The meaning of first_logical_block in do_mpage_readpage

hi, it may be a stupid question, but I am not quite clear about what
"first_logical_block" is used for as one of parameters of
"do_mpage_readpage".

I searched all the source code and I found that "first_logical_block"
always equals zero when passed to "do_mpage_readpage" and is  used
only in scope of "do_mpage_readpage".

This makes me wonder about:
1. when should "first_logical_block" passed as non-zero values ?
2. why "first_logical_block" is one of args of "do_mpage_readpage"
instead of being a local variable of it ? ( it seems that no one uses
"first_logical_block" outside  "do_mpage_readpage")

Anyone can help me out .
Thanks in advance!
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