From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.cn.fujitsu.com ([183.91.158.132]:12723 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752223AbeAXDdO (ORCPT ); Tue, 23 Jan 2018 22:33:14 -0500 Date: Wed, 24 Jan 2018 11:33:08 +0800 From: Lu Fengqi To: Zygo Blaxell CC: Subject: Re: [PATCH v2] btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes Message-ID: <20180124033308.GF532@fnst.localdomain> References: <20180121190848.7wmps6q5ng6b42ki@hungrycats.org> <20180124032203.3dszkkof5f36qpmf@hungrycats.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" In-Reply-To: <20180124032203.3dszkkof5f36qpmf@hungrycats.org> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Tue, Jan 23, 2018 at 10:22:09PM -0500, Zygo Blaxell wrote: >Until v4.14, this warning was very infrequent: > > WARNING: CPU: 3 PID: 18172 at fs/btrfs/backref.c:1391 find_parent_nodes+0xc41/0x14e0 > Modules linked in: [...] > CPU: 3 PID: 18172 Comm: bees Tainted: G D W L 4.11.9-zb64+ #1 > Hardware name: System manufacturer System Product Name/M5A78L-M/USB3, BIOS 2101 12/02/2014 > Call Trace: > dump_stack+0x85/0xc2 > __warn+0xd1/0xf0 > warn_slowpath_null+0x1d/0x20 > find_parent_nodes+0xc41/0x14e0 > __btrfs_find_all_roots+0xad/0x120 > ? extent_same_check_offsets+0x70/0x70 > iterate_extent_inodes+0x168/0x300 > iterate_inodes_from_logical+0x87/0xb0 > ? iterate_inodes_from_logical+0x87/0xb0 > ? extent_same_check_offsets+0x70/0x70 > btrfs_ioctl+0x8ac/0x2820 > ? lock_acquire+0xc2/0x200 > do_vfs_ioctl+0x91/0x700 > ? __fget+0x112/0x200 > SyS_ioctl+0x79/0x90 > entry_SYSCALL_64_fastpath+0x23/0xc6 > ? trace_hardirqs_off_caller+0x1f/0x140 > >Starting with v4.14 (specifically 86d5f9944252 ("btrfs: convert prelimary >reference tracking to use rbtrees")) the WARN_ON occurs three orders of >magnitude more frequently--almost once per second while running workloads >like bees. > >Replace the WARN_ON() with a comment rationale for its removal. >The rationale is paraphrased from an explanation by Edmund Nadolski > on the linux-btrfs mailing list. > >Fixes: 8da6d5815c59 ("Btrfs: added btrfs_find_all_roots()") >Signed-off-by: Zygo Blaxell Reviewed-by: Lu Fengqi -- Thanks, Lu >--- >v2: > Replace WARN_ON with rationale instead of merely deleting it. > Trim irrelevant detail from the backtrace. Add Fixes reference. > Fix subject line (missing "< 0"). > > fs/btrfs/backref.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > >diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c >index 7d0dc100a09a..06597c5f9f4b 100644 >--- a/fs/btrfs/backref.c >+++ b/fs/btrfs/backref.c >@@ -1263,7 +1263,16 @@ static int find_parent_nodes(struct btrfs_trans_handle *trans, > while (node) { > ref = rb_entry(node, struct prelim_ref, rbnode); > node = rb_next(&ref->rbnode); >- WARN_ON(ref->count < 0); >+ /* >+ * ref->count < 0 can happen here if there are delayed >+ * refs with a node->action of BTRFS_DROP_DELAYED_REF. >+ * prelim_ref_insert() relies on this when merging >+ * identical refs to keep the overall count correct. >+ * prelim_ref_insert() will merge only those refs >+ * which compare identically. Any refs having >+ * e.g. different offsets would not be merged, >+ * and would retain their original ref->count < 0. >+ */ > if (roots && ref->count && ref->root_id && ref->parent == 0) { > if (sc && sc->root_objectid && > ref->root_id != sc->root_objectid) { >-- >2.11.0 >