<div dir="auto">Can u help me </div> <div class="gmail_quote"><div dir="ltr">On Sat, Jan 19, 2019, 12:00 PM <<a href="mailto:dm-devel-request@redhat.com">dm-devel-request@redhat.com</a>> wrote: </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Send dm-devel mailing list submissions to <a href="mailto:dm-devel@redhat.com" target="_blank" rel="noreferrer">dm-devel@redhat.com</a> To subscribe or unsubscribe via the World Wide Web, visit <a href="https://www.redhat.com/mailman/listinfo/dm-devel" rel="noreferrer noreferrer" target="_blank">https://www.redhat.com/mailman/listinfo/dm-devel</a> or, via email, send a message with subject or body 'help' to <a href="mailto:dm-devel-request@redhat.com" target="_blank" rel="noreferrer">dm-devel-request@redhat.com</a> You can reach the person managing the list at <a href="mailto:dm-devel-owner@redhat.com" target="_blank" rel="noreferrer">dm-devel-owner@redhat.com</a> When replying, please edit your Subject line so it is more specific than "Re: Contents of dm-devel digest..." Today's Topics: 1. Re: [PATCH] bcache: never writeback a discard operation (Coly Li) ---------------------------------------------------------------------- Message: 1 Date: Sat, 19 Jan 2019 19:03:36 +0800 From: Coly Li <<a href="mailto:colyli@suse.de" target="_blank" rel="noreferrer">colyli@suse.de</a>> To: Daniel Axtens <<a href="mailto:dja@axtens.net" target="_blank" rel="noreferrer">dja@axtens.net</a>> Cc: Michael Lyle <<a href="mailto:mlyle@lyle.org" target="_blank" rel="noreferrer">mlyle@lyle.org</a>>, "Guilherme G . Piccoli" <<a href="mailto:gpiccoli@canonical.com" target="_blank" rel="noreferrer">gpiccoli@canonical.com</a>>, <a href="mailto:linux-bcache@vger.kernel.org" target="_blank" rel="noreferrer">linux-bcache@vger.kernel.org</a>, <a href="mailto:linux-block@vger.kernel.org" target="_blank" rel="noreferrer">linux-block@vger.kernel.org</a>, <a href="mailto:dm-devel@redhat.com" target="_blank" rel="noreferrer">dm-devel@redhat.com</a>, Mauricio Oliveira <<a href="mailto:mauricio.oliveira@canonical.com" target="_blank" rel="noreferrer">mauricio.oliveira@canonical.com</a>>, Kent Overstreet <<a href="mailto:kent.overstreet@gmail.com" target="_blank" rel="noreferrer">kent.overstreet@gmail.com</a>> Subject: Re: [dm-devel] [PATCH] bcache: never writeback a discard operation Message-ID: <<a href="mailto:b269c2f9-91fe-3b61-75e7-f319ba2b39de@suse.de" target="_blank" rel="noreferrer">b269c2f9-91fe-3b61-75e7-f319ba2b39de@suse.de</a>> Content-Type: text/plain; charset=gbk ? 2019/1/18 ??1:18, Daniel Axtens ??: > 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: <a href="https://www.spinics.net/lists/linux-bcache/msg06996.html" rel="noreferrer noreferrer" target="_blank">https://www.spinics.net/lists/linux-bcache/msg06996.html</a> > > Previous reports: > - <a href="https://bugzilla.kernel.org/show_bug.cgi?id=201051" rel="noreferrer noreferrer" target="_blank">https://bugzilla.kernel.org/show_bug.cgi?id=201051</a> > - <a href="https://bugzilla.kernel.org/show_bug.cgi?id=196103" rel="noreferrer noreferrer" target="_blank">https://bugzilla.kernel.org/show_bug.cgi?id=196103</a> > - <a href="https://www.spinics.net/lists/linux-bcache/msg06885.html" rel="noreferrer noreferrer" target="_blank">https://www.spinics.net/lists/linux-bcache/msg06885.html</a> > > Cc: Kent Overstreet <<a href="mailto:koverstreet@google.com" target="_blank" rel="noreferrer">koverstreet@google.com</a>> > Cc: <a href="mailto:stable@vger.kernel.org" target="_blank" rel="noreferrer">stable@vger.kernel.org</a> > Fixes: 72c270612bd3 ("bcache: Write out full stripes") > Signed-off-by: Daniel Axtens <<a href="mailto:dja@axtens.net" target="_blank" rel="noreferrer">dja@axtens.net</a>> > --- > 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. -- Coly Li ------------------------------ -- dm-devel mailing list <a href="mailto:dm-devel@redhat.com" target="_blank" rel="noreferrer">dm-devel@redhat.com</a> <a href="https://www.redhat.com/mailman/listinfo/dm-devel" rel="noreferrer noreferrer" target="_blank">https://www.redhat.com/mailman/listinfo/dm-devel</a> End of dm-devel Digest, Vol 179, Issue 15 ***************************************** </blockquote></div>