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,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 C394BC34031 for ; Wed, 19 Feb 2020 00:59:22 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 858072173E for ; Wed, 19 Feb 2020 00:59:22 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 858072173E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=fromorbit.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 2CAB46B0003; Tue, 18 Feb 2020 19:59:22 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 27B556B0006; Tue, 18 Feb 2020 19:59:22 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 16AAE6B0007; Tue, 18 Feb 2020 19:59:22 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0231.hostedemail.com [216.40.44.231]) by kanga.kvack.org (Postfix) with ESMTP id EEC176B0003 for ; Tue, 18 Feb 2020 19:59:21 -0500 (EST) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id A7CA2181AEF10 for ; Wed, 19 Feb 2020 00:59:21 +0000 (UTC) X-FDA: 76505068122.15.sack55_8bd6e89451011 X-HE-Tag: sack55_8bd6e89451011 X-Filterd-Recvd-Size: 6200 Received: from mail105.syd.optusnet.com.au (mail105.syd.optusnet.com.au [211.29.132.249]) by imf37.hostedemail.com (Postfix) with ESMTP for ; Wed, 19 Feb 2020 00:59:20 +0000 (UTC) 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 368A83A2380; Wed, 19 Feb 2020 11:59:16 +1100 (AEDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1j4Dhr-0004cn-HF; Wed, 19 Feb 2020 11:59:15 +1100 Date: Wed, 19 Feb 2020 11:59:15 +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 v6 07/19] mm: Put readahead pages in cache earlier Message-ID: <20200219005915.GV10776@dread.disaster.area> References: <20200217184613.19668-1-willy@infradead.org> <20200217184613.19668-12-willy@infradead.org> <20200218061459.GM10776@dread.disaster.area> <20200218154222.GQ7778@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218154222.GQ7778@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=JfrnYn6hAAAA:8 a=7-415B0cAAAA:8 a=vUdR-S3ouboEXt6xVngA:9 a=CjuIK1q_8ugA:10 a=1CNFftbPRP8L7MoqJWF3:22 a=biEYGPWJfzWAr4FL6Ov7:22 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Feb 18, 2020 at 07:42:22AM -0800, Matthew Wilcox wrote: > On Tue, Feb 18, 2020 at 05:14:59PM +1100, Dave Chinner wrote: > > On Mon, Feb 17, 2020 at 10:45:52AM -0800, Matthew Wilcox wrote: > > > From: "Matthew Wilcox (Oracle)" > > > > > > At allocation time, put the pages in the cache unless we're using > > > ->readpages. Add the readahead_for_each() iterator for the benefit of > > > the ->readpage fallback. This iterator supports huge pages, even though > > > none of the filesystems to be converted do yet. > > > > This could be better written - took me some time to get my head > > around it and the code. > > > > "When populating the page cache for readahead, mappings that don't > > use ->readpages need to have their pages added to the page cache > > before ->readpage is called. Do this insertion earlier so that the > > pages can be looked up immediately prior to ->readpage calls rather > > than passing them on a linked list. This early insert functionality > > is also required by the upcoming ->readahead method that will > > replace ->readpages. > > > > Optimise and simplify the readpage loop by adding a > > readahead_for_each() iterator to provide the pages we need to read. > > This iterator also supports huge pages, even though none of the > > filesystems have been converted to use them yet." > > Thanks, I'll use that. > > > > +static inline struct page *readahead_page(struct readahead_control *rac) > > > +{ > > > + struct page *page; > > > + > > > + if (!rac->_nr_pages) > > > + return NULL; > > > > Hmmmm. > > > > > + > > > + page = xa_load(&rac->mapping->i_pages, rac->_start); > > > + VM_BUG_ON_PAGE(!PageLocked(page), page); > > > + rac->_batch_count = hpage_nr_pages(page); > > > > So we could have rac->_nr_pages = 2, and then we get an order 2 > > large page returned, and so rac->_batch_count = 4. > > Well, no, we couldn't. rac->_nr_pages is incremented by 4 when we add > an order-2 page to the readahead. I don't see any code that does that. :) i.e. we aren't actually putting high order pages into the page cache here - page_alloc() allocates order-0 pages) - so there's nothing in the patch that tells me how rac->_nr_pages behaves when allocating large pages... IOWs, we have an undocumented assumption in the implementation... > I can put a > BUG_ON(rac->_batch_count > rac->_nr_pages) > in here to be sure to catch any logic error like that. Definitely necessary given that we don't insert large pages for readahead yet. A comment explaining the assumptions that the code makes for large pages is probably in order, too. > > > - page->index = offset; > > > - list_add(&page->lru, &page_pool); > > > + if (use_list) { > > > + page->index = offset; > > > + list_add(&page->lru, &page_pool); > > > + } else if (add_to_page_cache_lru(page, mapping, offset, > > > + gfp_mask) < 0) { > > > + put_page(page); > > > + goto read; > > > + } > > > > Ok, so that's why you put read code at the end of the loop. To turn > > the code into spaghetti :/ > > > > How much does this simplify down when we get rid of ->readpages and > > can restructure the loop? This really seems like you're trying to > > flatten two nested loops into one by the use of goto.... > > I see it as having two failure cases in this loop. One for "page is > already present" (which already existed) and one for "allocated a page, > but failed to add it to the page cache" (which used to be done later). > I didn't want to duplicate the "call read_pages()" code. So I reshuffled > the code rather than add a nested loop. I don't think the nested loop > is easier to read (we'll be at 5 levels of indentation for some statements). > Could do it this way ... Can we move the update of @rac inside read_pages()? The next start offset^Windex we start at is rac._start + rac._nr_pages, right? so read_pages() could do: { if (readahead_count(rac)) { /* do readahead */ } /* advance the readahead cursor */ rac->_start += rac->_nr_pages; rac._nr_pages = 0; } and then we only need to call read_pages() in these cases and so the requirement for avoiding duplicating code is avoided... Cheers, Dave. -- Dave Chinner david@fromorbit.com