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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 BCC04C3815B for ; Mon, 20 Apr 2020 20:11:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 755F120782 for ; Mon, 20 Apr 2020 20:11:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="v80VgvMh" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 755F120782 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linuxfoundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 0FF468E0005; Mon, 20 Apr 2020 16:11:05 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0B0968E0003; Mon, 20 Apr 2020 16:11:05 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id EE0DA8E0005; Mon, 20 Apr 2020 16:11:04 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0106.hostedemail.com [216.40.44.106]) by kanga.kvack.org (Postfix) with ESMTP id D661D8E0003 for ; Mon, 20 Apr 2020 16:11:04 -0400 (EDT) Received: from smtpin27.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 9767A180ACF62 for ; Mon, 20 Apr 2020 20:11:04 +0000 (UTC) X-FDA: 76729327248.27.cows44_438b46556be35 X-HE-Tag: cows44_438b46556be35 X-Filterd-Recvd-Size: 6286 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf43.hostedemail.com (Postfix) with ESMTP for ; Mon, 20 Apr 2020 20:11:04 +0000 (UTC) Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D007820736; Mon, 20 Apr 2020 20:11:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587413463; bh=KLlWVaAqNkpR57zpuZ37ho0Y/eR5zLBukf7yTdX44Vc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=v80VgvMhfFBBd0i7hOVtPAGu8wGHazDbzENqiH4/g1SHRqDbY8fA9nxNIAzJNpZsE ouLoSvUSHSEkMTIF5OjNKZ5ofeEFVQwwVNwIzdqq7Vu8DW60oa5ttJgjGj4+RrmEKp ZvMmHcF94i5x8g4Hv6qf6bzoQvJHYNIr53k423Z4= Date: Mon, 20 Apr 2020 22:11:01 +0200 From: Greg KH To: Luis Chamberlain Cc: Bart Van Assche , axboe@kernel.dk, viro@zeniv.linux.org.uk, rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 08/10] blktrace: add checks for created debugfs files on setup Message-ID: <20200420201101.GB302402@kroah.com> References: <20200419194529.4872-1-mcgrof@kernel.org> <20200419194529.4872-9-mcgrof@kernel.org> <38240225-e48e-3035-0baa-4929948b23a3@acm.org> <20200419230537.GG11244@42.do-not-panic.com> <20200420114038.GE3906674@kroah.com> <20200420184445.GK11244@42.do-not-panic.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420184445.GK11244@42.do-not-panic.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Apr 20, 2020 at 06:44:45PM +0000, Luis Chamberlain wrote: > On Mon, Apr 20, 2020 at 01:40:38PM +0200, Greg KH wrote: > > On Sun, Apr 19, 2020 at 04:17:46PM -0700, Bart Van Assche wrote: > > > On 4/19/20 4:05 PM, Luis Chamberlain wrote: > > > > On Sun, Apr 19, 2020 at 03:57:58PM -0700, Bart Van Assche wrote: > > > > > On 4/19/20 12:45 PM, Luis Chamberlain wrote: > > > > > > Even though debugfs can be disabled, enabling BLK_DEV_IO_TRACE will > > > > > > select DEBUG_FS, and blktrace exposes an API which userspace uses > > > > > > relying on certain files created in debugfs. If files are not created > > > > > > blktrace will not work correctly, so we do want to ensure that a > > > > > > blktrace setup creates these files properly, and otherwise inform > > > > > > userspace. > > > > > > > > > > > > Signed-off-by: Luis Chamberlain > > > > > > --- > > > > > > kernel/trace/blktrace.c | 8 +++++--- > > > > > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > > > > > > > > > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c > > > > > > index 9cc0153849c3..fc32a8665ce8 100644 > > > > > > --- a/kernel/trace/blktrace.c > > > > > > +++ b/kernel/trace/blktrace.c > > > > > > @@ -552,17 +552,19 @@ static int blk_trace_create_debugfs_files(struct blk_user_trace_setup *buts, > > > > > > struct dentry *dir, > > > > > > struct blk_trace *bt) > > > > > > { > > > > > > - int ret = -EIO; > > > > > > - > > > > > > bt->dropped_file = debugfs_create_file("dropped", 0444, dir, bt, > > > > > > &blk_dropped_fops); > > > > > > + if (!bt->dropped_file) > > > > > > + return -ENOMEM; > > > > > > bt->msg_file = debugfs_create_file("msg", 0222, dir, bt, &blk_msg_fops); > > > > > > + if (!bt->msg_file) > > > > > > + return -ENOMEM; > > > > > > bt->rchan = relay_open("trace", dir, buts->buf_size, > > > > > > buts->buf_nr, &blk_relay_callbacks, bt); > > > > > > if (!bt->rchan) > > > > > > - return ret; > > > > > > + return -EIO; > > > > > > return 0; > > > > > > } > > > > > > > > > > I should have had a look at this patch before I replied to the previous > > > > > patch. > > > > > > > > > > Do you agree that the following code can be triggered by > > > > > debugfs_create_file() and also that debugfs_create_file() never returns > > > > > NULL? > > > > > > > > If debugfs is enabled, and not that we know it is in this blktrace code, > > > > as we select it, it can return ERR_PTR(-ERROR) if an error occurs. > > > > > > This is what I found in include/linux/debugfs.h in case debugfs is disabled: > > > > > > static inline struct dentry *debugfs_create_file(const char *name, > > > umode_t mode, struct dentry *parent, void *data, > > > const struct file_operations *fops) > > > { > > > return ERR_PTR(-ENODEV); > > > } > > > > > > I have not found any code path that can cause debugfs_create_file() to > > > return NULL. Did I perhaps overlook something? If not, it's not clear to me > > > why the above patch adds checks that check whether debugfs_create_file() > > > returns NULL? > > > > Short answer, yes, it can return NULL. Correct answer is, you don't > > care, don't check the value and don't do anything about it. It's > > debugging code, userspace doesn't care, so just keep moving on. > > Thing is this code *exposes* knobs to userspace for an API that *does* > exepect those files to exist. That is, blktrace *relies* on these > debugfs files to exist. So the kconfig which enables blktrace > CONFIG_BLK_DEV_IO_TRACE selects DEBUG_FS. That's nice, but again, no kernel code should do anything different depending on what debugfs happens to be doing at that point in time. > So typically we don't care if these files were created or not on regular > drivers, but in this case this code is only compiled when debugfs is > enabled and CONFIG_BLK_DEV_IO_TRACE, and the userspace interaction with > debugfs *expects* these files. > > So what do you recommend? Make sure that userspace can handle the files not being there and keep on working properly if they aren't. As you can't "recover" from debugfs failing, there's no need to check anything with it. thanks, greg k-h