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 D140BC77B75 for ; Wed, 10 May 2023 01:07:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229966AbjEJBHo (ORCPT ); Tue, 9 May 2023 21:07:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48102 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229561AbjEJBHn (ORCPT ); Tue, 9 May 2023 21:07:43 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8228E46B7; Tue, 9 May 2023 18:07:40 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 2D33021B82; Wed, 10 May 2023 01:07:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1683680859; h=from:from:reply-to: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=KDOIbYCc8cEVMo43jDWz1q3omhbKLsZ5bmHLW9Q3nx0=; b=KmfsshDJwbLSI9cQhJej3WeTjpKKdwn/WDh8CVfdB1s7174IS5BgrWvujONqgpG8GL4ZOC yry0mnSO/Z3LYH6RZla3Lo/Nbiw/q5jeTbO2R1U2rsUsSUAXxLZ8ulMPM0yYjOzbGHT6xG GoQlrWw52E/pYd/iAK+o6rXO21lF9Ks= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1683680859; h=from:from:reply-to: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=KDOIbYCc8cEVMo43jDWz1q3omhbKLsZ5bmHLW9Q3nx0=; b=lhGqPBcFXZod3cdJf8lThYcIRHii/80QYIW+AUgflS4q86KK+kz0naRXucwEwl0lcStpKR Td9br/xgIgxz9XCA== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id F1D0113416; Wed, 10 May 2023 01:07:38 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 66AEO1ruWmTVMwAAMHmgww (envelope-from ); Wed, 10 May 2023 01:07:38 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 490FDA0745; Wed, 10 May 2023 03:07:37 +0200 (CEST) Date: Wed, 10 May 2023 03:07:37 +0200 From: Jan Kara To: Kent Overstreet Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-bcachefs@vger.kernel.org, Kent Overstreet , Jan Kara , "Darrick J . Wong" Subject: Re: [PATCH 06/32] sched: Add task_struct->faults_disabled_mapping Message-ID: <20230510010737.heniyuxazlprrbd6@quack3> References: <20230509165657.1735798-1-kent.overstreet@linux.dev> <20230509165657.1735798-7-kent.overstreet@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230509165657.1735798-7-kent.overstreet@linux.dev> Precedence: bulk List-ID: X-Mailing-List: linux-bcachefs@vger.kernel.org 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. 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. [1] https://lore.kernel.org/all/20191218124052.GB19387@quack2.suse.cz > --------------------------------- > > The closest current VFS equivalent is mapping->invalidate_lock, which > comes from XFS. However, it's not used by direct IO. Instead, direct IO > paths shoot down the page cache twice - before starting the IO and at > the end, and they're still technically racy w.r.t. page cache coherency. > > This is a more complete approach: in the future we might consider > replacing mapping->invalidate_lock with the bcachefs code. Yes, and this is because we never provided 100% consistent buffered VS direct IO behavior on the same file exactly because we never found the complexity worth the usefulness... Honza > > Signed-off-by: Kent Overstreet > Cc: Jan Kara > Cc: Darrick J. Wong > Cc: linux-fsdevel@vger.kernel.org > --- > include/linux/sched.h | 1 + > init/init_task.c | 1 + > 2 files changed, 2 insertions(+) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 63d242164b..f2a56f64f7 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -869,6 +869,7 @@ struct task_struct { > > struct mm_struct *mm; > struct mm_struct *active_mm; > + struct address_space *faults_disabled_mapping; > > int exit_state; > int exit_code; > diff --git a/init/init_task.c b/init/init_task.c > index ff6c4b9bfe..f703116e05 100644 > --- a/init/init_task.c > +++ b/init/init_task.c > @@ -85,6 +85,7 @@ struct task_struct init_task > .nr_cpus_allowed= NR_CPUS, > .mm = NULL, > .active_mm = &init_mm, > + .faults_disabled_mapping = NULL, > .restart_block = { > .fn = do_no_restart_syscall, > }, > -- > 2.40.1 > -- Jan Kara SUSE Labs, CR