On Mon, Jan 22, 2018 at 11:34:52AM +0800, Lu Fengqi wrote: > On Sun, Jan 21, 2018 at 02:08:58PM -0500, Zygo Blaxell wrote: > >This warning appears during execution of the LOGICAL_INO ioctl and > >appears to be spurious: > > > > ------------[ cut here ]------------ > > WARNING: CPU: 3 PID: 18172 at fs/btrfs/backref.c:1391 > > find_parent_nodes+0xc41/0x14e0 > > Modules linked in: ib_iser rdma_cm iw_cm ib_cm ib_core configfs > > iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi overlay r8169 ufs qnx4 > > hfsplus hfs minix ntfs vfat msdos fat jfs xfs cpuid rpcsec_gss_krb5 nfsv4 > > nfsv3 nfs fscache algif_skcipher af_alg softdog nfsd auth_rpcgss nfs_acl > > lockd grace sunrpc bnep cpufreq_userspace cpufreq_powersave > > cpufreq_conservative nfnetlink_queue nfnetlink_log nfnetlink bluetooth > > rfkill snd_seq_dummy snd_hrtimer snd_seq_midi snd_seq_oss > > snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device binfmt_misc fuse nbd > > xt_REDIRECT nf_nat_redirect ipt_REJECT nf_reject_ipv4 xt_nat xt_conntrack > > xt_tcpudp nf_log_ipv4 nf_log_common xt_LOG ip6table_nat nf_conntrack_ipv6 > > nf_defrag_ipv6 nf_nat_ipv6 iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 > > nf_nat_ipv4 nf_nat nf_conntrack > > ip6table_mangle iptable_mangle ip6table_filter ip6_tables > > iptable_filter ip_tables x_tables tcp_cubic dummy lp dm_crypt edac_mce_amd > > edac_core snd_hda_codec_hdmi ppdev kvm_amd kvm irqbypass crct10dif_pclmul > > crc32_pclmul ghash_clmulni_intel snd_hda_codec_via pcbc amdkfd > > snd_hda_codec_generic amd_iommu_v2 aesni_intel snd_hda_intel radeon > > snd_hda_codec aes_x86_64 snd_hda_core snd_hwdep crypto_simd glue_helper sg > > snd_pcm_oss cryptd input_leds joydev pcspkr serio_raw snd_mixer_oss > > rtc_cmos snd_pcm parport_pc parport shpchp wmi acpi_cpufreq evdev snd_timer > > asus_atk0110 k10temp fam15h_power snd soundcore sp5100_tco hid_generic ipv6 > > af_packet crc_ccitt raid10 raid456 async_raid6_recov async_memcpy async_pq > > async_xor async_tx libcrc32c raid0 multipath linear dm_mod raid1 md_mod > > ohci_pci ide_pci_generic > > sr_mod cdrom pdc202xx_new ohci_hcd crc32c_intel atiixp ehci_pci > > psmouse ide_core i2c_piix4 ehci_hcd xhci_pci mii xhci_hcd [last unloaded: > > r8169] > > 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 > > RIP: 0033:0x7f727b20be07 > > RSP: 002b:00007f7279f1e018 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 > > RAX: ffffffffffffffda RBX: ffffffff9c0f4d7f RCX: 00007f727b20be07 > > RDX: 00007f7279f1e118 RSI: 00000000c0389424 RDI: 0000000000000003 > > RBP: 0000000000000035 R08: 00007f72581bf340 R09: 0000000000000000 > > R10: 0000000000000020 R11: 0000000000000246 R12: 0000000000000040 > > R13: 00007f725818d230 R14: 00007f7279f1b640 R15: 00007f7258000020 > > ? trace_hardirqs_off_caller+0x1f/0x140 > > ---[ end trace 5de243350f6762c6 ]--- > > ------------[ cut here ]------------ > > > >ref->count can be below zero under normal conditions (for delayed refs), > >so there is no need to spam dmesg when it happens. > > > > Added Edmund. > > Hi, > > I've also encountered the same problem when running the test case > xfstests/btrfs/004. However, I'm not sure whether the negative ref->count > is reasonable. > > IMO, these functions (such as add_delayed_refs, add_delayed_refs, > add_delayed_refs, add_missing_keys and resolve_indirect_refs) have been > executed at this point in time. Hence, these references not only include > these refs in the memory (delayed) but also include those refs in the disk > (inline/keyed).
I don't have the complete picture, but while looking at other code, comments,
and git log messages surrounding ref->count in btrfs, I found:
* ref->count starts off at -1 (for a delayed ref). During the process
of becoming a non-delayed ref, ref->count is incremented until it
is positive.
* refs are only deleted during a positive-to-zero transition of
ref->count, not a negative-to-zero transition.
* ref->count is sometimes incremented by more than one, specifically 2
in some cases (e.g. when the ref is attached to an inode?).
This would skip directly from ref->count == -1 to ref->count ==
1 without touching 0 in between.
So the condition ref->count < 0 seems to be both possible and (at least
in some parts of btrfs) expected. Years of observing this warning
without correlation to bad side-effects seems to suggest the condition
is also harmless (though it does not exclude the possibility that there
is something elsewhere that is unintentionally mitigating the harm).
> I would appreciate it if you could explain to me why the
> reference count can be reduced to less than zero.
I don't think this happens (the ref would be destroyed, possibly
deallocated in that case). I think ref->count starts below zero
(suggested by comments in other parts of btrfs) and is still below zero
when we hit the WARN_ON().
> >On kernel v4.14 this warning occurs 100-1000 times more frequently than
> >on kernels v4.2..v4.12. In the worst case, one test machine had 59020
> >warnings in 24 hours on v4.14.14 compared to 55 on v4.12.14.
>
> According to my bisect result, The frequency of the warning occurrence
> increased to the detectable degree after this patch
> 86d5f9944252 ("btrfs: convert prelimary reference tracking to use rbtrees")
> is committed. I understand that this does not mean that this patch caused
> the problem, but maybe Edmund can give us some help, so I added him to the
> recipient.
>
> --
> Thanks,
> Lu
>
> >
> >Signed-off-by: Zygo Blaxell <ce3g8...@umail.furryterror.org>
> >---
> > fs/btrfs/backref.c | 1 -
> > 1 file changed, 1 deletion(-)
> >
> >diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> >index 7d0dc100a09a..57e8d2562ed5 100644
> >--- a/fs/btrfs/backref.c
> >+++ b/fs/btrfs/backref.c
> >@@ -1263,7 +1263,6 @@ 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);
> > if (roots && ref->count && ref->root_id && ref->parent == 0) {
> > if (sc && sc->root_objectid &&
> > ref->root_id != sc->root_objectid) {
> >--
> >2.11.0
> >
> >--
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> >
> >
>
>
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