From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from esa1.hgst.iphmx.com ([68.232.141.245]:14197 "EHLO esa1.hgst.iphmx.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750720AbcIEEcj (ORCPT ); Mon, 5 Sep 2016 00:32:39 -0400 From: Naohiro Aota To: "jbacik@fb.com" , "linux-btrfs@vger.kernel.org" CC: "linux-kernel@vger.kernel.org" , "dsterba@suse.com" , "clm@fb.com" Subject: Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs Date: Mon, 5 Sep 2016 04:32:37 +0000 Message-ID: <1473049955.14093.37.camel@hgst.com> References: <1472802392-10851-1-git-send-email-naohiro.aota@hgst.com> In-Reply-To: Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Sender: linux-btrfs-owner@vger.kernel.org List-ID: 2016-09-02 (金) の 09:35 -0400 に Josef Bacik さんは書きました: > On 09/02/2016 03:46 AM, Naohiro Aota wrote: > > > > Currently, btrfs_relocate_chunk() is removing relocated BG by > > itself. But > > the work can be done by btrfs_delete_unused_bgs() (and it's better > > since it > > trim the BG). Let's dedupe the code. > > > > While btrfs_delete_unused_bgs() is already hitting the relocated > > BG, it > > skip the BG since the BG has "ro" flag set (to keep balancing BG > > intact). > > On the other hand, btrfs cannot drop "ro" flag here to prevent > > additional > > writes. So this patch make use of "removed" flag. > > btrfs_delete_unused_bgs() now detect the flag to distinguish > > whether a > > read-only BG is relocating or not. > > > > This seems racey to me.  We remove the last part of the block group, > it ends up  > on the unused_bgs_list, we process this list, see that removed isn't > set and we  > skip it, then later we set removed, but it's too late.  I think the > right way is  > to actually do a transaction, set ->removed, manually add it to the  > unused_bgs_list if it's not already, then end the transaction.  This > way we are  > guaranteed to have the bg on the list when it is ready to be > removed.  This is  > my analysis after looking at it for 10 seconds after being awake for > like 30  > minutes so if I'm missing something let me know.  Thanks, I don't think a race will happen. Since we are holding delete_unused_bgs_mutex here, btrfs_delte_unused_bgs() checks ->removed flag after we unlock the mutex i.e. we setup the flag properly. For a case btrfs_delete_usused_bgs() checks the BG before we hold delte_unused_bgs_mutex, then that BG is removed by it (if it's empty) and btrfs_relocate_chunk() should never see it. Regards, Naohiro {.n++%ݶw{.n+{k~^nrzh&zzޗ++zfh~iz_j:+v)ߣm