[PATCH V2 1/1] loop: introduce LO_FLAGS_NO_DEALLOC

[Zhang Boyang <zhangboyang.id@gmail.com>]

Previously, for file-backed loop devices, REQ_OP_DISCARD and
REQ_OP_WRITE_ZEROES (without REQ_NOUNMAP) are implemented using
fallocate(FALLOC_FL_PUNCH_HOLE), which will cause the underlying file to
be sparse and disk space freed. The users have no choice to prevent this
this from happening.

This patch introduces LO_FLAGS_NO_DEALLOC. With this flag set,
REQ_OP_DISCARD and REQ_OP_WRITE_ZEROES are forced to use
fallocate(FALLOC_FL_ZERO_RANGE). The disk space of underlying file is
kept allocated. This is useful if users, for example, want to use a
preallocated file as the backing file.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
---
 drivers/block/loop.c      | 17 +++++++++++++++--
 include/uapi/linux/loop.h | 15 +++++++++++----
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 084f9b8a0ba3..36bd9906a154 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -483,11 +483,15 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
 		 * write zeroes the range.  Otherwise, punch them out.
 		 */
 		return lo_fallocate(lo, rq, pos,
-			(rq->cmd_flags & REQ_NOUNMAP) ?
+			((rq->cmd_flags & REQ_NOUNMAP) ||
+			 (lo->lo_flags & LO_FLAGS_NO_DEALLOC)) ?
 				FALLOC_FL_ZERO_RANGE :
 				FALLOC_FL_PUNCH_HOLE);
 	case REQ_OP_DISCARD:
-		return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
+		return lo_fallocate(lo, rq, pos,
+			(lo->lo_flags & LO_FLAGS_NO_DEALLOC) ?
+				FALLOC_FL_ZERO_RANGE :
+				FALLOC_FL_PUNCH_HOLE);
 	case REQ_OP_WRITE:
 		if (cmd->use_aio)
 			return lo_rw_aio(lo, cmd, pos, WRITE);
@@ -719,12 +723,20 @@ static ssize_t loop_attr_dio_show(struct loop_device *lo, char *buf)
 	return sysfs_emit(buf, "%s\n", dio ? "1" : "0");
 }
 
+static ssize_t loop_attr_no_dealloc_show(struct loop_device *lo, char *buf)
+{
+	int no_dealloc = (lo->lo_flags & LO_FLAGS_NO_DEALLOC);
+
+	return sysfs_emit(buf, "%s\n", no_dealloc ? "1" : "0");
+}
+
 LOOP_ATTR_RO(backing_file);
 LOOP_ATTR_RO(offset);
 LOOP_ATTR_RO(sizelimit);
 LOOP_ATTR_RO(autoclear);
 LOOP_ATTR_RO(partscan);
 LOOP_ATTR_RO(dio);
+LOOP_ATTR_RO(no_dealloc);
 
 static struct attribute *loop_attrs[] = {
 	&loop_attr_backing_file.attr,
@@ -733,6 +745,7 @@ static struct attribute *loop_attrs[] = {
 	&loop_attr_autoclear.attr,
 	&loop_attr_partscan.attr,
 	&loop_attr_dio.attr,
+	&loop_attr_no_dealloc.attr,
 	NULL,
 };
 
diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
index 6f63527dd2ed..91a0a8b1f298 100644
--- a/include/uapi/linux/loop.h
+++ b/include/uapi/linux/loop.h
@@ -18,17 +18,24 @@ enum {
 	LO_FLAGS_AUTOCLEAR	= 4,
 	LO_FLAGS_PARTSCAN	= 8,
 	LO_FLAGS_DIRECT_IO	= 16,
+	LO_FLAGS_NO_DEALLOC	= 32,
 };
 
 /* LO_FLAGS that can be set using LOOP_SET_STATUS(64) */
-#define LOOP_SET_STATUS_SETTABLE_FLAGS (LO_FLAGS_AUTOCLEAR | LO_FLAGS_PARTSCAN)
+#define LOOP_SET_STATUS_SETTABLE_FLAGS (LO_FLAGS_AUTOCLEAR		\
+					| LO_FLAGS_PARTSCAN		\
+					| LO_FLAGS_NO_DEALLOC)
 
 /* LO_FLAGS that can be cleared using LOOP_SET_STATUS(64) */
-#define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR)
+#define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR		\
+					 | LO_FLAGS_NO_DEALLOC)
 
 /* LO_FLAGS that can be set using LOOP_CONFIGURE */
-#define LOOP_CONFIGURE_SETTABLE_FLAGS (LO_FLAGS_READ_ONLY | LO_FLAGS_AUTOCLEAR \
-				       | LO_FLAGS_PARTSCAN | LO_FLAGS_DIRECT_IO)
+#define LOOP_CONFIGURE_SETTABLE_FLAGS (LO_FLAGS_READ_ONLY		\
+				       | LO_FLAGS_AUTOCLEAR		\
+				       | LO_FLAGS_PARTSCAN		\
+				       | LO_FLAGS_DIRECT_IO		\
+				       | LO_FLAGS_NO_DEALLOC)
 
 #include <asm/posix_types.h>	/* for __kernel_old_dev_t */
 #include <linux/types.h>	/* for __u64 */
-- 
2.30.2