btrfs: add zeroout mode to CLEAR_FREE ioctl

Add new type of clearing that will write zeros to the unused space
(similar to what trim/discard would do).

The mode is implemented by blkdev_issue_zeroout() that can write zeros
to the blocks explicitly unless the hardware implements UNMAP command
that unmaps the blocks that effectively appear as zeroed. This is
handled transparently.

As a special case of thin provisioning device, the UNMAP is usually
handled and can free the underlying space.

Signed-off-by: David Sterba <[email protected]>

Author: kdave

fs/btrfs/extent-tree.c

@@ -1250,10 +1250,16 @@ static int remove_extent_backref(struct btrfs_trans_handle *trans,
 static int btrfs_issue_clear_op(struct block_device *bdev, u64 start, u64 size,
 				enum btrfs_clear_op_type clear)
 {
+	unsigned int flags = BLKDEV_ZERO_KILLABLE;
+
 	switch (clear) {
 	case BTRFS_CLEAR_OP_DISCARD:
 		return blkdev_issue_discard(bdev, start >> SECTOR_SHIFT,
 					    size >> SECTOR_SHIFT, GFP_NOFS);
+	case BTRFS_CLEAR_OP_ZERO:
+		return blkdev_issue_zeroout(bdev, start >> SECTOR_SHIFT,
+					    size >> SECTOR_SHIFT, GFP_NOFS,
+					    flags);
 	default:
 		return -EOPNOTSUPP;
 	}

include/uapi/linux/btrfs.h

@@ -1101,6 +1101,12 @@ enum btrfs_err_code {
  */
 enum btrfs_clear_op_type {
 	BTRFS_CLEAR_OP_DISCARD,
+	/*
+	 * Write zeros to the range, either overwrite or with hardware offload
+	 * that can unmap the blocks internally.
+	 * (Same as blkdev_issue_zeroout() with 0 flags).
+	 */
+	BTRFS_CLEAR_OP_ZERO,
 	BTRFS_NR_CLEAR_OP_TYPES,
 };