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[dm-devel] [PATCH] dm snapshot: don't use zero chunksize

When playing with invalid chunksizes, I have found another bug: under 
certain conditions (too small devices or too big chunksize) the calculated 
hash table size is zero. This triggers invalid C operation (a shift by -1) 
and causes "out of memory" messages --- when there's really nothing out of 

This patch changes it to use minimum 64 hashsize.



Under some special conditions (too big chunk size or zero-sized device),
the resulting hash_size is calculated as zero.

rounddown_pow_of_two(0) is undefined operation (it expands to shift by -1).
And init_exception_table with zero argument would fail with -ENOMEM.

This patch makes minimum chunk size 64, just like for pending exception table.

Signed-off-by: Mikulas Patocka <mpatocka redhat com>

 drivers/md/dm-snap.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6.31-fast-new/drivers/md/dm-snap.c
--- linux-2.6.31-fast-new.orig/drivers/md/dm-snap.c	2009-10-07 00:32:09.000000000 +0200
+++ linux-2.6.31-fast-new/drivers/md/dm-snap.c	2009-10-07 00:33:04.000000000 +0200
@@ -570,6 +570,8 @@ static int init_hash_tables(struct dm_sn
 	hash_size = min(origin_dev_size, cow_dev_size) >> s->store->chunk_shift;
 	hash_size = min(hash_size, max_buckets);
+	if (hash_size < 64)
+		hash_size = 64;
 	hash_size = rounddown_pow_of_two(hash_size);
 	if (init_exception_table(&s->complete, hash_size,

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