slab: sheaf prefilling for guaranteed allocations

slab: sheaf prefilling for guaranteed allocations

Add functions for efficient guaranteed allocations e.g. in a critical
section that cannot sleep, when the exact number of allocations is not
known beforehand, but an upper limit can be calculated.

kmem_cache_prefill_sheaf() returns a sheaf containing at least given
number of objects.

kmem_cache_alloc_from_sheaf() will allocate an object from the sheaf
and is guaranteed not to fail until depleted.

kmem_cache_return_sheaf() is for giving the sheaf back to the slab
allocator after the critical section. This will also attempt to refill
it to cache's sheaf capacity for better efficiency of sheaves handling,
but it's not stricly necessary to succeed.

kmem_cache_refill_sheaf() can be used to refill a previously obtained
sheaf to requested size. If the current size is sufficient, it does
nothing. If the requested size exceeds cache's sheaf_capacity and the
sheaf's current capacity, the sheaf will be replaced with a new one,
hence the indirect pointer parameter.

kmem_cache_sheaf_size() can be used to query the current size.

The implementation supports requesting sizes that exceed cache's
sheaf_capacity, but it is not efficient - such sheaves are allocated
fresh in kmem_cache_prefill_sheaf() and flushed and freed immediately by
kmem_cache_return_sheaf(). kmem_cache_refill_sheaf() might be expecially
ineffective when replacing a sheaf with a new one of a larger capacity.
It is therefore better to size cache's sheaf_capacity accordingly.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>