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 E9E6FC33CA9 for ; Mon, 13 Jan 2020 22:27:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BC39D2084D for ; Mon, 13 Jan 2020 22:27:06 +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="fX/j+1Ow" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726536AbgAMW1G (ORCPT ); Mon, 13 Jan 2020 17:27:06 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:47614 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726530AbgAMW1G (ORCPT ); Mon, 13 Jan 2020 17:27:06 -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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=SxeMrcw8VfwoTo9Q+GzAW5ii85Q073aK0lgVbFxIoE4=; b=fX/j+1OwGRaQw1c6OulAfDSmc vF070DKVrwwzQje2r/Ium//k5I3u9hwozV+Xoa4216mfL+YrUwXzxsJrQMDOIkCkCRw5Ct9LS8p9j AkAjnyuhUi4vpw7Nm8PKBgtAmM3TlMP2BA/2l7bdHal6njOrB193U4SxKHpSIDARKyrB+ONzZiDnw UKf2El+WNSNNn16tg9v+RbOknmcaG4kM7NeLckDlBDewswSWtw1XrqnB81tsyErYzirIttekNgVrP 70YlWvPwuqWdZ0zE8jKHMy+LTiuwvl9eWBFZZmsN2It2mJmUiFmrAsaTbjPEeLnvksPaRvmoyTS77 HMOmJcTMg==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1ir8Aq-0006Ri-Nz; Mon, 13 Jan 2020 22:27:04 +0000 Date: Mon, 13 Jan 2020 14:27:04 -0800 From: Matthew Wilcox To: Jens Axboe Cc: Chris Mason , "linux-xfs@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "jlayton@kernel.org" , "hch@infradead.org" Subject: Re: [RFC 0/8] Replacing the readpages a_op Message-ID: <20200113222704.GC18216@bombadil.infradead.org> References: <20200113153746.26654-1-willy@infradead.org> <6CA4CD96-0812-4261-8FF9-CD28AA2EC38A@fb.com> <20200113174008.GB332@bombadil.infradead.org> <15C84CC9-3196-441D-94DE-F3FD7AC364F0@fb.com> <20200113215811.GA18216@bombadil.infradead.org> <910af281-4e2b-3e5d-5533-b5ceafd59665@kernel.dk> <20200113221047.GB18216@bombadil.infradead.org> <1b94e6b6-29dc-2e90-d1ca-982accd3758c@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1b94e6b6-29dc-2e90-d1ca-982accd3758c@kernel.dk> Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Mon, Jan 13, 2020 at 03:14:26PM -0700, Jens Axboe wrote: > On 1/13/20 3:10 PM, Matthew Wilcox wrote: > > On Mon, Jan 13, 2020 at 03:00:40PM -0700, Jens Axboe wrote: > >> On 1/13/20 2:58 PM, Matthew Wilcox wrote: > >>> On Mon, Jan 13, 2020 at 06:00:52PM +0000, Chris Mason wrote: > >>>> This is true, I didn't explain that part well ;) Depending on > >>>> compression etc we might end up poking the xarray inside the actual IO > >>>> functions, but the main difference is that btrfs is building a single > >>>> bio. You're moving the plug so you'll merge into single bio, but I'd > >>>> rather build 2MB bios than merge them. > >>> > >>> Why don't we store a bio pointer inside the plug? You're opencoding that, > >>> iomap is opencoding that, and I bet there's a dozen other places where > >>> we pass a bio around. Then blk_finish_plug can submit the bio. > >> > >> Plugs aren't necessarily a bio, they can be callbacks too. > > > > I'm thinking something as simple as this: > > It's a little odd imho, the plugging generally collect requests. Sounds > what you're looking for is some plug owner private data, which just > happens to be a bio in this case? > > Is this over repeated calls to some IO generating helper? Would it be > more efficient if that helper could generate the full bio in one go, > instead of piecemeal? The issue is around ->readpages. Take a look at how iomap_readpages works, for example. We're under a plug (taken in mm/readahead.c), but we still go through the rigamarole of keeping a pointer to the bio in ctx.bio and passing ctx around so that we don't end up with many fragments which have to be recombined into a single bio at the end. I think what I want is a bio I can reach from current, somehow. And the plug feels like a natural place to keep it because it's basically saying "I want to do lots of little IOs and have them combined". The fact that the iomap code has a bio that it precombines fragments into suggests to me that the existing antifragmentation code in the plugging mechanism isn't good enough. So let's make it better by storing a bio in the plug and then we can get rid of the bio in the iomap code.