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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_PASS,T_DKIMWL_WL_HIGH 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 89982C5CFC1 for ; Tue, 19 Jun 2018 10:40:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3C38420874 for ; Tue, 19 Jun 2018 10:40:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="FQ11rQRE" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3C38420874 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937586AbeFSKk2 (ORCPT ); Tue, 19 Jun 2018 06:40:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:49062 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756608AbeFSKk0 (ORCPT ); Tue, 19 Jun 2018 06:40:26 -0400 Received: from tleilax.poochiereds.net (cpe-71-70-156-158.nc.res.rr.com [71.70.156.158]) (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 D68142075E; Tue, 19 Jun 2018 10:40:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1529404825; bh=fyJeNmOKrZI9cShy89XOz/7PAwt5F5C5rUsrocv+I+8=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=FQ11rQRE+JbiX82L5jWx5f1REREB+d5Xs9/fRJli/au/QwSsigYWuKBiSzOPcNGo8 Xs/7kQYMuqFUR1Ble2IGwkvq6M+V8H3y5t2MagXKFTLPkJ/R7I3atvNak1QYptmO3j xRzpgNQ4WoL9Gndx2Ze1f3LwQ12rBIVNe15/dq5s= Message-ID: <0ddda59286e0be135cf133dc653da54f66c264a7.camel@kernel.org> Subject: Re: [PATCH 2/5] buffer: record blockdev write errors in super_block that backs them From: Jeff Layton To: viro@ZenIV.linux.org.uk, dhowells@redhat.com, Jens Axboe , David Howells Cc: willy@infradead.org, andres@anarazel.de, cmaiolino@redhat.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, "linux-block@vger.kernel.org" Date: Tue, 19 Jun 2018 06:40:23 -0400 In-Reply-To: <81a365a631279f8b0ad0ed71b222c19817045704.camel@kernel.org> References: <20180604180304.9662-1-jlayton@kernel.org> <20180604180304.9662-3-jlayton@kernel.org> <81a365a631279f8b0ad0ed71b222c19817045704.camel@kernel.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.2 (3.28.2-1.fc28) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2018-06-06 at 11:56 -0400, Jeff Layton wrote: > On Mon, 2018-06-04 at 14:03 -0400, Jeff Layton wrote: > > From: Jeff Layton > > > > When syncing out a block device (a'la __sync_blockdev), any error > > encountered will only be recorded in the bd_inode's mapping. When the > > blockdev contains a filesystem however, we'd like to also record the > > error in the super_block that's stored there. > > > > Make mark_buffer_write_io_error also record the error in the > > corresponding super_block when a writeback error occurs and the block > > device contains a mounted superblock. > > > > Signed-off-by: Jeff Layton > > --- > > fs/buffer.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/fs/buffer.c b/fs/buffer.c > > index 249b83fafe48..dae2a857d5bc 100644 > > --- a/fs/buffer.c > > +++ b/fs/buffer.c > > @@ -1117,6 +1117,8 @@ void mark_buffer_write_io_error(struct buffer_head *bh) > > mapping_set_error(bh->b_page->mapping, -EIO); > > if (bh->b_assoc_map) > > mapping_set_error(bh->b_assoc_map, -EIO); > > + if (bh->b_bdev->bd_super) > > + errseq_set(&bh->b_bdev->bd_super->s_wb_err, -EIO); > > } > > EXPORT_SYMBOL(mark_buffer_write_io_error); > > > > (cc'ing linux-block and Jens) > > I'm wondering whether this patch might turn out to be racy. For > instance, could a call to __sync_blockdev race with an unmount in such > a way that bd_super goes NULL after we check it but before errseq_set > is called? > > If so, what can we do to ensure that that doesn't happen? Any insight > here would be appreciated. > > Thanks, Jens, ping? I never got a response on the above. After looking over it some more, I suspect that this may be racy with some filesystems. Some of them seem to just flush out data to the bd_inode on unmount, and trust the system to take care of the rest. One possible fix there might be to turn bd_super into an RCU managed pointer. We already free super_blocks under RCU, so we could do something there like: rcu_read_lock(); sb = rcu_dereference(bh->b_bdev->bd_super); if (sb) errseq_set(&sb->s_wb_err, -EIO); rcu_read_unlock(); There aren't that many accessors of bd_super, so that seems like it'd be fairly simple to do. Still, I'd like someone to sanity check me here. Is there something that would prevent the above race that I'm not seeing? Thanks, -- Jeff Layton