We may fail to reclaim because a region has reclaim disabled (usually because
it is in an allocating_section. Failed reclaims can cause high CPU usage
if all of the lower addresses happen to be in a reclaim-disabled region (this
is somewhat mitigated by the fact that checking for reclaim disabled is very
cheap), but worse, failing a segment reclaim can lead to reclaimed memory
being fragmented. This results in the original allocation continuing to fail.
To combat that, we limit the number of failed reclaims. If we reach the limit,
we fail the reclaim. The surrounding allocating_section will release the
reclaim_lock, and increase reserves, which will result in reclaim being
retried with all regions being reclaimable, and succeed in allocating
contiguous memory.
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