This changes the basic search algorithm from a single search up and down
the tree to a full depth-first traversal to handle conditions where the
tree matches at a higher level but not a lower level.
Normally higher level blocks always point to matching blocks, but there
are cases where this does not happen:
1. Racing block pointer updates from dbuf_write_ready.
Before f664f1ee7f (#8946), both dbuf_write_ready and
dnode_next_offset held dn_struct_rwlock which protected against
pointer writes from concurrent syncs.
This no longer applies, so sync context can f.e. clear or fill all
L1->L0 BPs before the L2->L1 BP and higher BP's are updated.
dnode_free_range in particular can reach this case and skip over L1
blocks that need to be dirtied. Later, sync will panic in
free_children when trying to clear a non-dirty indirect block.
This case was found with ztest.
2. txg > 0, non-hole case. This is #11196.
Freeing blocks/dnodes breaks the assumption that a match at a higher
level implies a match at a lower level when filtering txg > 0.
Whenever some but not all L0 blocks are freed, the parent L1 block is
rewritten. Its updated L2->L1 BP reflects a newer birth txg.
Later when searching by txg, if the L1 block matches since the txg is
newer, it is possible that none of the remaining L1->L0 BPs match if
none have been updated.
The same behavior is possible with dnode search at L0.
This is reachable from dsl_destroy_head for synchronous freeing.
When this happens open context fails to free objects leaving sync
context stuck freeing potentially many objects.
This is also reachable from traverse_pool for extreme rewind where it
is theoretically possible that datasets not dirtied after txg are
skipped if the MOS has high enough indirection to trigger this case.
In both of these cases, without backtracking the search ends prematurely
as ESRCH result implies no more matches in the entire object.
Signed-off-by: Robert Evans <evansr@google.com>
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