uma: Avoid depleting keg reserves when filling a bucket

[markj]

Summary

zone_import() fetches a free or partially free slab from the keg and
then uses its items to populate an array, typically filling a bucket.
If a single allocation causes the keg to drop below its minimum reserve,
the inner loop ends. However, if the bucket is still not full and
M_USE_RESERVE is specified, the outer loop will continue to fetch items
from the keg.

If M_USE_RESERVE is specified and the number of free items is below the
reserved limit, we should return only a single item. Otherwise, if the
bucket size is larger than the reserve, all of the reserved items may
end up in a single per-CPU bucket, invisible to other CPUs.

[rlibby]

sys/vm/uma_core.c

3755–3759 ↗

(On Diff #78193)

Should this be conditional on actually being at or below the reserve limit?

[markj]

Modify zone_import() to return as soon as the free count dips below the
reserve. This is still not perfect, since for round-robin zones we could
potentially continue allocating from other domains.