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ext2 and ext3 per-blockgroup metadata needs locking. An fs-wide lock is
expensive, and a per-blockgroup lock consumes too much storage (up to 32768
blockgroups per filesystem). We need something in-between.
blockgroup_locks are very simple hashed spinlocks which provide this
compromise. The size of the lock is scaled by NR_CPUS to implement an
additional speed/space tradeoff.
These locks are actually fairly generic. However I presented it as something
which is specific to ext2 and ext3 so that people wouldn't go using them all
over the place. They consume a lot of storage.
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