s390/dasd: Add discard support for FBA devices

The z/VM hypervisor provides virtual disks (VDISK) which are backed by
main memory of the hypervisor. Those devices are seen as DASD FBA disks
within the Linux guest.

Whenever data is written to such a device, memory is allocated
on-the-fly by z/VM accordingly. This memory, however, is not being freed
if data on the device is deleted by the guest OS.

In order to make memory usable after deletion again, add discard support
to the FBA discipline.

While at it, update comments regarding the DASD_FEATURE_* flags.

Reviewed-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 199 insertions, 3 deletions

diff --git a/drivers/s390/block/dasd_fba.c b/drivers/s390/block/dasd_fba.c
index 462cab5d4302..6168ccdb389c 100644
--- a/drivers/s390/block/dasd_fba.c
+++ b/drivers/s390/block/dasd_fba.c
@@ -174,6 +174,9 @@ dasd_fba_check_characteristics(struct dasd_device *device)
 	if (readonly)
 		set_bit(DASD_FLAG_DEVICE_RO, &device->flags);
 
+	/* FBA supports discard, set the according feature bit */
+	dasd_set_feature(cdev, DASD_FEATURE_DISCARD, 1);
+
 	dev_info(&device->cdev->dev,
 		 "New FBA DASD %04X/%02X (CU %04X/%02X) with %d MB "
 		 "and %d B/blk%s\n",
@@ -247,9 +250,192 @@ static void dasd_fba_check_for_device_change(struct dasd_device *device,
 		dasd_generic_handle_state_change(device);
 };
 
-static struct dasd_ccw_req *dasd_fba_build_cp(struct dasd_device * memdev,
-					      struct dasd_block *block,
-					      struct request *req)
+
+/*
+ * Builds a CCW with no data payload
+ */
+static void ccw_write_no_data(struct ccw1 *ccw)
+{
+	ccw->cmd_code = DASD_FBA_CCW_WRITE;
+	ccw->flags |= CCW_FLAG_SLI;
+	ccw->count = 0;
+}
+
+/*
+ * Builds a CCW that writes only zeroes.
+ */
+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));
+}
+
+/*
+ * Helper function to count the amount of necessary CCWs within a given range
+ * with 4k alignment and command chaining in mind.
+ */
+static int count_ccws(sector_t first_rec, sector_t last_rec,
+		      unsigned int blocks_per_page)
+{
+	sector_t wz_stop = 0, d_stop = 0;
+	int cur_pos = 0;
+	int count = 0;
+
+	if (first_rec % blocks_per_page != 0) {
+		wz_stop = first_rec + blocks_per_page -
+			(first_rec % blocks_per_page) - 1;
+		if (wz_stop > last_rec)
+			wz_stop = last_rec;
+		cur_pos = wz_stop - first_rec + 1;
+		count++;
+	}
+
+	if (last_rec - (first_rec + cur_pos) + 1 >= blocks_per_page) {
+		if ((last_rec - blocks_per_page + 1) % blocks_per_page != 0)
+			d_stop = last_rec - ((last_rec - blocks_per_page + 1) %
+					     blocks_per_page);
+		else
+			d_stop = last_rec;
+
+		cur_pos += d_stop - (first_rec + cur_pos) + 1;
+		count++;
+	}
+
+	if (cur_pos == 0 || first_rec + cur_pos - 1 < last_rec)
+		count++;
+
+	return count;
+}
+
+/*
+ * This function builds a CCW request for block layer discard requests.
+ * Each page in the z/VM hypervisor that represents certain records of an FBA
+ * device will be padded with zeros. This is a special behaviour of the WRITE
+ * command which is triggered when no data payload is added to the CCW.
+ *
+ * Note: Due to issues in some z/VM versions, we can't fully utilise this
+ * special behaviour. We have to keep a 4k (or 8 block) alignment in mind to
+ * work around those issues and write actual zeroes to the unaligned parts in
+ * the request. This workaround might be removed in the future.
+ */
+static struct dasd_ccw_req *dasd_fba_build_cp_discard(
+						struct dasd_device *memdev,
+						struct dasd_block *block,
+						struct request *req)
+{
+	struct LO_fba_data *LO_data;
+	struct dasd_ccw_req *cqr;
+	struct ccw1 *ccw;
+
+	sector_t wz_stop = 0, d_stop = 0;
+	sector_t first_rec, last_rec;
+
+	unsigned int blksize = block->bp_block;
+	unsigned int blocks_per_page;
+	int wz_count = 0;
+	int d_count = 0;
+	int cur_pos = 0; /* Current position within the extent */
+	int count = 0;
+	int cplength;
+	int datasize;
+	int nr_ccws;
+
+	first_rec = blk_rq_pos(req) >> block->s2b_shift;
+	last_rec =
+		(blk_rq_pos(req) + blk_rq_sectors(req) - 1) >> block->s2b_shift;
+	count = last_rec - first_rec + 1;
+
+	blocks_per_page = BLOCKS_PER_PAGE(blksize);
+	nr_ccws = count_ccws(first_rec, last_rec, blocks_per_page);
+
+	/* define extent + nr_ccws * locate record + nr_ccws * single CCW */
+	cplength = 1 + 2 * nr_ccws;
+	datasize = sizeof(struct DE_fba_data) +
+		nr_ccws * (sizeof(struct LO_fba_data) + sizeof(struct ccw1));
+
+	cqr = dasd_smalloc_request(DASD_FBA_MAGIC, cplength, datasize, memdev);
+	if (IS_ERR(cqr))
+		return cqr;
+
+	ccw = cqr->cpaddr;
+
+	define_extent(ccw++, cqr->data, WRITE, blksize, first_rec, count);
+	LO_data = cqr->data + sizeof(struct DE_fba_data);
+
+	/* First part is not aligned. Calculate range to write zeroes. */
+	if (first_rec % blocks_per_page != 0) {
+		wz_stop = first_rec + blocks_per_page -
+			(first_rec % blocks_per_page) - 1;
+		if (wz_stop > last_rec)
+			wz_stop = last_rec;
+		wz_count = wz_stop - first_rec + 1;
+
+		ccw[-1].flags |= CCW_FLAG_CC;
+		locate_record(ccw++, LO_data++, WRITE, cur_pos, wz_count);
+
+		ccw[-1].flags |= CCW_FLAG_CC;
+		ccw_write_zero(ccw++, wz_count * blksize);
+
+		cur_pos = wz_count;
+	}
+
+	/* We can do proper discard when we've got at least blocks_per_page blocks. */
+	if (last_rec - (first_rec + cur_pos) + 1 >= blocks_per_page) {
+		/* is last record at page boundary? */
+		if ((last_rec - blocks_per_page + 1) % blocks_per_page != 0)
+			d_stop = last_rec - ((last_rec - blocks_per_page + 1) %
+					     blocks_per_page);
+		else
+			d_stop = last_rec;
+
+		d_count = d_stop - (first_rec + cur_pos) + 1;
+
+		ccw[-1].flags |= CCW_FLAG_CC;
+		locate_record(ccw++, LO_data++, WRITE, cur_pos, d_count);
+
+		ccw[-1].flags |= CCW_FLAG_CC;
+		ccw_write_no_data(ccw++);
+
+		cur_pos += d_count;
+	}
+
+	/* We might still have some bits left which need to be zeroed. */
+	if (cur_pos == 0 || first_rec + cur_pos - 1 < last_rec) {
+		if (d_stop != 0)
+			wz_count = last_rec - d_stop;
+		else if (wz_stop != 0)
+			wz_count = last_rec - wz_stop;
+		else
+			wz_count = count;
+
+		ccw[-1].flags |= CCW_FLAG_CC;
+		locate_record(ccw++, LO_data++, WRITE, cur_pos, wz_count);
+
+		ccw[-1].flags |= CCW_FLAG_CC;
+		ccw_write_zero(ccw++, wz_count * blksize);
+	}
+
+	if (blk_noretry_request(req) ||
+	    block->base->features & DASD_FEATURE_FAILFAST)
+		set_bit(DASD_CQR_FLAGS_FAILFAST, &cqr->flags);
+
+	cqr->startdev = memdev;
+	cqr->memdev = memdev;
+	cqr->block = block;
+	cqr->expires = memdev->default_expires * HZ;	/* default 5 minutes */
+	cqr->retries = memdev->default_retries;
+	cqr->buildclk = get_tod_clock();
+	cqr->status = DASD_CQR_FILLED;
+
+	return cqr;
+}
+
+static struct dasd_ccw_req *dasd_fba_build_cp_regular(
+						struct dasd_device *memdev,
+						struct dasd_block *block,
+						struct request *req)
 {
 	struct dasd_fba_private *private = block->base->private;
 	unsigned long *idaws;
@@ -372,6 +558,16 @@ static struct dasd_ccw_req *dasd_fba_build_cp(struct dasd_device * memdev,
 	return cqr;
 }
 
+static struct dasd_ccw_req *dasd_fba_build_cp(struct dasd_device *memdev,
+					      struct dasd_block *block,
+					      struct request *req)
+{
+	if (req_op(req) == REQ_OP_DISCARD || req_op(req) == REQ_OP_WRITE_ZEROES)
+		return dasd_fba_build_cp_discard(memdev, block, req);
+	else
+		return dasd_fba_build_cp_regular(memdev, block, req);
+}
+
 static int
 dasd_fba_free_cp(struct dasd_ccw_req *cqr, struct request *req)
 {