linux-kernel - [PATCH v2 7/7] KVM: arm64: Use less bits for hyp

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Message-Id: <20210602094347.3730846-8-qperret@google.com>
Date:   Wed,  2 Jun 2021 09:43:47 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     maz@...nel.org, will@...nel.org, james.morse@....com,
        alexandru.elisei@....com, catalin.marinas@....com,
        suzuki.poulose@....com
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org
Subject: [PATCH v2 7/7] 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@...gle.com>
---
 arch/arm64/kvm/hyp/include/nvhe/memory.h | 2 +-
 arch/arm64/kvm/hyp/nvhe/page_alloc.c     | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h
index 3fe34fa30ea4..592b7edb3edb 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/memory.h
+++ b/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;
 };
 
diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
index be07055bbc10..41fc25bdfb34 100644
--- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
+++ b/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++;
 }
 
-- 
2.32.0.rc0.204.g9fa02ecfa5-goog