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 259CEC34044 for ; Tue, 18 Feb 2020 16:10:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EF81D21D56 for ; Tue, 18 Feb 2020 16:10:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="ipLlBOxg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726594AbgBRQKG (ORCPT ); Tue, 18 Feb 2020 11:10:06 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:44246 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726360AbgBRQKF (ORCPT ); Tue, 18 Feb 2020 11:10:05 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=5KDaFTW+pz8VfJMmd/150jcSlwmVWcMm/3HMnI/xfvg=; b=ipLlBOxgYKVfRZo7Sgo48aXcMn g+YvOK1M9fWayNYCWbwGsZLPH92YapUqcU3V5/r6tbkb9ft/cqAo3wSGowvvZcx4rWcc7/FZqrjgU C3Fut73k2NJ3qoka036sCPRkrwE5zKTESvinIrwcv+q0cgEnEAW9BFRIcBy7P/qW64o6xFymjBnd6 8c5ms/UIfeyFwZwXVtAUiNakXaQvESAYSksXwpI8XKuUP0K9NptGJs83qh5VWlLhzAz7zqZR5fA4t Wop5PUNx/CWSP8yDQssgxeF8hMUqj5MF3HuUkauFwvDvTC8SaXnbvBgLZnpTizJzcd+E709oxa5QK FdKMzcpg==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1j45Rk-0001Tm-Vv; Tue, 18 Feb 2020 16:10:04 +0000 Date: Tue, 18 Feb 2020 08:10:04 -0800 From: Matthew Wilcox To: Dave Chinner 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 08/19] mm: Add readahead address space operation Message-ID: <20200218161004.GR7778@bombadil.infradead.org> References: <20200217184613.19668-1-willy@infradead.org> <20200217184613.19668-14-willy@infradead.org> <20200218062147.GN10776@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218062147.GN10776@dread.disaster.area> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Feb 18, 2020 at 05:21:47PM +1100, Dave Chinner wrote: > On Mon, Feb 17, 2020 at 10:45:54AM -0800, Matthew Wilcox wrote: > > From: "Matthew Wilcox (Oracle)" > > > > This replaces ->readpages with a saner interface: > > - Return void instead of an ignored error code. > > - Pages are already in the page cache when ->readahead is called. > > Might read better as: > > - Page cache is already populates with locked pages when > ->readahead is called. Will do. > > - Implementation looks up the pages in the page cache instead of > > having them passed in a linked list. > > Add: > > - cleanup of unused readahead handled by ->readahead caller, not > the method implementation. The readpages caller does that cleanup too, so it's not an advantage to the readahead interface. if (mapping->a_ops->readpages) { ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages); /* Clean up the remaining pages */ put_pages_list(pages); goto out; } > > ``readpages`` > > called by the VM to read pages associated with the address_space > > object. This is essentially just a vector version of readpage. > > Instead of just one page, several pages are requested. > > readpages is only used for read-ahead, so read errors are > > ignored. If anything goes wrong, feel free to give up. > > + This interface is deprecated; implement readahead instead. > > What is the removal schedule for the deprecated interface? I mentioned that in the cover letter; once Dave Howells has the fscache branch merged, I'll do the remaining filesystems. Should be within the next couple of merge windows. > > +/* The byte offset into the file of this readahead block */ > > +static inline loff_t readahead_offset(struct readahead_control *rac) > > +{ > > + return (loff_t)rac->_start * PAGE_SIZE; > > +} > > Urk. Didn't an early page use "offset" for the page index? That > was was "mm: Remove 'page_offset' from readahead loop" did, right? > > That's just going to cause confusion to have different units for > readahead "offsets".... We are ... not consistent anywhere in the VM/VFS with our naming. Unfortunately. $ grep -n offset mm/filemap.c 391: * @start: offset in bytes where the range starts ... 815: pgoff_t offset = old->index; ... 2020: unsigned long offset; /* offset into pagecache page */ ... 2257: *ppos = ((loff_t)index << PAGE_SHIFT) + offset; That last one's my favourite. Not to mention the fine distinction you and I discussed recently between offset_in_page() and page_offset(). Best of all, even our types encode the ambiguity of an 'offset'. We have pgoff_t and loff_t (replacing the earlier off_t). So, new rule. 'pos' is the number of bytes into a file. 'index' is the number of PAGE_SIZE pages into a file. We don't use the word 'offset' at all. 'length' as a byte count and 'count' as a page count seem like fine names to me. > > - if (aops->readpages) { > > + if (aops->readahead) { > > + aops->readahead(rac); > > + readahead_for_each(rac, page) { > > + unlock_page(page); > > + put_page(page); > > + } > > This needs a comment to explain the unwinding that needs to be done > here. I'm not going to remember in a year's time that this is just > for the pages that weren't submitted by ->readahead.... ACK.