powerpc/perf/hv-24x7: use kmem_cache instead of aligned stack allocations

Ian pointed out the use of __aligned(4096) caused rather large stack
consumption in single_24x7_request(), so use the kmem_cache
hv_page_cache (which we've already got set up for other allocations)
insead of allocating locally.

CC: Haren Myneni <hbabu@us.ibm.com>
Reported-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Cody P Schafer <dev@codyps.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/perf/hv-24x7.c

@@ -294,7 +294,7 @@ static unsigned long single_24x7_request(u8 domain, u32 offset, u16 ix,
 					 u16 lpar, u64 *res,
 					 bool success_expected)
 {
-	unsigned long ret;
+	unsigned long ret = -ENOMEM;
 
 	/*
 	 * request_buffer and result_buffer are not required to be 4k aligned,
@@ -304,7 +304,27 @@ static unsigned long single_24x7_request(u8 domain, u32 offset, u16 ix,
 	struct reqb {
 		struct hv_24x7_request_buffer buf;
 		struct hv_24x7_request req;
-	} __packed __aligned(4096) request_buffer = {
+	} __packed *request_buffer;
+
+	struct {
+		struct hv_24x7_data_result_buffer buf;
+		struct hv_24x7_result res;
+		struct hv_24x7_result_element elem;
+		__be64 result;
+	} __packed *result_buffer;
+
+	BUILD_BUG_ON(sizeof(*request_buffer) > 4096);
+	BUILD_BUG_ON(sizeof(*result_buffer) > 4096);
+
+	request_buffer = kmem_cache_zalloc(hv_page_cache, GFP_USER);
+	if (!request_buffer)
+		goto out;
+
+	result_buffer = kmem_cache_zalloc(hv_page_cache, GFP_USER);
+	if (!result_buffer)
+		goto out_free_request_buffer;
+
+	*request_buffer = (struct reqb) {
 		.buf = {
 			.interface_version = HV_24X7_IF_VERSION_CURRENT,
 			.num_requests = 1,
@@ -320,28 +340,27 @@ static unsigned long single_24x7_request(u8 domain, u32 offset, u16 ix,
 		}
 	};
 
-	struct resb {
-		struct hv_24x7_data_result_buffer buf;
-		struct hv_24x7_result res;
-		struct hv_24x7_result_element elem;
-		__be64 result;
-	} __packed __aligned(4096) result_buffer = {};
-
 	ret = plpar_hcall_norets(H_GET_24X7_DATA,
-			virt_to_phys(&request_buffer), sizeof(request_buffer),
-			virt_to_phys(&result_buffer),  sizeof(result_buffer));
+			virt_to_phys(request_buffer), sizeof(*request_buffer),
+			virt_to_phys(result_buffer),  sizeof(*result_buffer));
 
 	if (ret) {
 		if (success_expected)
-			pr_err_ratelimited("hcall failed: %d %#x %#x %d => 0x%lx (%ld) detail=0x%x failing ix=%x\n",
-					domain, offset, ix, lpar,
-					ret, ret,
-					result_buffer.buf.detailed_rc,
-					result_buffer.buf.failing_request_ix);
-		return ret;
+			pr_err_ratelimited("hcall failed: %d %#x %#x %d => "
+				"0x%lx (%ld) detail=0x%x failing ix=%x\n",
+				domain, offset, ix, lpar, ret, ret,
+				result_buffer->buf.detailed_rc,
+				result_buffer->buf.failing_request_ix);
+		goto out_free_result_buffer;
 	}
 
-	*res = be64_to_cpu(result_buffer.result);
+	*res = be64_to_cpu(result_buffer->result);
+
+out_free_result_buffer:
+	kfree(result_buffer);
+out_free_request_buffer:
+	kfree(request_buffer);
+out:
 	return ret;
 }