why clear_buffer_uptodate in end_buffer_write_sync on write error?

[Robert W. Fuller]

I'm trying to understand the file systems.  I'm making good progress, 
but I don't get this.  If the buffer is marked not up to date because 
the write failed, won't this cause block_prepare_write to try and read 
the buffer using ll_rw_block thereby overwriting the data that couldn't 
be written?  Is this desirable?  Perhaps I'm confused?
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