2022-08-06 15:58:33

by Zhang Boyang

[permalink] [raw]
Subject: [PATCH V2 0/1] loop: introduce LO_FLAGS_NO_DEALLOC

Hi,

This patch gives userspace ability to prevent underlying file of loop
device to be sparse. Currently, if loop device is 'trimmed'
(BLKDISCARD), then underlying file will always become sparse and disk
space freed. This behaviour is good for thin provisioning but not good
for preallocated disk images. This patch introduces LO_FLAGS_NO_DEALLOC
flag, which will write zeroes to underlying file instead of punching
holes when BLKDISCARD is requested. Thus reducing file fragmentation of
preallocated disk images and improve performance.

I will also submit patches to util-linux to provide userspace support if
this patch is merged.

Changes in V1->V2:
Renamed NODEALLOC to NO_DEALLOC, to avoid confusion between NO_DEALLOC
and NODE_ALLOC. Suggested by Jens Axboe.

Best Regards,
Zhang Boyang



2022-08-06 16:08:51

by Zhang Boyang

[permalink] [raw]
Subject: [PATCH V2 1/1] loop: introduce LO_FLAGS_NO_DEALLOC

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 <[email protected]>
---
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