KVM: arm64: Use less bits for hyp_page refcount

The hyp_page refcount is currently encoded on 4 bytes even though we
never need to count that many objects in a page. Make it 2 bytes to save
some space in the vmemmap.

As overflows are more likely to happen as well, make sure to catch those
with a BUG in the increment function.
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210608114518.748712-8-qperret@google.com

arch/arm64/kvm/hyp/include/nvhe/memory.h

@@ -8,7 +8,7 @@
 #include <linux/types.h>
 
 struct hyp_page {
-	unsigned int refcount;
+	unsigned short refcount;
 	unsigned short order;
 };
 

arch/arm64/kvm/hyp/nvhe/page_alloc.c

@@ -146,6 +146,7 @@ static struct hyp_page *__hyp_extract_page(struct hyp_pool *pool,
 
 static inline void hyp_page_ref_inc(struct hyp_page *p)
 {
+	BUG_ON(p->refcount == USHRT_MAX);
 	p->refcount++;
 }