From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from james.kirk.hungrycats.org ([174.142.39.145]:42524 "EHLO james.kirk.hungrycats.org" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1752009AbeAXDWS (ORCPT ); Tue, 23 Jan 2018 22:22:18 -0500 Date: Tue, 23 Jan 2018 22:22:09 -0500 From: Zygo Blaxell To: linux-btrfs@vger.kernel.org Subject: [PATCH v2] btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes Message-ID: <20180124032203.3dszkkof5f36qpmf@hungrycats.org> References: <20180121190848.7wmps6q5ng6b42ki@hungrycats.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="wm32q42yjtttn4wp" In-Reply-To: <20180121190848.7wmps6q5ng6b42ki@hungrycats.org> Sender: linux-btrfs-owner@vger.kernel.org List-ID: --wm32q42yjtttn4wp Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Until v4.14, this warning was very infrequent: WARNING: CPU: 3 PID: 18172 at fs/btrfs/backref.c:1391 find_parent_nodes+0x= c41/0x14e0 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 --- v2:=20 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_hand= le *trans, while (node) { ref =3D rb_entry(node, struct prelim_ref, rbnode); node =3D 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 =3D=3D 0) { if (sc && sc->root_objectid && ref->root_id !=3D sc->root_objectid) { --=20 2.11.0 --wm32q42yjtttn4wp Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iF0EABECAB0WIQSnOVjcfGcC/+em7H2B+YsaVrMbnAUCWmf71gAKCRCB+YsaVrMb nLlRAKCoFVpNFH9ET1LiGGmjf35h+173KwCg3EbKA6kLB6sLqvsSfffmbsCaBHw= =4Qng -----END PGP SIGNATURE----- --wm32q42yjtttn4wp--