[PATCH] device-mapper: mirror log bitset fix

The linux bitset operators (test_bit, set_bit etc) work on arrays of "unsigned
long".  dm-log uses such bitsets but treats them as arrays of uint32_t, only
allocating and zeroing a multiple of 4 bytes (as 'clean_bits' is a uint32_t).

The patch below fixes this problem.

The problem is specific to 64-bit big endian machines such as s390x or ppc-64
and can prevent pvmove terminating.

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

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/md/dm-log.c

@@ -333,10 +333,10 @@ static int core_ctr(struct dirty_log *log, struct dm_target *ti,
 	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;