Full buffer cache not working as expected

[Thanos Makatos <mcatos@ics.forth.gr>]

I run several times the following test and what I've seen is that when
the buffer cache becomes full, unneeded dirty buffer heads are not evicted
and no other memory allocation can happen (including reading a block
from the disk to the buffer cache). Should this happen?
Here's the code that reproduces this issue:

for(i = 0; i < get_capacity(bdev->bd_disk)/8; i++) {
        bh = __bread(bdev, i, PAGE_SIZE);
        lock_buffer(bh);
        memset(bh->b_data, 0, PAGE_SIZE);
        set_buffer_dirty(bh);
        unlock_buffer(bh);
        __brelse(bh);
}

I added some printks printing the buffer head's reference counter: after
the '__bread' ref count is 2, after '__brelse' ref count is 1. Shouldn't
these
numbers be 1 and 0 respectively? I added an extra 'put_bh' before (or
after) the '__brelse' but I get a BUG().