Re: [PATCH] btrfs: remove spurious WARN_ON(ref->count) in find_parent_nodes

[Edmund Nadolski <enadolski@suse.de>]

On 01/21/2018 08:34 PM, 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 would appreciate it if you could explain to me why the
> reference count can be reduced to less than zero.

It’s not clear to me that a negative count in this case is expected. For
direct refs the code gets the count from the btrfs_shared_data_ref (in
the item) or the  ref_mod (in the delayed ref). For the latter it
_should_ convert a negative count to positive when using as a
prelim_ref.  An indirect prelim_ref can also be converted from a direct
in which case it uses the count from the indirect.

I haven't seen this previously but will continue to test.  Please let me
know of any additional ways to trigger this.

Thanks,
Ed

> 
>> 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.
>