potential for buffer_head shrinkage.
From: Dave Jones
Date: Fri Dec 29 2006 - 21:47:27 EST
Looking at struct buffer_head, it seems that b_state
uses at most 15 bits, where it's defined as a 64bit entity
due to it being used by bit_spin_lock and friends.
Given it's not uncommon for a few hundred thousand of these
to be present, I wonder if it's worth the effort of folding
b_count into the upper bits of b_state, thus shrinking
buffer_head by 16 bits? This would still leave 32 bits
'wasted' for further bh_state_bits expansion if necessary.
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