KVM: ppc: Use a read lock around MMU operations, and release it on error

gfn_to_page() and kvm_release_page_clean() are called from other contexts with
mmap_sem locked only for reading.
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/powerpc/kvm/44x_tlb.c

@@ -142,18 +142,19 @@ void kvmppc_mmu_map(struct kvm_vcpu *vcpu, u64 gvaddr, gfn_t gfn, u64 asid,
 	stlbe = &vcpu->arch.shadow_tlb[victim];
 
 	/* Get reference to new page. */
-	down_write(&current->mm->mmap_sem);
+	down_read(&current->mm->mmap_sem);
 	new_page = gfn_to_page(vcpu->kvm, gfn);
 	if (is_error_page(new_page)) {
 		printk(KERN_ERR "Couldn't get guest page!\n");
 		kvm_release_page_clean(new_page);
+		up_read(&current->mm->mmap_sem);
 		return;
 	}
 	hpaddr = page_to_phys(new_page);
 
 	/* Drop reference to old page. */
 	kvmppc_44x_shadow_release(vcpu, victim);
-	up_write(&current->mm->mmap_sem);
+	up_read(&current->mm->mmap_sem);
 
 	vcpu->arch.shadow_pages[victim] = new_page;