From time to time people report space map corruption resulting in panic

(ss == NULL) on pool import. I had such a panic recently. With current version
of ZFS it is still possible to import the pool in readonly mode and backup
all the data, but in case it is impossible for some reason add tunable
vfs.zfs.space_map_last_hope, which when set to '1' will tell ZFS to remove
colliding range and retry. This seems to have worked for me, but I consider
it highly risky to use.

MFC after:	1 week

1 files changed, 25 insertions, 1 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/space_map.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/space_map.c
index 1ce7b2a..a8df1c7 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/space_map.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/space_map.c
@@ -29,6 +29,13 @@
 #include <sys/zio.h>
 #include <sys/space_map.h>
 
+SYSCTL_DECL(_vfs_zfs);
+static int space_map_last_hope;
+TUNABLE_INT("vfs.zfs.space_map_last_hope", &space_map_last_hope);
+SYSCTL_INT(_vfs_zfs, OID_AUTO, space_map_last_hope, CTLFLAG_RDTUN,
+    &space_map_last_hope, 0,
+    "If kernel panic in space_map code on pool import, import the pool in readonly mode and backup all your data before trying this option.");
+
 /*
  * Space map routines.
  * NOTE: caller is responsible for all locking.
@@ -93,7 +100,7 @@ space_map_add(space_map_t *sm, uint64_t start, uint64_t size)
 	VERIFY(sm->sm_space + size <= sm->sm_size);
 	VERIFY(P2PHASE(start, 1ULL << sm->sm_shift) == 0);
 	VERIFY(P2PHASE(size, 1ULL << sm->sm_shift) == 0);
-
+again:
 	ssearch.ss_start = start;
 	ssearch.ss_end = end;
 	ss = avl_find(&sm->sm_root, &ssearch, &where);
@@ -104,6 +111,23 @@ space_map_add(space_map_t *sm, uint64_t start, uint64_t size)
 		    (longlong_t)start, (longlong_t)size);
 		return;
 	}
+	if (ss != NULL && space_map_last_hope) {
+		uint64_t sstart, ssize;
+
+		if (ss->ss_start > start)
+			sstart = ss->ss_start;
+		else
+			sstart = start;
+		if (ss->ss_end > end)
+			ssize = end - sstart;
+		else
+			ssize = ss->ss_end - sstart;
+		ZFS_LOG(0,
+		    "Removing colliding space_map range (start=%ju end=%ju). Good luck!",
+		    (uintmax_t)sstart, (uintmax_t)(sstart + ssize));
+		space_map_remove(sm, sstart, ssize);
+		goto again;
+	}
 
 	/* Make sure we don't overlap with either of our neighbors */
 	VERIFY(ss == NULL);