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 F2EF9C77B7D for ; Wed, 10 May 2023 06:18:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235676AbjEJGS5 (ORCPT ); Wed, 10 May 2023 02:18:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60122 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235524AbjEJGSx (ORCPT ); Wed, 10 May 2023 02:18:53 -0400 Received: from out-10.mta1.migadu.com (out-10.mta1.migadu.com [IPv6:2001:41d0:203:375::a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7AC8E2125 for ; Tue, 9 May 2023 23:18:51 -0700 (PDT) Date: Wed, 10 May 2023 02:18:45 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1683699529; 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=55Q8X6t02662mXQC8+LwTPOHKnb3j6o+rlHjx+N49Hg=; b=mLR/oq1oA26vtjEchErKnUDqmPN/zAImxyvJCM190/MgRMWxACL21IkwwDIIHANTb5MyGI eAZCVOVSeEGXkHMgEUGClnRcpt57PHHrZFosb2W1ZpoA1M3hKKmPirWTOte064phHcWjmO nmPp4HM7hyukvVDlP4nm4bsTyjg8fq4= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Kent Overstreet To: Jan Kara Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-bcachefs@vger.kernel.org, Kent Overstreet , "Darrick J . Wong" , dhowells@redhat.com Subject: Re: [PATCH 06/32] sched: Add task_struct->faults_disabled_mapping Message-ID: References: <20230509165657.1735798-1-kent.overstreet@linux.dev> <20230509165657.1735798-7-kent.overstreet@linux.dev> <20230510010737.heniyuxazlprrbd6@quack3> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230510010737.heniyuxazlprrbd6@quack3> X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, May 10, 2023 at 03:07:37AM +0200, Jan Kara wrote: > On Tue 09-05-23 12:56:31, Kent Overstreet wrote: > > From: Kent Overstreet > > > > This is used by bcachefs to fix a page cache coherency issue with > > O_DIRECT writes. > > > > Also relevant: mapping->invalidate_lock, see below. > > > > O_DIRECT writes (and other filesystem operations that modify file data > > while bypassing the page cache) need to shoot down ranges of the page > > cache - and additionally, need locking to prevent those pages from > > pulled back in. > > > > But O_DIRECT writes invoke the page fault handler (via get_user_pages), > > and the page fault handler will need to take that same lock - this is a > > classic recursive deadlock if userspace has mmaped the file they're DIO > > writing to and uses those pages for the buffer to write from, and it's a > > lock ordering deadlock in general. > > > > Thus we need a way to signal from the dio code to the page fault handler > > when we already are holding the pagecache add lock on an address space - > > this patch just adds a member to task_struct for this purpose. For now > > only bcachefs is implementing this locking, though it may be moved out > > of bcachefs and made available to other filesystems in the future. > > It would be nice to have at least a link to the code that's actually using > the field you are adding. Bit of a trick to link to a _later_ patch in the series from a commit message, but... https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/fs-io.c#n975 https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/fs-io.c#n2454 > Also I think we were already through this discussion [1] and we ended up > agreeing that your scheme actually solves only the AA deadlock but a > malicious userspace can easily create AB BA deadlock by running direct IO > to file A using mapped file B as a buffer *and* direct IO to file B using > mapped file A as a buffer. No, that's definitely handled (and you can see it in the code I linked), and I wrote a torture test for fstests as well. David Howells was also just running into a strange locking situation with iov_iters and recursive gups - I don't recall all the details, but it sounded like this might be a solution for that. David, did you have thoughts on that?