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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3E31AEE14AC for ; Wed, 6 Sep 2023 16:10:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241509AbjIFQKl (ORCPT ); Wed, 6 Sep 2023 12:10:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35398 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237777AbjIFQKk (ORCPT ); Wed, 6 Sep 2023 12:10:40 -0400 Received: from out-220.mta0.migadu.com (out-220.mta0.migadu.com [91.218.175.220]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 808291BCF; Wed, 6 Sep 2023 09:10:08 -0700 (PDT) Date: Wed, 6 Sep 2023 12:10:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1694016606; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=e1EALNeItIzFfx3mxa/QV2x2Sw2QpVyNa7EcoJAOGVM=; b=KIiyRKTMZngVdZODrrqFbK5sAmW4rt0Ea5oHDtkXH0BT+d/+oRQ6eiwGRW0123e1WfWXP5 syXFLYiGnQFVdRdfZtIIlrlGsdJRxffXqD9Ft6HgTqgEmXRf4uJJa2KCY9BAIPOuFye+9D Z+Eoba8lTG2nHrvU/X2pbqt41jJUJNQ= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Kent Overstreet To: Matthew Wilcox Cc: Christoph Hellwig , torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-bcachefs@vger.kernel.org, Christian Brauner Subject: Re: [GIT PULL] bcachefs Message-ID: <20230906161002.2ztelmyzgy3pbmcd@moria.home.lan> References: <20230903032555.np6lu5mouv5tw4ff@moria.home.lan> <20230906000007.ry5rmk35vt57kppx@moria.home.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Sep 06, 2023 at 01:41:22AM +0100, Matthew Wilcox wrote: > On Tue, Sep 05, 2023 at 08:00:07PM -0400, Kent Overstreet wrote: > > On Tue, Sep 05, 2023 at 06:24:47AM -0700, Christoph Hellwig wrote: > > > Hi Kent, > > > > > > I thought you'd follow Christians proposal to actually work with > > > people to try to use common APIs (i.e. to use iomap once it's been > > > even more iter-like, for which I'm still waiting for suggestions), > > > and make your new APIs used more widely if they are a good idea > > > (which also requires explaining them better) and aim for 6.7 once > > > that is done. > > > > Christoph, I get that iomap is your pet project and you want to make it > > better and see it more widely used. > > > > But the reasons bcachefs doesn't use iomap have been discussed at > > length, and I've posted and talked about the bcachefs equivalents of > > that code. You were AWOL on those discussions; you consistently say > > "bcachefs should use iomap" and then walk away, so those discussions > > haven't moved forwards. > > > > To recap, besides being more iterator like (passing data structures > > around with iterators, vs. indirect function calls into the filesystem), > > bcachefs also hangs a bit more state off the pagecache, due to being > > multi device and also using the same data structure for tracking disk > > reservations (because why make the buffered write paths look that up > > separately?). > > I /thought/ the proposal was to use iomap for bcachefs DIO and leave > buffered writes for a different day. I agree the iomap buffered write > path is inappropriate for bcachefs today. I'd like that to change, > but there's a lot of functionality that it would need to support. No, I'm not going to convert the bcachefs DIO path to iomap; not as it exitss now. Right now I've got a clean separation between the VFS level DIO code, and the lower level bcachefs code that walks the extents btree and issues IOs. I have to consider the iomap approach where the loop-up-mappings-and-issue loop is in iomap code but calling into filesystem code pretty gross. I was talking about this /years/ ago when I did the work to make it possible to efficiently split bios - iomap could have taken the approach bcachefs did, the prereqs were in place when iomap was started, but it didn't happen - iomap ended up being a more conservative approach, a cleaned up version of buffer heads and fs/direct-IO.c. That's fine, iomap is certainly an improvement over what it was replacing, but it would not be an improvement for bcachefs. I think it might be more fruitful to look at consolidating the buffered IO code in bcachefs and iomap. The conceptual models are a bit closer, bcachefs's buffered IO code is just a bit more fully-featured in that it does the dirty block tracking in a unified way. That was a project that turned out pretty nicely.