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 88300C67871 for ; Mon, 16 Jan 2023 05:46:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231776AbjAPFqA (ORCPT ); Mon, 16 Jan 2023 00:46:00 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41072 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231735AbjAPFp5 (ORCPT ); Mon, 16 Jan 2023 00:45:57 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1B2D959C8; Sun, 15 Jan 2023 21:45:56 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.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=DyqbCFVVSRH9BdL+CtsL2Q0IUbjE2/UgdwZHHo8qQB0=; b=VpLfGMnSfQj8FrosrID5nRtle+ nzIT4ANfYFRckuOCdbv2oJ2HnpBcXVwqkfI/P1K9FPFM70xt8YwOt0UfDlcVci11nIxWv9Wgt1evQ rdOf6sTwJiLeX0Z6uD3pIY9oEcYwVZtaMAmBSZxLqLcKgX2/Me0xLNWLmX2P6h4dKu4t7g8bRZXqy Le5zW6+zPyY5sN2iyBiTMjLuZ3FiUerwjh4u8H1XUfZc4940SNTBY26wmHKnJtdrvVhX+BN1TNLqQ RaEf9hS4DtPfCb3mW/ffI/L1Yax3IOQY5bbo8l723yeF4pU2FduhlWFnNj1DoIm6cOglPov/O6q+0 BoQrNtNA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHIJl-008SpC-8k; Mon, 16 Jan 2023 05:46:01 +0000 Date: Mon, 16 Jan 2023 05:46:01 +0000 From: Matthew Wilcox To: "Darrick J. Wong" 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 09:06:50AM -0800, Darrick J. Wong wrote: > 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" ? Erm ... I would rather not! Part of me remembers that x86-64 has the rather nice calling convention of being able to return a struct containing two values in two registers: : Integer return values up to 64 bits in size are stored in RAX while : values up to 128 bit are stored in RAX and RDX. so maybe we can return: struct OptionFolio { int err; struct folio *folio; };