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 E1C9DC67871 for ; Sun, 15 Jan 2023 17:07:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231576AbjAORHR (ORCPT ); Sun, 15 Jan 2023 12:07:17 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51418 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231569AbjAORHA (ORCPT ); Sun, 15 Jan 2023 12:07:00 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7F9D010A90; Sun, 15 Jan 2023 09:06:52 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id E325360D58; Sun, 15 Jan 2023 17:06:51 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 41C40C433EF; Sun, 15 Jan 2023 17:06:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1673802411; bh=+crzPVi66jqHfKaNucFDjZ0PcQ/R8J1h/0R/3vs99TM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ER1gN5dVvBAlxtXU9pxQVLrJL0qLrAgzeM5h5kXJzIpLFiN57alMMvvYYv4bCzSKV /7N9J8NIKpYTa2GqaA37bXOvQgXrE/H30BnN/B7pwj5jbFSC+ahJeyx7H7Ve6SYczV /WvNzB6ElrG5WeKbgEhVzzI2GbaHp96DoXC/9AgTaBPjn4DneqRirl9MMgW9FpBM4/ qshX5Vex9vuAGoTGNvJQLCyQv/RWyWP8PsnJ3AIQwVDGHfBdhft60swAUN04EOXNpU 8ts+MOCUPLGFbnreCyhTAa7ZdRm9RJVd/rac1dWmjnaxp44gj/XH9/AHxb+qL1+2+E oxGGAlCSrn8qg== Date: Sun, 15 Jan 2023 09:06:50 -0800 From: "Darrick J. Wong" To: Matthew Wilcox Cc: Christoph Hellwig , Andreas Gruenbacher , Dave Chinner , Alexander Viro , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, cluster-devel@redhat.com, Christoph Hellwig Subject: Re: [RFC v6 04/10] iomap: Add iomap_get_folio helper Message-ID: References: <20230108213305.GO1971568@dread.disaster.area> <20230108194034.1444764-1-agruenba@redhat.com> <20230108194034.1444764-5-agruenba@redhat.com> <20230109124642.1663842-1-agruenba@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Sun, Jan 15, 2023 at 09:01:22AM -0800, Darrick J. Wong wrote: > On Tue, Jan 10, 2023 at 01:34:16PM +0000, Matthew Wilcox wrote: > > On Tue, Jan 10, 2023 at 12:46:45AM -0800, Christoph Hellwig wrote: > > > On Mon, Jan 09, 2023 at 01:46:42PM +0100, Andreas Gruenbacher wrote: > > > > We can handle that by adding a new IOMAP_NOCREATE iterator flag and > > > > checking for that in iomap_get_folio(). Your patch then turns into > > > > the below. > > > > > > Exactly. And as I already pointed out in reply to Dave's original > > > patch what we really should be doing is returning an ERR_PTR from > > > __filemap_get_folio instead of reverse-engineering the expected > > > error code. > > > > Ouch, we have a nasty problem. > > > > If somebody passes FGP_ENTRY, we can return a shadow entry. And the > > encodings for shadow entries overlap with the encodings for ERR_PTR, > > meaning that some shadow entries will look like errors. The way I > > solved this in the XArray code is by shifting the error values by > > two bits and encoding errors as XA_ERROR(-ENOMEM) (for example). > > > > I don't _object_ to introducing XA_ERROR() / xa_err() into the VFS, > > but so far we haven't, and I'd like to make that decision intentionally. > > Sorry, I'm not following this at all -- where in buffered-io.c does > anyone pass FGP_ENTRY? Andreas' code doesn't seem to introduce it > either...? Oh, never mind, I worked out that the conflict is between iomap not passing FGP_ENTRY and wanting a pointer or a negative errno; and someone who does FGP_ENTRY, in which case the xarray value can be confused for a negative errno. OFC now I wonder, can we simply say that the return value is "The found folio or NULL if you set FGP_ENTRY; or the found folio or a negative errno if you don't" ? --D > --D