uz_bucket_size_max is the maximum permitted bucket size. When filling a
new bucket to satisfy uma_zalloc(), the bucket is populated with at most
uz_bucket_size_max items. The maximum number of entries in the bucket
may be larger. When freeing items, however, we will fill per-CPPU
buckets up to their maximum number of entries, potentially exceeding
uz_bucket_size_max. This makes it difficult to precisely limit the
number of items that may be cached in a zone. For example, if one wants
to limit buckets to 1 entry for a particular zone, that's not possible
since the smallest bucket holds up to 2 entries.
Try to solve the problem by using uz_bucket_size_max to limit the number
of entries in a bucket. Note that the ub_entries field is initialized
upon every bucket allocation. Most zones are not affected since they do
not impose any specific limit on the maximum bucket size.
While here, remove the UMA_ZONE_MINBUCKET flag. It was unused and we
now have uma_zone_set_maxcache() to control the zone's cache size more
precisely.
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27167
f8b6c51538