Comment 0 for bug 1505948

Hello everybody,

Linux 4.1, 4.2 or 4.3-rc leads to an immediate kernel panic in our setup when trying to start a Qemu process on top of a fuse-based mount. Here is an example stacktrace:

[ 739.807817] BUG: unable to handle kernel paging request at ffff8800a4104ea0
[ 739.840201] IP: [<ffffffff811cc95a>] kmem_cache_alloc_trace+0x7a/0x1f0
[ 739.870309] PGD 2fee067 PUD 2fbf4dd063 PMD 0
[ 739.890418] Oops: 0000 [#1] SMP
[ 739.905265] Modules linked in: nbd vport_vxlan vport_gre gre ebtable_filter ebtables openvswitch ib_iser rdma_cm iw_cm ib_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ipt_REJECT nf_reject_ipv4 nf_conntrack_ipv4 nf_defrag_ipv4 iptable_filter xt_CT iptable_raw ip_tables xt_tcpudp ip6t_REJECT nf_reject_ipv6 xt_limit nf_conntrack_ipv6 nf_defrag_ipv6 xt_multiport xt_conntrack nf_conntrack ip6table_filter ip6_tables x_tables dm_crypt ipmi_ssif intel_rapl iosf_mbi x86_pkg_temp_thermal intel_powerclamp coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd kvm_intel kvm ipmi_devintf vhost_net vhost macvtap macvlan joydev input_leds dm_multipath scsi_dh bonding sb_edac 8021q garp hpilo mrp stp ipmi_si llc edac_core lpc_ich ioatdma 8250_fintek ipmi_msghandler lp shpchp acpi_power_meter mac_hid parport nls_iso8859_1 sch_fq_codel xfs libcrc32c btrfs xor raid6_pq ixgbe ses enclosure hid_generic dca vxlan usbhid ip6_udp_tunnel tg3 udp_tunnel ptp hid pps_core hpsa mdio wmi
[ 740.345300] CPU: 8 PID: 10550 Comm: qemu-system-x86 Not tainted 4.2.0-040200-generic #201508301530
[ 740.386879] Hardware name: HP ProLiant DL380 Gen9, BIOS P89 05/06/2015
[ 740.416827] task: ffff882f8e958dc0 ti: ffff882f28c20000 task.ti: ffff882f28c20000
[ 740.451672] RIP: 0010:[<ffffffff811cc95a>] [<ffffffff811cc95a>] kmem_cache_alloc_trace+0x7a/0x1f0
[ 740.494047] RSP: 0018:ffff882f28c23c68 EFLAGS: 00010286
[ 740.518425] RAX: 0000000000000000 RBX: 00000000000000d0 RCX: 00000000000026b3
[ 740.551611] RDX: 00000000000026b2 RSI: 00000000000000d0 RDI: ffff882fbf407840
[ 740.584846] RBP: ffff882f28c23ca8 R08: 0000000000019920 R09: ffffe8d000200ab0
[ 740.618287] R10: ffffffff812e8dcd R11: ffffea00bca0ac00 R12: 00000000000000d0
[ 740.651320] R13: ffff882fbf407840 R14: ffff8800a4104ea0 R15: ffff882fbf407840
[ 740.684195] FS: 00007f2642ffd700(0000) GS:ffff882fbfa00000(0000) knlGS:0000000000000000
[ 740.722030] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 740.749469] CR2: ffff8800a4104ea0 CR3: 0000002f26f83000 CR4: 00000000001426e0
[ 740.783390] Stack:
[ 740.792577] ffffffff812e8dcd 0000000000000048 0000000000000002 ffff882f908c8468
[ 740.827003] 0000000001bef000 ffff882f928e4600 ffff882f28c23e48 ffff882f28c23d70
[ 740.860971] ffff882f28c23d38 ffffffff812e8dcd 0000000000000001 ffff882f908c8300
[ 740.894994] Call Trace:
[ 740.906211] [<ffffffff812e8dcd>] ? fuse_direct_IO+0xdd/0x280
[ 740.932940] [<ffffffff812e8dcd>] fuse_direct_IO+0xdd/0x280
[ 740.958866] [<ffffffff8117750e>] generic_file_direct_write+0x9e/0x150
[ 740.989318] [<ffffffff812e96bc>] fuse_file_write_iter+0x15c/0x2e0
[ 741.017725] [<ffffffff811e94a7>] __vfs_write+0xa7/0xf0
[ 741.041787] [<ffffffff811e9b09>] vfs_write+0xa9/0x190
[ 741.065307] [<ffffffff811ea9d9>] SyS_pwrite64+0x69/0xa0
[ 741.090141] [<ffffffff81085b57>] ? SyS_rt_sigprocmask+0x67/0xb0
[ 741.135924] [<ffffffff817a8e32>] entry_SYSCALL_64_fastpath+0x16/0x75
[ 741.183478] Code: 4c 03 05 32 d8 e3 7e 4d 8b 30 49 8b 40 10 4d 85 f6 0f 84 22 01 00 00 48 85 c0 0f 84 19 01 00 00 49 63 47 20 48 8d 4a 01 4d 8b 07 <49> 8b 1c 06 4c 89 f0 65 49 0f c7 08 0f 94 c0 84 c0 74 b9 49 63
[ 741.306817] RIP [<ffffffff811cc95a>] kmem_cache_alloc_trace+0x7a/0x1f0

The problem has also been documented by somebody else in the Fedora bug tracker at
https://bugzilla.redhat.com/show_bug.cgi?id=1254310

This behaviour is 100% reproducible. I have asked the fuse-devel mailinglist for advice, but up to this point with no success:

http://sourceforge.net/p/fuse/mailman/message/34537139/

We are still investigating if this issue is also happening with 4.0 and will add the information to this bug report once we have it. Any help on debugging will be greatly appreciated.