s390/dasd: Fix zero write for FBA devices

A discard request that writes zeros using the global kernel internal
ZERO_PAGE will fail for machines with more than 2GB of memory due to the
location of the ZERO_PAGE.

Fix this by using a driver owned global zero page allocated with GFP_DMA
flag set.

Fixes: 28b841b3 ("s390/dasd: Add discard support for FBA devices")
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Cc: <stable@vger.kernel.org> # 4.14+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/s390/block/dasd_fba.c

@@ -40,6 +40,7 @@
 MODULE_LICENSE("GPL");
 
 static struct dasd_discipline dasd_fba_discipline;
+static void *dasd_fba_zero_page;
 
 struct dasd_fba_private {
 	struct dasd_fba_characteristics rdc_data;
@@ -270,7 +271,7 @@ static void ccw_write_zero(struct ccw1 *ccw, int count)
 	ccw->cmd_code = DASD_FBA_CCW_WRITE;
 	ccw->flags |= CCW_FLAG_SLI;
 	ccw->count = count;
-	ccw->cda = (__u32) (addr_t) page_to_phys(ZERO_PAGE(0));
+	ccw->cda = (__u32) (addr_t) dasd_fba_zero_page;
 }
 
 /*
@@ -830,6 +831,11 @@ dasd_fba_init(void)
 	int ret;
 
 	ASCEBC(dasd_fba_discipline.ebcname, 4);
+
+	dasd_fba_zero_page = (void *)get_zeroed_page(GFP_KERNEL | GFP_DMA);
+	if (!dasd_fba_zero_page)
+		return -ENOMEM;
+
 	ret = ccw_driver_register(&dasd_fba_driver);
 	if (!ret)
 		wait_for_device_probe();
@@ -841,6 +847,7 @@ static void __exit
 dasd_fba_cleanup(void)
 {
 	ccw_driver_unregister(&dasd_fba_driver);
+	free_page((unsigned long)dasd_fba_zero_page);
 }
 
 module_init(dasd_fba_init);