Daniel Axtens 9951379b0c bcache: never writeback a discard operation ... 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 We have observed it where both: 1) LVM/devmapper is involved (bcache backing device is LVM volume) and 2) writeback cache is involved (bcache cache_mode is writeback) On one machine, we can reliably reproduce it with: # echo writeback > /sys/block/bcache0/bcache/cache_mode (not sure whether above line is required) # 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 Observing this with tracepoints on, we see the following writes: fstrim-18019 [022] .... 91107.302026: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0 DS 4260112 + 196352 hit 0 bypass 1 fstrim-18019 [022] .... 91107.302050: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0 DS 4456464 + 262144 hit 0 bypass 1 fstrim-18019 [022] .... 91107.302075: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0 DS 4718608 + 81920 hit 0 bypass 1 fstrim-18019 [022] .... 91107.302094: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0 DS 5324816 + 180224 hit 0 bypass 1 fstrim-18019 [022] .... 91107.302121: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0 DS 5505040 + 262144 hit 0 bypass 1 fstrim-18019 [022] .... 91107.302145: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0 DS 5767184 + 81920 hit 0 bypass 1 fstrim-18019 [022] .... 91107.308777: bcache_write: 73f95583-561c-408f-a93a-4cbd2498f5c8 inode 0 DS 6373392 + 180224 hit 1 bypass 0 <crash> Note the final one has different hit/bypass flags. This is because in should_writeback(), we were hitting a case where the partial stripe condition was returning true and so should_writeback() was returning true early. If that hadn't been the case, it would have hit the would_skip test, and as would_skip == s->iop.bypass == true, should_writeback() would have returned false. Looking at the git history from 'commit 72c270612b ("bcache: Write out full stripes")', it looks like the idea was to optimise for raid5/6: * If a stripe is already dirty, force writes to that stripe to writeback mode - to help build up full stripes of dirty data To fix this issue, make sure that should_writeback() on a discard op never returns true. More details of debugging: https://www.spinics.net/lists/linux-bcache/msg06996.html Previous reports: - https://bugzilla.kernel.org/show_bug.cgi?id=201051 - https://bugzilla.kernel.org/show_bug.cgi?id=196103 - https://www.spinics.net/lists/linux-bcache/msg06885.html (Coly Li: minor modification to follow maximum 75 chars per line rule) Cc: Kent Overstreet <koverstreet@google.com> Cc: stable@vger.kernel.org Fixes: 72c270612b ("bcache: Write out full stripes") Signed-off-by: Daniel Axtens <dja@axtens.net> Signed-off-by: Coly Li <colyli@suse.de> Signed-off-by: Jens Axboe <axboe@kernel.dk>		2019-02-09 07:18:31 -07:00
..
alloc.c	bcache: use MAX_CACHES_PER_SET instead of magic number 8 in __bch_bucket_alloc_set	2018-10-08 08:19:56 -06:00
bcache.h	bcache: option to automatically run gc thread after writeback	2018-12-13 08:15:54 -07:00
bset.c	bcache: fix code comments style	2018-08-11 15:46:42 -06:00
bset.h	bcache: style fixes for lines over 80 characters	2018-08-11 15:46:41 -06:00
btree.c	bcache: add comment for cache_set->fill_iter	2018-12-13 08:15:54 -07:00
btree.h	bcache: introduce force_wake_up_gc()	2018-12-13 08:15:54 -07:00
closure.c	bcache: add missing SPDX header	2018-08-11 15:46:42 -06:00
closure.h	bcache: fix typo in code comments of closure_return_with_destructor()	2018-10-08 08:19:43 -06:00
debug.c	bcache: do not check if debug dentry is ERR or NULL explicitly on remove	2018-12-13 08:15:54 -07:00
debug.h	bcache: add identifier names to arguments of function definitions	2018-08-11 15:46:41 -06:00
extents.c	bcache: replace hard coded number with BUCKET_GC_GEN_MAX	2018-10-08 08:19:55 -06:00
extents.h	bcache: add identifier names to arguments of function definitions	2018-08-11 15:46:41 -06:00
io.c	bcache: fix typo 'succesfully' to 'successfully'	2018-08-11 15:46:42 -06:00
journal.c	bcache: print number of keys in trace_bcache_journal_write	2018-12-13 08:15:54 -07:00
journal.h	bcache: add identifier names to arguments of function definitions	2018-08-11 15:46:41 -06:00
Kconfig	for-4.19/post-20180822	2018-08-22 13:38:05 -07:00
Makefile	License cleanup: add SPDX GPL-2.0 license identifier to files with no license	2017-11-02 11:10:55 +01:00
movinggc.c	bcache: style fix to add a blank line after declarations	2018-08-11 15:46:41 -06:00
request.c	bcache: update comment for bch_data_insert	2018-12-13 08:15:54 -07:00
request.h	bcache: remove unused bch_passthrough_cache	2018-10-08 08:19:52 -06:00
stats.c	bcache: style fix to add a blank line after declarations	2018-08-11 15:46:41 -06:00
stats.h	bcache: add identifier names to arguments of function definitions	2018-08-11 15:46:41 -06:00
super.c	bcache: make cutoff_writeback and cutoff_writeback_sync tunable	2018-12-13 08:15:54 -07:00
sysfs.c	bcache: set writeback_percent in a flexible range	2018-12-13 08:15:54 -07:00
sysfs.h	bcache: replace Symbolic permissions by octal permission numbers	2018-08-11 15:46:41 -06:00
trace.c	License cleanup: add SPDX GPL-2.0 license identifier to files with no license	2017-11-02 11:10:55 +01:00
util.c	for-4.19/post-20180822	2018-08-22 13:38:05 -07:00
util.h	for-4.19/post-20180822	2018-08-22 13:38:05 -07:00
writeback.c	bcache: option to automatically run gc thread after writeback	2018-12-13 08:15:54 -07:00
writeback.h	bcache: never writeback a discard operation	2019-02-09 07:18:31 -07:00