Re: [PATCH] bcache: treat stale && dirty keys as bad keys

[Kent Overstreet <kent.overstreet@gmail.com>]

On Wed, Dec 19, 2018 at 09:32:55AM +0800, Junhui Tang wrote:
> hello Kent
> 
> Long long no see, glad to hear you again.
> 
> >> Then two steps:
> >> A) update k1 to k2 in btree node memory;
> >>    bch_btree_insert_keys(b, op, insert_keys, replace_key)
> >> B) Write the bset(contains k2) to cache disk by a 30s delay work
> >>    bch_btree_leaf_dirty(b, journal_ref).
> >> But before the 30s delay work write the bset to cache device,
> >> these things happend:
> >> A) GC works, and reclaim the bucket k2 point to;
> >> B) Allocator works, and invalidate the bucket k2 point to,
> >>    and increase the gen of the bucket, and place it into free_inc
> >>    fifo;
> >> C) Until now, the 30s delay work still does not finish work,
> >>    so in the disk, the key still is k1, it is dirty and stale
> >>    (its gen is smaller than the gen of the bucket). and then the
> >>    machine power off suddenly happens;
> >> D) When the machine power on again, after the btree reconstruction,
> >>    the stale dirty key appear.
> 
> > Only prior to journal replay, right? Or did you uncover something more severe?
> No, it's after the journal replay, and in write_dirty_finish(), when
> replace a dirty key with a clean key by calling bch_btree_insert(),
> no journal will write.

Holy crap you're right, this was from before I moved journalling to be driven by
the btree update path.

I think a better fix here would be to journal the btree updates writeback does,
but given that we haven't been journalling those updates all this time your fix
does make sense too.

> 
> >> In bch_extent_bad(), when expensive_debug_checks is off, it would
> >> treat the dirty key as good even it is stale keys, and it would
> >> cause bellow probelms:
> >> A) In read_dirty() it would cause machine crash:
> >>    BUG_ON(ptr_stale(dc->disk.c, &w->key, 0));
> >> B) It could be worse when reads hits stale dirty keys, it would
> >>    read old incorrect data.
> 
> >Neither of these can happen until after journal replay is finished. Prior to
> >journal replay we expect to find stale dirty keys - if we find any after journal
> >replay then it's indicative of a real bug.
> As I said previous, since no journal writes after inserting a replace key in
> writeback, so this issue has nothing to do with journal.
> 
> This is a real problem in my environment, after running IO sometimes, I turn off
> the power suddenly,  then turn on the power, and the machine crash in
> read_dirty() due to the stale && dirty keys.