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=-3.8 required=3.0 tests=BAYES_00, 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 3CD87C43214 for ; Sun, 29 Aug 2021 01:24:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 15D9C604AC for ; Sun, 29 Aug 2021 01:24:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233244AbhH2BBa (ORCPT ); Sat, 28 Aug 2021 21:01:30 -0400 Received: from zeniv-ca.linux.org.uk ([142.44.231.140]:38374 "EHLO zeniv-ca.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229725AbhH2BBa (ORCPT ); Sat, 28 Aug 2021 21:01:30 -0400 Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mK99Q-00Gvaz-Dk; Sun, 29 Aug 2021 00:58:20 +0000 Date: Sun, 29 Aug 2021 00:58:20 +0000 From: Al Viro To: Linus Torvalds Cc: Andreas Gruenbacher , Christoph Hellwig , "Darrick J. Wong" , Jan Kara , Matthew Wilcox , cluster-devel , linux-fsdevel , Linux Kernel Mailing List , ocfs2-devel@oss.oracle.com Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable Message-ID: References: <20210827164926.1726765-1-agruenba@redhat.com> <20210827164926.1726765-6-agruenba@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Aug 27, 2021 at 09:48:55PM +0000, Al Viro wrote: > So we have 3 callers where we want all-or-nothing semantics - two in > arch/x86/kernel/fpu/signal.c and one in btrfs. HWPOISON will be a problem > for all 3, AFAICS... > > IOW, it looks like we have two different things mixed here - one that wants > to try and fault stuff in, with callers caring only about having _something_ > faulted in (most of the users) and one that wants to make sure we *can* do > stores or loads on each byte in the affected area. > > Just accessing a byte in each page really won't suffice for the second kind. > Neither will g-u-p use, unless we teach it about HWPOISON and other fun > beasts... Looks like we want that thing to be a separate primitive; for > btrfs I'd probably replace fault_in_pages_writeable() with clear_user() > as a quick fix for now... Looks like out of these 3 we have * x86 restoring FPU state on sigreturn: correct, if somewhat obfuscated; HWPOISON is not an issue. We want full fault-in there (1 or 2 pages) * x86 saving FPU state into sigframe: not really needed; we do __clear_user() on any error anyway, and taking it into the caller past the pagefault_enable() will serve just fine instead of fault-in of the same for write. * btrfs search_ioctl(): HWPOISON is not an issue (no #MC on stores), but arm64 side of the things very likely is a problem with MTE; there we can have successful store in some bytes in a page with faults on stores elsewhere in it. With such setups that thing will loop indefinitely. And unlike x86 FPU handling, btrfs is arch-independent. IOW, unless I'm misreading the situation, we have one caller where "all or nothing" semantics is correct and needed, several where fault-in is pointless, one where the current use of fault-in is actively wrong (ppc kvm, patch from ppc folks exists), another place where neither semantics is right (btrfs on arm64) and a bunch where "can we access at least the first byte?" should be fine...