; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Xz434+emhPfWjjKXYErvQLCog0qnyrqkr9U47Pj8EAE=; b=eMGJ6i/Tp+yHljfYp7aMYd/mUsv2EnqVE+fCxu3XOH5yGdB0h61gy+ScRLFSyuRmgb qURUHX6I6Ir8eepIVf+Ri2YviGMLt6DWjxys1zOQJEVUfWPlsKPJUpkbtTkdGn4w0n0B 1MP04jnPqpFSU10941aKvkRA4UrFsDmfSRMik= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Xz434+emhPfWjjKXYErvQLCog0qnyrqkr9U47Pj8EAE=; b=S/BaLV9Xy3jARac6XI2ICPBRKglAfLWJOBevBYogWhiJiy8jOsvbKsGeCOmAyYS96y FfVOgno1Ol2DFg/6w5igBGPzNdxP6BipEdWY4XJjKwqfx+SwA0pC59ERK/EULbh8WBWX B+xFhVZJzgknezKgV+mNJdet2RiniXWpm2YRq9xmeBL2QhZjEBPAcCot8ZBfpCuuiH6m LXbTHs4jmL8PjcBKVem1hlNyYWH/rvOnDz39V7tV/2J5TbQbPn2RkTRf5LJpuSr9365L 5wC+Sz0Y6viayn2z30BXVUXyKC3+CdT0iMRPYUllIjIsgCWANSf//sgkIs8P/rLKAsAj /5fg== X-Gm-Message-State: AGi0Pub9RN0MSibkSfSN7oRaYjysuMGVjve1m8A2YpQT/lBbkiOtuj86 a1Pv9+IV6qxBZHQDbhm5IUetko0Z9qAYLWvwaeYAZw== X-Google-Smtp-Source: APiQypLbTn+lSf9tXQjoSINzOkViouA+EuTM208dkpb3K/JiwqGLK/tHyeL8eS/wVDrlTm/f0ngTgrLv3Gua9FB3DGg= X-Received: by 2002:a17:906:8549:: with SMTP id h9mr15204554ejy.145.1587383692787; Mon, 20 Apr 2020 04:54:52 -0700 (PDT) MIME-Version: 1.0 References: <20200414150233.24495-1-willy@infradead.org> <20200414150233.24495-25-willy@infradead.org> <20200420114300.GB5820@bombadil.infradead.org> In-Reply-To: <20200420114300.GB5820@bombadil.infradead.org> From: Miklos Szeredi Date: Mon, 20 Apr 2020 13:54:41 +0200 Message-ID: Subject: Re: [PATCH v11 24/25] fuse: Convert from readpages to readahead To: Matthew Wilcox Cc: Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, ocfs2-devel@oss.oracle.com, linux-xfs , Dave Chinner , William Kucharski Content-Type: text/plain; charset="UTF-8" Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Mon, Apr 20, 2020 at 1:43 PM Matthew Wilcox wrote: > > On Mon, Apr 20, 2020 at 01:14:17PM +0200, Miklos Szeredi wrote: > > > + for (;;) { > > > + struct fuse_io_args *ia; > > > + struct fuse_args_pages *ap; > > > + > > > + nr_pages = readahead_count(rac) - nr_pages; > > > > Hmm. I see what's going on here, but it's confusing. Why is > > __readahead_batch() decrementing the readahead count at the start, > > rather than at the end? > > > > At the very least it needs a comment about why nr_pages is calculated this way. > > Because usually that's what we want. See, for example, fs/mpage.c: > > while ((page = readahead_page(rac))) { > prefetchw(&page->flags); > args.page = page; > args.nr_pages = readahead_count(rac); > args.bio = do_mpage_readpage(&args); > put_page(page); > } > > fuse is different because it's trying to allocate for the next batch, > not for the batch we're currently on. > > I'm a little annoyed because I posted almost this exact loop here: > > https://lore.kernel.org/linux-fsdevel/CAJfpegtrhGamoSqD-3Svfj3-iTdAbfD8TP44H_o+HE+g+CAnCA@mail.gmail.com/ > > and you said "I think that's fine", modified only by your concern > for it not being obvious that nr_pages couldn't be decremented by > __readahead_batch(), so I modified the loop slightly to assign to > nr_pages. The part you're now complaining about is unchanged. Your annoyance is perfectly understandable. This is something I noticed now, not back then. > > > > + if (nr_pages > max_pages) > > > + nr_pages = max_pages; > > > + if (nr_pages == 0) > > > + break; > > > + ia = fuse_io_alloc(NULL, nr_pages); > > > + if (!ia) > > > + return; > > > + ap = &ia->ap; > > > + nr_pages = __readahead_batch(rac, ap->pages, nr_pages); > > > + for (i = 0; i < nr_pages; i++) { > > > + fuse_wait_on_page_writeback(inode, > > > + readahead_index(rac) + i); > > > > What's wrong with ap->pages[i]->index? Are we trying to wean off using ->index? > > It saves reading from a cacheline? I wouldn't be surprised if the > compiler hoisted the read from rac->_index to outside the loop and just > iterated from rac->_index to rac->_index + nr_pages. Hah, if such optimizations were worth anything with codepaths involving roundtrips to userspace... Anyway, I'll let these be, and maybe clean them up later. Acked-by: Miklos Szeredi Thanks, Miklos