| Message ID | 20190118051825.18196-1-dja@axtens.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | bcache: never writeback a discard operation | expand |
在 2019/1/18 下午1:18, Daniel Axtens 写道: > 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 if this 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 72c270612bd3 ("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 > > Cc: Kent Overstreet <koverstreet@google.com> > Cc: stable@vger.kernel.org > Fixes: 72c270612bd3 ("bcache: Write out full stripes") > Signed-off-by: Daniel Axtens <dja@axtens.net> > --- > drivers/md/bcache/writeback.h | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h > index 6a743d3bb338..4e4c6810dc3c 100644 > --- a/drivers/md/bcache/writeback.h > +++ b/drivers/md/bcache/writeback.h > @@ -71,6 +71,9 @@ static inline bool should_writeback(struct cached_dev *dc, struct bio *bio, > in_use > bch_cutoff_writeback_sync) > return false; > > + if (bio_op(bio) == REQ_OP_DISCARD) > + return false; > + > if (dc->partial_stripes_expensive && > bcache_dev_stripe_dirty(dc, bio->bi_iter.bi_sector, > bio_sectors(bio))) > Hi Daniel, Nice catch! I add this one to my for-next directory, for v5.1 merge window. Thanks.
Hi Sasha, > This commit has been processed because it contains a "Fixes:" tag, > fixing commit: 72c270612bd3 bcache: Write out full stripes. I added that fixes tag because that was the commit that added the code. However, I noticed that one of the bug reports mentions that the problem only arising after v4.8. [1] I don't quite know what to make of this: perhaps it is a consequence of another change enabling the broken path. Maybe someone on one of the lists will have an idea. > v4.4.171: Build failed! Errors: > drivers/md/bcache/writeback.h:71:6: error: implicit declaration of function ‘bio_op’; did you mean ‘bio_rw’? [-Werror=implicit-function-declaration] > drivers/md/bcache/writeback.h:71:21: error: ‘REQ_OP_DISCARD’ undeclared (first use in this function); did you mean ‘REQ_DISCARD’? > > v3.18.132: Build failed! Errors: > drivers/md/bcache/writeback.h:71:6: error: implicit declaration of function ‘bio_op’; did you mean ‘bio_rw’? [-Werror=implicit-function-declaration] > drivers/md/bcache/writeback.h:71:21: error: ‘REQ_OP_DISCARD’ undeclared (first use in this function); did you mean ‘REQ_DISCARD’? > > > How should we proceed with this patch? The patch seems reasonably easy to backport. Compile-tested only, and only against v4.4.171. Regards, Daniel [1] https://bugzilla.kernel.org/show_bug.cgi?id=196103 From 4d66224bdc8c3e715872ca8b0711f89d1eb8245f Mon Sep 17 00:00:00 2001 From: Daniel Axtens <dja@axtens.net> Date: Fri, 18 Jan 2019 15:04:51 +1100 Subject: [PATCH] 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 if this 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 72c270612bd3 ("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 Cc: Kent Overstreet <koverstreet@google.com> Fixes: 72c270612bd3 ("bcache: Write out full stripes") (backported: replace bio_op with bio_rw bitmasking) Signed-off-by: Daniel Axtens <dja@axtens.net> --- drivers/md/bcache/writeback.h | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h index daec4fd782ea..4bb2843c0009 100644 --- a/drivers/md/bcache/writeback.h +++ b/drivers/md/bcache/writeback.h @@ -68,6 +68,9 @@ static inline bool should_writeback(struct cached_dev *dc, struct bio *bio, in_use > CUTOFF_WRITEBACK_SYNC) return false; + if (bio->bi_rw & REQ_DISCARD) + return false; + if (dc->partial_stripes_expensive && bcache_dev_stripe_dirty(dc, bio->bi_iter.bi_sector, bio_sectors(bio)))
在 2019/1/23 上午8:10, Daniel Axtens 写道: > Hi Sasha, > >> This commit has been processed because it contains a "Fixes:" tag, >> fixing commit: 72c270612bd3 bcache: Write out full stripes. > > I added that fixes tag because that was the commit that added the > code. However, I noticed that one of the bug reports mentions that the > problem only arising after v4.8. [1] I don't quite know what to make of > this: perhaps it is a consequence of another change enabling the broken > path. Maybe someone on one of the lists will have an idea. > From code logic, bcache should not have a discard bio in cache device, otherwise it is a bug. Therefore no matter whether the issue happens before v4.8 kernel, I'd like to have the fix in. >> v4.4.171: Build failed! Errors: >> drivers/md/bcache/writeback.h:71:6: error: implicit declaration of function ‘bio_op’; did you mean ‘bio_rw’? [-Werror=implicit-function-declaration] >> drivers/md/bcache/writeback.h:71:21: error: ‘REQ_OP_DISCARD’ undeclared (first use in this function); did you mean ‘REQ_DISCARD’? >> >> v3.18.132: Build failed! Errors: >> drivers/md/bcache/writeback.h:71:6: error: implicit declaration of function ‘bio_op’; did you mean ‘bio_rw’? [-Werror=implicit-function-declaration] >> drivers/md/bcache/writeback.h:71:21: error: ‘REQ_OP_DISCARD’ undeclared (first use in this function); did you mean ‘REQ_DISCARD’? >> >> >> How should we proceed with this patch? > > The patch seems reasonably easy to backport. Compile-tested only, and > only against v4.4.171. I don't have idea whether stable kernels accept rebased patches, for SUSE kernel I will do back port for all necessary kernel versions.
diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h index 6a743d3bb338..4e4c6810dc3c 100644 --- a/drivers/md/bcache/writeback.h +++ b/drivers/md/bcache/writeback.h @@ -71,6 +71,9 @@ static inline bool should_writeback(struct cached_dev *dc, struct bio *bio, in_use > bch_cutoff_writeback_sync) return false; + if (bio_op(bio) == REQ_OP_DISCARD) + return false; + if (dc->partial_stripes_expensive && bcache_dev_stripe_dirty(dc, bio->bi_iter.bi_sector, bio_sectors(bio)))
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 if this 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 72c270612bd3 ("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 Cc: Kent Overstreet <koverstreet@google.com> Cc: stable@vger.kernel.org Fixes: 72c270612bd3 ("bcache: Write out full stripes") Signed-off-by: Daniel Axtens <dja@axtens.net> --- drivers/md/bcache/writeback.h | 3 +++ 1 file changed, 3 insertions(+)