[dm-devel] [lvm-devel] kernel BUG at drivers/md/persistent-data/dm-space-map-disk.c:178
Joe Thornber
thornber at redhat.com
Tue Jan 7 10:46:27 UTC 2020
On Tue, Jan 07, 2020 at 10:35:46AM +0000, Joe Thornber wrote:
> On Sat, Dec 28, 2019 at 02:13:07AM +0000, Eric Wheeler wrote:
> > On Fri, 27 Dec 2019, Eric Wheeler wrote:
>
> > > Just hit the bug again without mq-scsi (scsi_mod.use_blk_mq=n), all other
> > > times MQ has been turned on.
> > >
> > > I'm trying the -ENOSPC hack which will flag the pool as being out of space
> > > so I can recover more gracefully than a BUG_ON. Here's a first-draft
> > > patch, maybe the spinlock will even prevent the issue.
> > >
> > > Compile tested, I'll try on a real system tomorrow.
> > >
> > > Comments welcome:
>
> Both sm_ll_find_free_block() and sm_ll_inc() can trigger synchronous IO. So you
> absolutely cannot use a spin lock.
>
> dm_pool_alloc_data_block() holds a big rw semaphore which should prevent anything
> else trying to allocate at the same time.
I suspect the problem is to do with the way we search for the new block in the
space map for the previous transaction (sm->old_ll), and then increment in the current
transaction (sm->ll).
We keep old_ll around so we can ensure we never (re) allocate a block that's used in
the previous transaction. This gives us our crash resistance, since if anything goes
wrong we effectively rollback to the previous transaction.
What I think we should be doing is running find_free on the old_ll, then double checking
it's not actually used in the current transaction. ATM we're relying on smd->begin being
set properly everywhere, and I suspect this isn't the case. A quick look shows sm_disk_inc_block()
doesn't adjust it. sm_disk_inc_block() can be called when breaking sharing of a neighbouring entry
in a leaf btree node ... and we know you use snapshots very heavily.
I'll get a patch to you later today.
- Joe
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