Comment 48 for bug 1867916

Focal / Testing
=====

modified
--------

$ uname -rv
5.4.0-41-generic #45+lp1867916.1 SMP Mon Jul 6 16:41:46 -03 2020

$ sudo make-bcache --bdev $DEV --block 8k
[ 29.593270] bcache: bcache_device_init() bcache0: sb/logical block size (8192) greater than page size (4096) falling back to device logical block size (512)
[ 29.596872] bcache: register_bdev() registered backing device loop0

original
--------

$ uname -rv
5.4.0-41-generic #45-Ubuntu SMP Fri Jul 3 10:57:47 UTC 2020

$ sudo make-bcache --bdev $DEV --block 8k
[ 37.880016] bcache: register_bdev() registered backing device loop0
[ 37.883376] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 37.884789] #PF: supervisor write access in kernel mode
[ 37.885862] #PF: error_code(0x0002) - not-present page
[ 37.886899] PGD 8000000129ee4067 P4D 8000000129ee4067 PUD 129ee5067 PMD 0
[ 37.888273] Oops: 0002 [#1] SMP PTI
[ 37.889014] CPU: 0 PID: 1585 Comm: bcache-register Not tainted 5.4.0-41-generic #45-Ubuntu
[ 37.890614] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 37.892334] RIP: 0010:create_empty_buffers+0x24/0x110
[ 37.893317] Code: 00 00 00 0f 1f 00 0f 1f 44 00 00 55 48 89 e5 41 55 49 89 d5 ba 01 00 00 00 41 54 53 48 89 fb e8 32 f4 ff ff 49 89 c4 48 89 c2 <4c> 09 2a 48 89 d0 48 8b 52 08 48 85 d2 75 f1 4c 89 60 08 48 8b 43
[ 37.896260] RSP: 0018:ffffb4cb40347820 EFLAGS: 00010286
[ 37.897132] RAX: 0000000000000000 RBX: fffff9a944a7ad80 RCX: ffffa032b78e6000
[ 37.898304] RDX: 0000000000000000 RSI: ffffa032b78e6000 RDI: fffff9a944a7ad80
[ 37.899451] RBP: ffffb4cb40347838 R08: ffffa032b78e6000 R09: 0000000000000000
[ 37.900597] R10: 0000000000000001 R11: ffffa032bac48758 R12: 0000000000000000
[ 37.901747] R13: 0000000000000000 R14: ffffa032b7205c60 R15: 0000000000000000
[ 37.902901] FS: 00007f08ed221700(0000) GS:ffffa032bba00000(0000) knlGS:0000000000000000
[ 37.904327] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 37.905347] CR2: 0000000000000000 CR3: 0000000129eb0000 CR4: 00000000000006f0
[ 37.906560] Call Trace:
[ 37.907083] create_page_buffers+0x52/0x60
[ 37.907841] block_read_full_page+0x4e/0x3b0
[ 37.908650] ? blkdev_direct_IO+0x70/0x70
[ 37.909402] ? __add_to_page_cache_locked+0x2e5/0x340
[ 37.910299] ? scan_shadow_nodes+0x30/0x30
[ 37.911050] blkdev_readpage+0x18/0x20
[ 37.911753] do_read_cache_page+0x2f6/0x830
[ 37.912523] ? prep_new_page+0x128/0x160
[ 37.913260] read_cache_page+0x12/0x20
[ 37.913957] read_dev_sector+0x27/0xd0
[ 37.914655] read_lba+0xbd/0x220
[ 37.915276] ? kmem_cache_alloc_trace+0x16c/0x240
[ 37.916114] efi_partition+0x1e0/0x6fd
[ 37.916818] ? vsnprintf+0x39e/0x4e0
[ 37.917506] ? snprintf+0x49/0x60
[ 37.918142] check_partition+0x154/0x244
[ 37.918876] rescan_partitions+0xae/0x280
[ 37.919612] bdev_disk_changed+0x5f/0x70
[ 37.920325] __blkdev_get+0x3f8/0x550
[ 37.921011] blkdev_get+0x3d/0x140
[ 37.921654] __device_add_disk+0x329/0x480
[ 37.922396] device_add_disk+0x13/0x20
[ 37.923103] bch_cached_dev_run+0x66/0x190 [bcache]
[ 37.923969] register_bcache.cold+0x17a/0x1c6 [bcache]
[ 37.924861] ? __handle_mm_fault+0x4c5/0x7a0
[ 37.925649] kobj_attr_store+0x12/0x20
[ 37.926342] sysfs_kf_write+0x3e/0x50
[ 37.927030] kernfs_fop_write+0xda/0x1b0
[ 37.927753] __vfs_write+0x1b/0x40
[ 37.928412] vfs_write+0xb9/0x1a0
[ 37.929052] ksys_write+0x67/0xe0
[ 37.929688] __x64_sys_write+0x1a/0x20
[ 37.930383] do_syscall_64+0x57/0x190
[ 37.931065] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 37.931950] RIP: 0033:0x7f08ecd312c0
[ 37.932624] Code: 73 01 c3 48 8b 0d d8 cb 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d 89 24 2d 00 00 75 10 b8 01 00 00 00
 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 fe dd 01 00 48 89 04 24
[ 37.935631] RSP: 002b:00007fff3edd2f38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 37.936948] RAX: ffffffffffffffda RBX: 000000000000000b RCX: 00007f08ecd312c0
[ 37.938172] RDX: 000000000000000b RSI: 000000000185e010 RDI: 0000000000000003
[ 37.939332] RBP: 000000000185e010 R08: 0000000000000000 R09: 000000000000000b
[ 37.940504] R10: 000000000000000a R11: 0000000000000246 R12: 000000000000000b
[ 37.941679] R13: 0000000000000001 R14: 00007fff3edd2fb0 R15: 0000000000000000
[ 37.942860] Modules linked in: bcache crc64 isofs kvm_intel kvm joydev input_leds serio_raw ib_iser rdma_cm iw_cm ib_cm ib_core iscsi_tcp
 libiscsi_tcp libiscsi scsi_transport_iscsi autofs4 btrfs xor zstd_compress raid6_pq libcrc32c virtio_net psmouse net_failover failover virt
io_blk floppy
[ 37.946927] CR2: 0000000000000000
[ 37.947597] ---[ end trace 8bf15a8fb80acaa0 ]---
[ 37.948438] RIP: 0010:create_empty_buffers+0x24/0x110
[ 37.949318] Code: 00 00 00 0f 1f 00 0f 1f 44 00 00 55 48 89 e5 41 55 49 89 d5 ba 01 00 00 00 41 54 53 48 89 fb e8 32 f4 ff ff 49 89 c4 48 89 c2 <4c> 09 2a 48 89 d0 48 8b 52 08 48 85 d2 75 f1 4c 89 60 08 48 8b 43
[ 37.952321] RSP: 0018:ffffb4cb40347820 EFLAGS: 00010286
[ 37.953241] RAX: 0000000000000000 RBX: fffff9a944a7ad80 RCX: ffffa032b78e6000
[ 37.954436] RDX: 0000000000000000 RSI: ffffa032b78e6000 RDI: fffff9a944a7ad80
[ 37.955612] RBP: ffffb4cb40347838 R08: ffffa032b78e6000 R09: 0000000000000000
[ 37.956778] R10: 0000000000000001 R11: ffffa032bac48758 R12: 0000000000000000
[ 37.957964] R13: 0000000000000000 R14: ffffa032b7205c60 R15: 0000000000000000
[ 37.959162] FS: 00007f08ed221700(0000) GS:ffffa032bba00000(0000) knlGS:0000000000000000
[ 37.960527] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 37.961848] CR2: 0000000000000000 CR3: 0000000129eb0000 CR4: 00000000000006f0