dm: table detect io beyond device
This patch fixes a panic on shrinking a DM device if there is
outstanding I/O to the part of the device that is being removed.
(Normally this doesn't happen - a filesystem would be resized first,
for example.)
The bug is that __clone_and_map() assumes dm_table_find_target()
always returns a valid pointer. It may fail if a bio arrives from the
block layer but its target sector is no longer included in the DM
btree.
This patch appends an empty entry to table->targets[] which will
be returned by a lookup beyond the end of the device.
After calling dm_table_find_target(), __clone_and_map() and target_message()
check for this condition using
dm_target_is_valid().
Sample test script to trigger oops:
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index 4b3faa4..177297a 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -112,6 +112,11 @@
int dm_table_any_congested(struct dm_table *t, int bdi_bits);
void dm_table_unplug_all(struct dm_table *t);
+/*
+ * To check the return value from dm_table_find_target().
+ */
+#define dm_target_is_valid(t) ((t)->table)
+
/*-----------------------------------------------------------------
* A registry of target types.
*---------------------------------------------------------------*/