mempool: kvmalloc pool

Add mempool_init_kvmalloc_pool() and mempool_create_kvmalloc_pool(),
which wrap kvmalloc() instead of kmalloc() - kmalloc() with a vmalloc()
fallback.

This is part of a bcachefs cleanup - dropping an internal kvpmalloc()
helper (which predates kvmalloc()) along with mempool helpers; this
replaces the bcachefs-private kvpmalloc_pool.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: linux-mm@kvack.org

include/linux/mempool.h

@@ -95,6 +95,19 @@ static inline mempool_t *mempool_create_kmalloc_pool(int min_nr, size_t size)
 			      (void *) size);
 }
 
+void *mempool_kvmalloc(gfp_t gfp_mask, void *pool_data);
+void mempool_kvfree(void *element, void *pool_data);
+
+static inline int mempool_init_kvmalloc_pool(mempool_t *pool, int min_nr, size_t size)
+{
+	return mempool_init(pool, min_nr, mempool_kvmalloc, mempool_kvfree, (void *) size);
+}
+
+static inline mempool_t *mempool_create_kvmalloc_pool(int min_nr, size_t size)
+{
+	return mempool_create(min_nr, mempool_kvmalloc, mempool_kvfree, (void *) size);
+}
+
 /*
  * A mempool_alloc_t and mempool_free_t for a simple page allocator that
  * allocates pages of the order specified by pool_data

mm/mempool.c

@@ -590,6 +590,19 @@ void mempool_kfree(void *element, void *pool_data)
 }
 EXPORT_SYMBOL(mempool_kfree);
 
+void *mempool_kvmalloc(gfp_t gfp_mask, void *pool_data)
+{
+	size_t size = (size_t)pool_data;
+	return kvmalloc(size, gfp_mask);
+}
+EXPORT_SYMBOL(mempool_kvmalloc);
+
+void mempool_kvfree(void *element, void *pool_data)
+{
+	kvfree(element);
+}
+EXPORT_SYMBOL(mempool_kvfree);
+
 /*
  * A simple mempool-backed page allocator that allocates pages
  * of the order specified by pool_data.