[SRU X][PATCH 0/1] Fix bcache + fstrim panic (LP: #1793901)

[Daniel Axtens]

SRU Justification
=================

[Impact]

Some users see panics like the following when performing fstrim on a
bcached volume:

[ 529.803060] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[ 530.183928] #PF error: [normal kernel read fault]
[ 530.412392] PGD 8000001f42163067 P4D 8000001f42163067 PUD 1f42168067 PMD 0
[ 530.750887] Oops: 0000 [#1] SMP PTI
[ 530.920869] CPU: 10 PID: 4167 Comm: fstrim Kdump: loaded Not tainted 5.0.0-rc1+ #3
[ 531.290204] Hardware name: HP ProLiant DL360 Gen9/ProLiant DL360 Gen9, BIOS P89 12/27/2015
[ 531.693137] RIP: 0010:blk_queue_split+0x148/0x620
[ 531.922205] Code: 60 38 89 55 a0 45 31 db 45 31 f6 45 31 c9 31 ff 89 4d 98 85 db 0f 84 7f 04 00 00 44 8b 6d 98 4c 89 ee 48 c1 e6 04 49 03 70 78 <8b> 46 08 44 8b 56 0c 48
8b 16 44 29 e0 39 d8 48 89 55 a8 0f 47 c3
[ 532.838634] RSP: 0018:ffffb9b708df39b0 EFLAGS: 00010246
[ 533.093571] RAX: 00000000ffffffff RBX: 0000000000046000 RCX: 0000000000000000
[ 533.441865] RDX: 0000000000000200 RSI: 0000000000000000 RDI: 0000000000000000
[ 533.789922] RBP: ffffb9b708df3a48 R08: ffff940d3b3fdd20 R09: 0000000000000000
[ 534.137512] R10: ffffb9b708df3958 R11: 0000000000000000 R12: 0000000000000000
[ 534.485329] R13: 0000000000000000 R14: 0000000000000000 R15: ffff940d39212020
[ 534.833319] FS: 00007efec26e3840(0000) GS:ffff940d1f480000(0000) knlGS:0000000000000000
[ 535.224098] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 535.504318] CR2: 0000000000000008 CR3: 0000001f4e256004 CR4: 00000000001606e0
[ 535.851759] Call Trace:
[ 535.970308] ? mempool_alloc_slab+0x15/0x20
[ 536.174152] ? bch_data_insert+0x42/0xd0 [bcache]
[ 536.403399] blk_mq_make_request+0x97/0x4f0
[ 536.607036] generic_make_request+0x1e2/0x410
[ 536.819164] submit_bio+0x73/0x150
[ 536.980168] ? submit_bio+0x73/0x150
[ 537.149731] ? bio_associate_blkg_from_css+0x3b/0x60
[ 537.391595] ? _cond_resched+0x1a/0x50
[ 537.573774] submit_bio_wait+0x59/0x90
[ 537.756105] blkdev_issue_discard+0x80/0xd0
[ 537.959590] ext4_trim_fs+0x4a9/0x9e0
[ 538.137636] ? ext4_trim_fs+0x4a9/0x9e0
[ 538.324087] ext4_ioctl+0xea4/0x1530
[ 538.497712] ? _copy_to_user+0x2a/0x40
[ 538.679632] do_vfs_ioctl+0xa6/0x600
[ 538.853127] ? __do_sys_newfstat+0x44/0x70
[ 539.051951] ksys_ioctl+0x6d/0x80
[ 539.212785] __x64_sys_ioctl+0x1a/0x20
[ 539.394918] do_syscall_64+0x5a/0x110
[ 539.568674] entry_SYSCALL_64_after_hwframe+0x44/0xa9

[Fix]

Under certain conditions, the test for whether an operation should be
written back to the underlying device was incorrect. Specifically, in
should_writeback(), we were hitting a case where an optimisation for
partial stripe conditions was returning true and so should_writeback()
was returning true early. This caused the code to go down an incorrect
path and create bios that contained NULL pointers.

To fix this issue, make sure that should_writeback() on a discard op
never returns true.

[Test Case]

We have observed it on some systems where both:
1) LVM/devmapper is involved (bcache backing device is LVM volume) and
2) writeback cache is involved (bcache cache_mode is writeback)

Not every machine exhibits the bug. On one machine that does exhibit
the bug, we can reliably reproduce it with:

 # echo writeback > /sys/block/bcache0/bcache/cache_mode
 # mount /dev/bcache0 /test
 # for i in {0..10}; do file="$(mktemp /test/zero.XXX)"; dd if=/dev/zero of="$file" bs=1M count=256; sync; rm $file; done; fstrim -v /test

[Regression Potential]

This could affect any device where bcache is used.

In mitigation, however: the patch is simple, is limited to considering
discard operations. The patch has been accepted upstream [1] and the
maintainer will be including it in SuSE kernels [2]. A Gentoo user
validated the upstream patch independently [3].

[1] https://www.spinics.net/lists/linux-bcache/msg06997.html
[2] https://www.spinics.net/lists/linux-bcache/msg06998.html
[3] https://bugzilla.kernel.org/show_bug.cgi?id=196103#c3

Daniel Axtens (1):
  UBUNTU: SAUCE: bcache: never writeback a discard operation

 drivers/md/bcache/writeback.h | 3 +++
 1 file changed, 3 insertions(+)

-- 
2.19.1