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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 820C9C35242 for ; Tue, 11 Feb 2020 20:08:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6C7D520714 for ; Tue, 11 Feb 2020 20:08:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729580AbgBKUIf (ORCPT ); Tue, 11 Feb 2020 15:08:35 -0500 Received: from mail105.syd.optusnet.com.au ([211.29.132.249]:38631 "EHLO mail105.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728537AbgBKUIf (ORCPT ); Tue, 11 Feb 2020 15:08:35 -0500 Received: from dread.disaster.area (pa49-179-138-28.pa.nsw.optusnet.com.au [49.179.138.28]) by mail105.syd.optusnet.com.au (Postfix) with ESMTPS id 2D0293A3899; Wed, 12 Feb 2020 07:08:28 +1100 (AEDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1j1bpa-0002l0-DL; Wed, 12 Feb 2020 07:08:26 +1100 Date: Wed, 12 Feb 2020 07:08:26 +1100 From: Dave Chinner To: Matthew Wilcox Cc: 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@vger.kernel.org Subject: Re: [PATCH v5 04/13] mm: Add readahead address space operation Message-ID: <20200211200826.GK10776@dread.disaster.area> References: <20200211010348.6872-1-willy@infradead.org> <20200211010348.6872-5-willy@infradead.org> <20200211045230.GD10776@dread.disaster.area> <20200211125413.GU8731@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211125413.GU8731@bombadil.infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=X6os11be c=1 sm=1 tr=0 a=zAxSp4fFY/GQY8/esVNjqw==:117 a=zAxSp4fFY/GQY8/esVNjqw==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=kj9zAlcOel0A:10 a=l697ptgUJYAA:10 a=7-415B0cAAAA:8 a=EnYlyWvjhy6VBD_eMqkA:9 a=CdacwtsPoHkD4rhW:21 a=vlsg44Ume0T2P6Xz:21 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, Feb 11, 2020 at 04:54:13AM -0800, Matthew Wilcox wrote: > On Tue, Feb 11, 2020 at 03:52:30PM +1100, Dave Chinner wrote: > > > +struct readahead_control { > > > + struct file *file; > > > + struct address_space *mapping; > > > +/* private: use the readahead_* accessors instead */ > > > + pgoff_t start; > > > + unsigned int nr_pages; > > > + unsigned int batch_count; > > > +}; > > > + > > > +static inline struct page *readahead_page(struct readahead_control *rac) > > > +{ > > > + struct page *page; > > > + > > > + if (!rac->nr_pages) > > > + return NULL; > > > + > > > + page = xa_load(&rac->mapping->i_pages, rac->start); > > > + VM_BUG_ON_PAGE(!PageLocked(page), page); > > > + rac->batch_count = hpage_nr_pages(page); > > > + rac->start += rac->batch_count; > > > > There's no mention of large page support in the patch description > > and I don't recall this sort of large page batching in previous > > iterations. > > > > This seems like new functionality to me, not directly related to > > the initial ->readahead API change? What have I missed? > > I had a crisis of confidence when I was working on this -- the loop > originally looked like this: > > #define readahead_for_each(rac, page) \ > for (; (page = readahead_page(rac)); rac->nr_pages--) > > and then I started thinking about what I'd need to do to support large > pages, and that turned into > > #define readahead_for_each(rac, page) \ > for (; (page = readahead_page(rac)); \ > rac->nr_pages -= hpage_nr_pages(page)) > > but I realised that was potentially a use-after-free because 'page' has > certainly had put_page() called on it by then. I had a brief period > where I looked at moving put_page() away from being the filesystem's > responsibility and into the iterator, but that would introduce more > changes into the patchset, as well as causing problems for filesystems > that want to break out of the loop. > > By this point, I was also looking at the readahead_for_each_batch() > iterator that btrfs uses, and so we have the batch count anyway, and we > might as well use it to store the number of subpages of the large page. > And so it became easier to just put the whole ball of wax into the initial > patch set, rather than introduce the iterator now and then fix it up in > the patch set that I'm basing on this. > > So yes, there's a certain amount of excess functionality in this patch > set ... I can remove it for the next release. I'd say "Just document it" as that was the main reason I noticed it. Or perhaps add the batching function as a stand-alone patch so it's clear that the batch interface solves two problems at once - large pages and the btrfs page batching implementation... Cheers, Dave. -- Dave Chinner david@fromorbit.com