base: statically check for reasonable Tslab block size

Per default Tslab checks that 8 slabs incl. overhead fit into one block.
If this is not desired the template parameter 'MIN_SLABS_PER_BLOCK' can
be used to control the minimum number of blocks.

Fixes #3834

repos/base/include/base/slab.h

@@ -109,13 +109,8 @@ class Genode::Slab : public Allocator
 		 */
 		~Slab();
 
-		/**
+		static constexpr size_t overhead_per_block() { return 4*sizeof(addr_t); }
-		 * Return number of bytes consumed per slab entry
+		static constexpr size_t overhead_per_entry() { return sizeof(addr_t) + 1; }
-		 *
-		 * The function takes the slab-internal meta-data needs and the actual
-		 * slab entry into account.
-		 */
-		static size_t entry_costs(size_t slab_size, size_t block_size);
 
 		/**
 		 * Return number of unused slab entries

repos/base/include/base/tslab.h

@@ -16,12 +16,16 @@
 
 #include <base/slab.h>
 
-namespace Genode { template <typename, size_t> struct Tslab; }
+namespace Genode { template <typename, size_t, unsigned> struct Tslab; }
 
 
-template <typename T, Genode::size_t BLOCK_SIZE>
+template <typename T, Genode::size_t BLOCK_SIZE, unsigned MIN_SLABS_PER_BLOCK = 8>
 struct Genode::Tslab : Slab
 {
+	/* check if reasonable amount of entries + overhead fits one block */
+	static_assert(MIN_SLABS_PER_BLOCK*(sizeof(T)+overhead_per_entry())
+	            + overhead_per_block() <= BLOCK_SIZE);
+
 	Tslab(Allocator *backing_store, void *initial_sb = 0)
 	: Slab(sizeof(T), BLOCK_SIZE, initial_sb, backing_store)
 	{ }

repos/base/src/lib/base/slab.cc

@@ -219,12 +219,14 @@ Slab::Slab(size_t slab_size, size_t block_size, void *initial_sb,
 	 */
 	_entries_per_block((_block_size - sizeof(Block) - sizeof(umword_t))
 	                   / (_slab_size + sizeof(Entry) + 1)),
-
 	_initial_sb((Block *)initial_sb),
 	_nested(false),
 	_curr_sb((Block *)initial_sb),
 	_backing_store(backing_store)
 {
+	static_assert(sizeof(Slab::Block) <= overhead_per_block());
+	static_assert(sizeof(Slab::Entry) <= overhead_per_entry());
+
 	/* if no initial slab block was specified, try to get one */
 	if (!_curr_sb && _backing_store)
 		_new_slab_block().with_result(