Hi,
I recently had to track down a problem with pvmove not terminating on
an s390x.
The patch below fixes the problem.
The problem is specific to 64-bit bigendian machines such as s390x or
ppc-64.
The linux bitset operators (test_bit, set_bit etc) work on arrays of
"unsigned long".
dm-log uses such bitsets but treats them often as arrays of uint32_t,
only allocateing a multiple of 4 bytes (as 'clean_bits' is a uint32_t).
Further, it only zeros a multiple of 4 bytes.
In the simplest case, if "region_count" were (say) 30, then
bitset_size (below) would be 4 and bitset_uint32_count would be 1.
Thus the memory for this butset, after allocation and zeroing would
be
0 0 0 0 X X X X
On a bigendian 64bit machine, bit 0 for this bitset is in the 8th
byte! (and every bit that dm-log would use would be in the X area).
0 0 0 0 X X X X
^
here
which hasn't been cleared properly.
As the dm-raid1 code only syncs and counts regions which have a 0 in
the 'sync_bits' bitset, and only finishes when it has counted high
enough, a large number of 1's among those 'X's will cause the sync to
not complete.
It is worth noting that the code uses the same bitsets for in-memory
and on-disk logs. As these bitsets are host-endian and host-sized,
this means that they cannot safely be moved between computers with
different architectures. I don't know if the dm doco makes this
clear...
NeilBrown
Signed-off-by: Neil Brown <neilb@xxxxxxx>
### Diffstat output
./drivers/md/dm-log.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff ./drivers/md/dm-log.c~current~ ./drivers/md/dm-log.c
--- ./drivers/md/dm-log.c~current~ 2005-09-14 14:19:51.000000000 +1000
+++ ./drivers/md/dm-log.c 2005-09-14 14:21:56.000000000 +1000
@@ -333,10 +333,10 @@ static int core_ctr(struct dirty_log *lo
lc->sync = sync;
/*
- * Work out how many words we need to hold the bitset.
+ * Work out how many "unsigned long"s we need to hold the bitset.
*/
bitset_size = dm_round_up(region_count,
- sizeof(*lc->clean_bits) << BYTE_SHIFT);
+ sizeof(unsigned long) << BYTE_SHIFT);
bitset_size >>= BYTE_SHIFT;
lc->bitset_uint32_count = bitset_size / 4;
