From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1423923AbcBQVIV (ORCPT ); Wed, 17 Feb 2016 16:08:21 -0500 Received: from mail-qg0-f43.google.com ([209.85.192.43]:36273 "EHLO mail-qg0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1423602AbcBQVHr (ORCPT ); Wed, 17 Feb 2016 16:07:47 -0500 Date: Wed, 17 Feb 2016 16:07:44 -0500 From: Tejun Heo To: Jan Kara Cc: Jens Axboe , Tahsin Erdogan , cgroups@vger.kernel.org, "Theodore Ts'o" , Nauman Rafique , linux-kernel@vger.kernel.org, Jan Kara Subject: Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned during cgroup writeback association switches Message-ID: <20160217210744.GA6479@mtj.duckdns.org> References: <20160215210047.GN3965@htj.duckdns.org> <20160216182457.GO3741@mtj.duckdns.org> <20160217205721.GE14140@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160217205721.GE14140@quack.suse.cz> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Jan. On Wed, Feb 17, 2016 at 09:57:21PM +0100, Jan Kara wrote: > Well, but this has the side-effect that trying to umount a filesystem while > migrations are happening will result in EBUSY error. Without obvious reason > why that happens. As an admin I would be rather upset when umount sometimes > returns EBUSY without apparent reason and you have to basically implement a > loop around umount to make it reliable. So a nack from me for this patch. I see. Can you please point me to the s_active check during umount? I first tried s_umount but couldn't transfer its ownership to the worker so ended up doing s_active. I looked at how s_active is used and couldn't find where it'd block umount. may_umount() checks mnt_count, not s_active, so it looked like holding s_active may delay destruction of the superblock but not prevent umount. > Traditionally, we have used sb->s_count and sb->s_umount semaphore to pin > superblock while writeback code was working on it. That makes umount block > until we can safely unmount the filesystem and thus doesn't result in these > spurious EBUSY errors. But from a quick look this can be problematic for the > cgroup setting. > > Alternatively, you could either cancel all the switching work when > unmounting filesystem or maybe just handle I_WB_SWITCH similarly to I_SYNC > - don't grab inode reference when switching is going on, just make > I_WB_SWITCH pin the inode and wait in evict() for it to be clear (similarly > as we call inode_wait_for_writeback() there). Yeah, this is an alternative but likely more involved. Thanks. -- tejun