Endless calls to xas_split_alloc() due to corrupted xarray entry

[Zhenyu Zhang <zhenyzha@redhat.com>]

Hi all,

we don't know how the xarray entry was corrupted. Maybe it's a known
issue to community.
Lets see.

Contents
--------
1. Problem Statement
2. The call trace
3. The captured data by bpftrace


1. Problem Statement
--------------------
With 4k guest and 64k host, on aarch64(Ampere's Altra Max CPU) hit Call trace:
    Steps:
    1) System setup hugepages on host.
       # echo 60 > /proc/sys/vm/nr_hugepages
    2) Mount this hugepage to /mnt/kvm_hugepage.
       # mount -t hugetlbfs -o pagesize=524288K none /mnt/kvm_hugepage
    3) HugePages didn't leak when using non-existent mem-path.
       # mkdir -p /mnt/tmp
    4) Boot guest.
       # /usr/libexec/qemu-kvm \
...
         -m 30720 \
-object '{"size": 32212254720, "mem-path": "/mnt/tmp", "qom-type":
"memory-backend-file"}'  \
-smp 4,maxcpus=4,cores=2,threads=1,clusters=1,sockets=2  \
         -blockdev '{"node-name": "file_image1", "driver": "file",
"auto-read-only": true, "discard": "unmap", "aio": "threads",
"filename": "/home/kvm_autotest_root/images/back_up_4k.qcow2",
"cache": {"direct": true, "no-flush": false}}' \
-blockdev '{"node-name": "drive_image1", "driver": "qcow2",
"read-only": false, "cache": {"direct": true, "no-flush": false},
"file": "file_image1"}' \
-device '{"driver": "scsi-hd", "id": "image1", "drive":
"drive_image1", "write-cache": "on"}' \

    5) Wait about 1 minute ------> hit Call trace

2. The call trace
--------------------
[   14.982751] block dm-0: the capability attribute has been deprecated.
[   15.690043] PEFILE: Unsigned PE binary


[   90.135676] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
[   90.136629] rcu: 3-...0: (3 ticks this GP)
idle=e6ec/1/0x4000000000000000 softirq=6847/6849 fqs=232
[   90.137293] rcu: (detected by 2, t=6012 jiffies, g=2085, q=2539 ncpus=4)
[   90.137796] Task dump for CPU 3:
[   90.138037] task:PK-Backend      state:R  running task     stack:0
   pid:2287  ppid:1      flags:0x00000202
[   90.138757] Call trace:
[   90.138940]  __switch_to+0xc8/0x110
[   90.139203]  0xb54a54f8c5fb0700

[  270.190849] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
[  270.191722] rcu: 3-...0: (3 ticks this GP)
idle=e6ec/1/0x4000000000000000 softirq=6847/6849 fqs=1020
[  270.192405] rcu: (detected by 1, t=24018 jiffies, g=2085, q=3104 ncpus=4)
[  270.192876] Task dump for CPU 3:
[  270.193099] task:PK-Backend      state:R  running task     stack:0
   pid:2287  ppid:1      flags:0x00000202
[  270.193774] Call trace:
[  270.193946]  __switch_to+0xc8/0x110
[  270.194336]  0xb54a54f8c5fb0700

[ 1228.068406] ------------[ cut here ]------------
[ 1228.073011] WARNING: CPU: 2 PID: 4496 at lib/xarray.c:1010
xas_split_alloc+0xf8/0x128
[ 1228.080828] Modules linked in: binfmt_misc vhost_net vhost
vhost_iotlb tap xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT
nf_reject_ipv4 nft_compat nft_chain_nat nf_nat nf_conntrack
nf_defrag_ipv6 nf_defrag_ipv4 nf_tables nfnetlink tun bridge stp llc
qrtr rfkill sunrpc vfat fat acpi_ipmi ipmi_ssif arm_spe_pmu
ipmi_devintf arm_cmn arm_dmc620_pmu ipmi_msghandler cppc_cpufreq
arm_dsu_pmu xfs libcrc32c ast drm_shmem_helper drm_kms_helper drm
crct10dif_ce ghash_ce igb nvme sha2_ce nvme_core sha256_arm64 sha1_ce
i2c_designware_platform sbsa_gwdt nvme_common i2c_algo_bit
i2c_designware_core xgene_hwmon dm_mirror dm_region_hash dm_log dm_mod
fuse
[ 1228.137630] CPU: 2 PID: 4496 Comm: qemu-kvm Kdump: loaded Tainted:
G        W          6.6.0-rc2-zhenyzha+ #5
[ 1228.147529] Hardware name: GIGABYTE R152-P31-00/MP32-AR1-00, BIOS
F31h (SCP: 2.10.20220810) 07/27/2022
[ 1228.156820] pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 1228.163767] pc : xas_split_alloc+0xf8/0x128
[ 1228.167938] lr : __filemap_add_folio+0x33c/0x4e0
[ 1228.172543] sp : ffff80008dd4f1c0
[ 1228.175844] x29: ffff80008dd4f1c0 x28: ffffd15825388c40 x27: 0000000000000001
[ 1228.182967] x26: 0000000000000001 x25: ffffffffffffc005 x24: 0000000000000000
[ 1228.190089] x23: ffff80008dd4f270 x22: ffffffc202b00000 x21: 0000000000000000
[ 1228.197211] x20: ffffffc2007f9600 x19: 000000000000000d x18: 0000000000000014
[ 1228.204334] x17: 00000000b21b8a3f x16: 0000000013a8aa94 x15: ffffd15824625944
[ 1228.211456] x14: ffffffffffffffff x13: 0000000000000030 x12: 0101010101010101
[ 1228.218578] x11: 7f7f7f7f7f7f7f7f x10: 000000000000000a x9 : ffffd158252dd3fc
[ 1228.225701] x8 : ffff80008dd4f1c0 x7 : ffff07ffa0945468 x6 : ffff80008dd4f1c0
[ 1228.232823] x5 : 0000000000000018 x4 : 0000000000000000 x3 : 0000000000012c40
[ 1228.239945] x2 : 000000000000000d x1 : 000000000000000c x0 : 0000000000000000
[ 1228.247067] Call trace:
[ 1228.249500]  xas_split_alloc+0xf8/0x128
[ 1228.253324]  __filemap_add_folio+0x33c/0x4e0
[ 1228.257582]  filemap_add_folio+0x48/0xd0
[ 1228.261493]  page_cache_ra_order+0x214/0x310
[ 1228.265750]  ondemand_readahead+0x1a8/0x320
[ 1228.269921]  page_cache_async_ra+0x64/0xa8
[ 1228.274005]  filemap_fault+0x238/0xaa8
[ 1228.277742]  __xfs_filemap_fault+0x60/0x3c0 [xfs]
[ 1228.282491]  xfs_filemap_fault+0x54/0x68 [xfs]
[ 1228.286979]  __do_fault+0x40/0x210
[ 1228.290368]  do_cow_fault+0xf0/0x300
[ 1228.293931]  do_pte_missing+0x140/0x238
[ 1228.297754]  handle_pte_fault+0x100/0x160
[ 1228.301751]  __handle_mm_fault+0x100/0x310
[ 1228.305835]  handle_mm_fault+0x6c/0x270
[ 1228.309659]  faultin_page+0x70/0x128
[ 1228.313221]  __get_user_pages+0xc8/0x2d8
[ 1228.317131]  get_user_pages_unlocked+0xc4/0x3b8
[ 1228.321648]  hva_to_pfn+0xf8/0x468
[ 1228.325037]  __gfn_to_pfn_memslot+0xa4/0xf8
[ 1228.329207]  user_mem_abort+0x174/0x7e8
[ 1228.333031]  kvm_handle_guest_abort+0x2dc/0x450
[ 1228.337548]  handle_exit+0x70/0x1c8
[ 1228.341024]  kvm_arch_vcpu_ioctl_run+0x224/0x5b8
[ 1228.345628]  kvm_vcpu_ioctl+0x28c/0x9d0
[ 1228.349450]  __arm64_sys_ioctl+0xa8/0xf0
[ 1228.353360]  invoke_syscall.constprop.0+0x7c/0xd0
[ 1228.358052]  do_el0_svc+0xb4/0xd0
[ 1228.361354]  el0_svc+0x50/0x228
[ 1228.364484]  el0t_64_sync_handler+0x134/0x150
[ 1228.368827]  el0t_64_sync+0x17c/0x180
[ 1228.372476] ---[ end trace 0000000000000000 ]---
[ 1228.377124] ------------[ cut here ]------------
[ 1228.381728] WARNING: CPU: 2 PID: 4496 at lib/xarray.c:1010
xas_split_alloc+0xf8/0x128
[ 1228.389546] Modules linked in: binfmt_misc vhost_net vhost
vhost_iotlb tap xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT
nf_reject_ipv4 nft_compat nft_chain_nat nf_nat nf_conntrack
nf_defrag_ipv6 nf_defrag_ipv4 nf_tables nfnetlink tun bridge stp llc
qrtr rfkill sunrpc vfat fat acpi_ipmi ipmi_ssif arm_spe_pmu
ipmi_devintf arm_cmn arm_dmc620_pmu ipmi_msghandler cppc_cpufreq
arm_dsu_pmu xfs libcrc32c ast drm_shmem_helper drm_kms_helper drm
crct10dif_ce ghash_ce igb nvme sha2_ce nvme_core sha256_arm64 sha1_ce
i2c_designware_platform sbsa_gwdt nvme_common i2c_algo_bit
i2c_designware_core xgene_hwmon dm_mirror dm_region_hash dm_log dm_mod
fuse
[ 1228.446348] CPU: 2 PID: 4496 Comm: qemu-kvm Kdump: loaded Tainted:
G        W          6.6.0-rc2-zhenyzha+ #5
[ 1228.456248] Hardware name: GIGABYTE R152-P31-00/MP32-AR1-00, BIOS
F31h (SCP: 2.10.20220810) 07/27/2022
[ 1228.465538] pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 1228.472486] pc : xas_split_alloc+0xf8/0x128
[ 1228.476656] lr : __filemap_add_folio+0x33c/0x4e0
[ 1228.481261] sp : ffff80008dd4f1c0
[ 1228.484563] x29: ffff80008dd4f1c0 x28: ffffd15825388c40 x27: 0000000000000001
[ 1228.491685] x26: 0000000000000001 x25: ffffffffffffc005 x24: 0000000000000000
[ 1228.498807] x23: ffff80008dd4f270 x22: ffffffc202b00000 x21: 0000000000000000
[ 1228.505930] x20: ffffffc2007f9600 x19: 000000000000000d x18: 0000000000000014
[ 1228.513052] x17: 00000000b21b8a3f x16: 0000000013a8aa94 x15: ffffd15824625944
[ 1228.520174] x14: ffffffffffffffff x13: 0000000000000030 x12: 0101010101010101
[ 1228.527297] x11: 7f7f7f7f7f7f7f7f x10: 000000000000000a x9 : ffffd158252dd3fc
[ 1228.534419] x8 : ffff80008dd4f1c0 x7 : ffff07ffa0945468 x6 : ffff80008dd4f1c0
[ 1228.541542] x5 : 0000000000000018 x4 : 0000000000000000 x3 : 0000000000012c40
[ 1228.548664] x2 : 000000000000000d x1 : 000000000000000c x0 : 0000000000000000
[ 1228.555786] Call trace:
[ 1228.558220]  xas_split_alloc+0xf8/0x128
[ 1228.562043]  __filemap_add_folio+0x33c/0x4e0
[ 1228.566300]  filemap_add_folio+0x48/0xd0
[ 1228.570211]  page_cache_ra_order+0x214/0x310
[ 1228.574469]  ondemand_readahead+0x1a8/0x320
[ 1228.578639]  page_cache_async_ra+0x64/0xa8
[ 1228.582724]  filemap_fault+0x238/0xaa8
[ 1228.586460]  __xfs_filemap_fault+0x60/0x3c0 [xfs]
[ 1228.591210]  xfs_filemap_fault+0x54/0x68 [xfs]



3. The captured data by bpftrace
(The following part is the crawl analysis of gshan@redhat.com )
--------------------
pid:  4475    task: qemu-kvm
file: /mnt/tmp/qemu_back_mem.mem-machine_mem.OdGYet (deleted)

-------------------- inode --------------------
i_flags:               0x0
i_ino:                 67333199
i_size:                32212254720

----------------- address_space ----------------
flags:                 040
invalidate_lock
  count:               256
  owner:               0xffff07fff6e759c1
    pid: 4496  task: qemu-kvm
  wait_list.next:      0xffff07ffa20422e0
  wait_list.prev:      0xffff07ffa20422e0

-------------------- xarray --------------------
entry[0]:       0xffff080f7eda0002
shift:          18
offset:         0
count:          2
nr_values:      0
parent:         0x0
slots[00]:      0xffff07ffa094546a
slots[01]:      0xffff07ffa1b09b22

entry[1]:       0xffff07ffa094546a
shift:          12
offset:         0
count:          20
nr_values:      0
parent:         0xffff080f7eda0000
slots[00]:      0xffffffc202880000
slots[01]:      0x2

entry[2]:       0xffffffc202880000
shift:          104
offset:         128
count:          0
nr_values:      0
parent:         0xffffffc20304c888
slots[00]:      0xffff08009a960000
slots[01]:      0x2001ffffffff

It seems the last xarray entry ("entry[2]") has been corrupted. "shift"
becomes 104 and "offset" becomes 128, which isn't reasonable.
It's explaining why we run into xas_split_alloc() in __filemap_add_folio()

        if (order > folio_order(folio))
                xas_split_alloc(&xas, xa_load(xas.xa, xas.xa_index),
                                order, gfp);

folio_order(folio) is likely 6 since the readahead window size on the BDI device
is 4MB.
However, @order figured from the corrupted xarray entry is much larger than 6.
log2(0x400000 / 0x10000) = log2(64) = 6

[root@ampere-mtsnow-altramax-28 ~]# uname -r
6.6.0-rc2-zhenyzha+
[root@ampere-mtsnow-altramax-28 ~]# cat
/sys/devices/virtual/bdi/253:0/read_ahead_kb
4096