linux-kernel - [PATCH v6 18/22] KVM: Dynamically size memslot array based on number of used slots

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Date:   Tue, 18 Feb 2020 13:07:32 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Christoffer Dall <christoffer.dall@....com>,
        Peter Xu <peterx@...hat.com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>
Subject: [PATCH v6 18/22] KVM: Dynamically size memslot array based on number of used slots

Now that the memslot logic doesn't assume memslots are always non-NULL,
dynamically size the array of memslots instead of unconditionally
allocating memory for the maximum number of memslots.

Note, because a to-be-deleted memslot must first be invalidated, the
array size cannot be immediately reduced when deleting a memslot.
However, consecutive deletions will realize the memory savings, i.e.
a second deletion will trim the entry.

Tested-by: Christoffer Dall <christoffer.dall@....com>
Tested-by: Marc Zyngier <maz@...nel.org>
Reviewed-by: Peter Xu <peterx@...hat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 include/linux/kvm_host.h |  2 +-
 virt/kvm/kvm_main.c      | 31 ++++++++++++++++++++++++++++---
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 72b46d6f7f4a..f85cc9771ade 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -431,11 +431,11 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
  */
 struct kvm_memslots {
 	u64 generation;
-	struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
 	/* The mapping table from slot id to the index in memslots[]. */
 	short id_to_index[KVM_MEM_SLOTS_NUM];
 	atomic_t lru_slot;
 	int used_slots;
+	struct kvm_memory_slot memslots[];
 };
 
 struct kvm {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2f2c434ab2c9..0583143aca90 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -566,7 +566,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
 		return NULL;
 
 	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
-		slots->id_to_index[i] = slots->memslots[i].id = -1;
+		slots->id_to_index[i] = -1;
 
 	return slots;
 }
@@ -1078,6 +1078,32 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
 	return old_memslots;
 }
 
+/*
+ * Note, at a minimum, the current number of used slots must be allocated, even
+ * when deleting a memslot, as we need a complete duplicate of the memslots for
+ * use when invalidating a memslot prior to deleting/moving the memslot.
+ */
+static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
+					     enum kvm_mr_change change)
+{
+	struct kvm_memslots *slots;
+	size_t old_size, new_size;
+
+	old_size = sizeof(struct kvm_memslots) +
+		   (sizeof(struct kvm_memory_slot) * old->used_slots);
+
+	if (change == KVM_MR_CREATE)
+		new_size = old_size + sizeof(struct kvm_memory_slot);
+	else
+		new_size = old_size;
+
+	slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT);
+	if (likely(slots))
+		memcpy(slots, old, old_size);
+
+	return slots;
+}
+
 static int kvm_set_memslot(struct kvm *kvm,
 			   const struct kvm_userspace_memory_region *mem,
 			   struct kvm_memory_slot *old,
@@ -1088,10 +1114,9 @@ static int kvm_set_memslot(struct kvm *kvm,
 	struct kvm_memslots *slots;
 	int r;
 
-	slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT);
+	slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
 	if (!slots)
 		return -ENOMEM;
-	memcpy(slots, __kvm_memslots(kvm, as_id), sizeof(struct kvm_memslots));
 
 	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
 		/*
-- 
2.24.1