to_cpy is the number of page pointers to copy from current cbt to new.
The following check:

>       if ((new_sz + bsz) >> cbt->block_bits <= cbt->block_max)
>               goto err_mtx;

ensures that the copy will be done only for new cbt bigger than current. So,
we have to calculate to_cpy based on the current (smaller) cbt. The rest of
new cbt is OK because it was nullified by do_cbt_alloc().

The bug existed since the very first version of CBT (commit ad7ba3dfe).

https://jira.sw.ru/browse/PSBM-48120

Signed-off-by: Maxim Patlasov <[email protected]>
---
 block/blk-cbt.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-cbt.c b/block/blk-cbt.c
index 001dbfd..3a2b197 100644
--- a/block/blk-cbt.c
+++ b/block/blk-cbt.c
@@ -448,7 +448,7 @@ void blk_cbt_update_size(struct block_device *bdev)
                set_bit(CBT_ERROR, &cbt->flags);
                goto err_mtx;
        }
-       to_cpy = NR_PAGES(new->block_max);
+       to_cpy = NR_PAGES(cbt->block_max);
        set_bit(CBT_NOCACHE, &cbt->flags);
        cbt_flush_cache(cbt);
        spin_lock_irq(&cbt->lock);

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