[PATCH v4 0/2] loop: Better discard for block devices

[PATCH v4 0/2] loop: Better discard for block devices

[Evan Green]

This series addresses some errors seen when using the loop
device directly backed by a block device. The first change plumbs
out the correct error message, and the second change prevents the
error from occurring in many cases.

The errors look like this:
[   90.880875] print_req_error: I/O error, dev loop5, sector 0

The errors occur when trying to do a discard or write zeroes operation
on a loop device backed by a block device that does not support write zeroes.
Firstly, the error itself is incorrectly reported as I/O error, but is
actually EOPNOTSUPP. The first patch plumbs out EOPNOTSUPP to properly
report the error.

The second patch prevents these errors from occurring by mirroring the
zeroing capabilities of the underlying block device into the loop device.
Before this change, discard was always reported as being supported, and
the loop device simply turns around and does an fallocate operation on the
backing device. After this change, backing block devices that do support
zeroing will continue to work as before, and continue to get all the
benefits of doing that. Backing devices that do not support zeroing will
fail earlier, avoiding hitting the loop device at all and ultimately
avoiding this error in the logs.

I can also confirm that this fixes test block/003 in the blktests, when
running blktests on a loop device backed by a block device.


Changes in v4:
- Mirror blkdev's write_zeroes into loopdev's discard_sectors.

Changes in v3:
- Updated tags
- Updated commit description

Changes in v2:
- Unnested error if statement (Bart)

Evan Green (2):
  loop: Report EOPNOTSUPP properly
  loop: Better discard support for block devices

 drivers/block/loop.c | 66 +++++++++++++++++++++++++++++---------------
 1 file changed, 44 insertions(+), 22 deletions(-)

-- 
2.20.1

[PATCH v4 1/2] loop: Report EOPNOTSUPP properly

[Evan Green]

Properly plumb out EOPNOTSUPP from loop driver operations, which may
get returned when for instance a discard operation is attempted but not
supported by the underlying block device. Before this change, everything
was reported in the log as an I/O error, which is scary and not
helpful in debugging.

Signed-off-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
---

Changes in v4: None
Changes in v3:
- Updated tags

Changes in v2:
- Unnested error if statement (Bart)

 drivers/block/loop.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index bf1c61cab8eb..bbf21ebeccd3 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -458,7 +458,9 @@ static void lo_complete_rq(struct request *rq)
 
 	if (!cmd->use_aio || cmd->ret < 0 || cmd->ret == blk_rq_bytes(rq) ||
 	    req_op(rq) != REQ_OP_READ) {
-		if (cmd->ret < 0)
+		if (cmd->ret == -EOPNOTSUPP)
+			ret = BLK_STS_NOTSUPP;
+		else if (cmd->ret < 0)
 			ret = BLK_STS_IOERR;
 		goto end_io;
 	}
@@ -1892,7 +1894,10 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
  failed:
 	/* complete non-aio request */
 	if (!cmd->use_aio || ret) {
-		cmd->ret = ret ? -EIO : 0;
+		if (ret == -EOPNOTSUPP)
+			cmd->ret = ret;
+		else
+			cmd->ret = ret ? -EIO : 0;
 		blk_mq_complete_request(rq);
 	}
 }
-- 
2.20.1

[PATCH v4 2/2] loop: Better discard support for block devices

[Evan Green]

If the backing device for a loop device is a block device,
then mirror the "write zeroes" capabilities of the underlying
block device into the loop device. Copy this capability into both
max_write_zeroes_sectors and max_discard_sectors of the loop device.

The reason for this is that REQ_OP_DISCARD on a loop device translates
into blkdev_issue_zeroout(), rather than blkdev_issue_discard(). This
presents a consistent interface for loop devices (that discarded data
is zeroed), regardless of the backing device type of the loop device.
There should be no behavior change for loop devices backed by regular
files.

While in there, differentiate between REQ_OP_DISCARD and
REQ_OP_WRITE_ZEROES, which are different for block devices,
but which the loop device had just been lumping together, since
they're largely the same for files.

This change fixes blktest block/003, and removes an extraneous
error print in block/013 when testing on a loop device backed
by a block device that does not support discard.

Signed-off-by: Evan Green <evgreen@chromium.org>
---

Changes in v4:
- Mirror blkdev's write_zeroes into loopdev's discard_sectors.

Changes in v3:
- Updated commit description

Changes in v2: None

 drivers/block/loop.c | 57 ++++++++++++++++++++++++++++----------------
 1 file changed, 37 insertions(+), 20 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index bbf21ebeccd3..ca6983a2c975 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -417,19 +417,14 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
 	return ret;
 }
 
-static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
+static int lo_discard(struct loop_device *lo, struct request *rq,
+		int mode, loff_t pos)
 {
-	/*
-	 * We use punch hole to reclaim the free space used by the
-	 * image a.k.a. discard. However we do not support discard if
-	 * encryption is enabled, because it may give an attacker
-	 * useful information.
-	 */
 	struct file *file = lo->lo_backing_file;
-	int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
+	struct request_queue *q = lo->lo_queue;
 	int ret;
 
-	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
+	if (!blk_queue_discard(q)) {
 		ret = -EOPNOTSUPP;
 		goto out;
 	}
@@ -599,8 +594,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
 	case REQ_OP_FLUSH:
 		return lo_req_flush(lo, rq);
 	case REQ_OP_DISCARD:
+		return lo_discard(lo, rq,
+			FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pos);
+
 	case REQ_OP_WRITE_ZEROES:
-		return lo_discard(lo, rq, pos);
+		return lo_discard(lo, rq,
+			FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE, pos);
+
 	case REQ_OP_WRITE:
 		if (lo->transfer)
 			return lo_write_transfer(lo, rq, pos);
@@ -854,6 +854,21 @@ static void loop_config_discard(struct loop_device *lo)
 	struct file *file = lo->lo_backing_file;
 	struct inode *inode = file->f_mapping->host;
 	struct request_queue *q = lo->lo_queue;
+	struct request_queue *backingq;
+
+	/*
+	 * If the backing device is a block device, mirror its zeroing
+	 * capability. REQ_OP_DISCARD translates to a zero-out even when backed
+	 * by block devices to keep consistent behavior with file-backed loop
+	 * devices.
+	 */
+	if (S_ISBLK(inode->i_mode)) {
+		backingq = bdev_get_queue(inode->i_bdev);
+		blk_queue_max_discard_sectors(q,
+			backingq->limits.max_write_zeroes_sectors);
+
+		blk_queue_max_write_zeroes_sectors(q,
+			backingq->limits.max_write_zeroes_sectors);
 
 	/*
 	 * We use punch hole to reclaim the free space used by the
@@ -861,22 +876,24 @@ static void loop_config_discard(struct loop_device *lo)
 	 * encryption is enabled, because it may give an attacker
 	 * useful information.
 	 */
-	if ((!file->f_op->fallocate) ||
-	    lo->lo_encrypt_key_size) {
+	} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
 		q->limits.discard_granularity = 0;
 		q->limits.discard_alignment = 0;
 		blk_queue_max_discard_sectors(q, 0);
 		blk_queue_max_write_zeroes_sectors(q, 0);
-		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
-		return;
-	}
 
-	q->limits.discard_granularity = inode->i_sb->s_blocksize;
-	q->limits.discard_alignment = 0;
+	} else {
+		q->limits.discard_granularity = inode->i_sb->s_blocksize;
+		q->limits.discard_alignment = 0;
+
+		blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
+		blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
+	}
 
-	blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
-	blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
-	blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
+	if (q->limits.max_write_zeroes_sectors)
+		blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
+	else
+		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
 }
 
 static void loop_unprepare_queue(struct loop_device *lo)
-- 
2.20.1

Re: [PATCH v4 2/2] loop: Better discard support for block devices

[Evan Green]

On Thu, May 2, 2019 at 10:44 AM Evan Green <evgreen@chromium.org> wrote:
>
> If the backing device for a loop device is a block device,
> then mirror the "write zeroes" capabilities of the underlying
> block device into the loop device. Copy this capability into both
> max_write_zeroes_sectors and max_discard_sectors of the loop device.
>
> The reason for this is that REQ_OP_DISCARD on a loop device translates
> into blkdev_issue_zeroout(), rather than blkdev_issue_discard(). This
> presents a consistent interface for loop devices (that discarded data
> is zeroed), regardless of the backing device type of the loop device.
> There should be no behavior change for loop devices backed by regular
> files.
>
> While in there, differentiate between REQ_OP_DISCARD and
> REQ_OP_WRITE_ZEROES, which are different for block devices,
> but which the loop device had just been lumping together, since
> they're largely the same for files.
>
> This change fixes blktest block/003, and removes an extraneous
> error print in block/013 when testing on a loop device backed
> by a block device that does not support discard.
>
> Signed-off-by: Evan Green <evgreen@chromium.org>
> ---
>
> Changes in v4:
> - Mirror blkdev's write_zeroes into loopdev's discard_sectors.
>
> Changes in v3:
> - Updated commit description
>
> Changes in v2: None
>
>  drivers/block/loop.c | 57 ++++++++++++++++++++++++++++----------------
>  1 file changed, 37 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index bbf21ebeccd3..ca6983a2c975 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -417,19 +417,14 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
>         return ret;
>  }
>
> -static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
> +static int lo_discard(struct loop_device *lo, struct request *rq,
> +               int mode, loff_t pos)
>  {
> -       /*
> -        * We use punch hole to reclaim the free space used by the
> -        * image a.k.a. discard. However we do not support discard if
> -        * encryption is enabled, because it may give an attacker
> -        * useful information.
> -        */
>         struct file *file = lo->lo_backing_file;
> -       int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
> +       struct request_queue *q = lo->lo_queue;
>         int ret;
>
> -       if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> +       if (!blk_queue_discard(q)) {
>                 ret = -EOPNOTSUPP;
>                 goto out;
>         }
> @@ -599,8 +594,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
>         case REQ_OP_FLUSH:
>                 return lo_req_flush(lo, rq);
>         case REQ_OP_DISCARD:
> +               return lo_discard(lo, rq,
> +                       FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pos);
> +
>         case REQ_OP_WRITE_ZEROES:
> -               return lo_discard(lo, rq, pos);
> +               return lo_discard(lo, rq,
> +                       FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE, pos);
> +
>         case REQ_OP_WRITE:
>                 if (lo->transfer)
>                         return lo_write_transfer(lo, rq, pos);
> @@ -854,6 +854,21 @@ static void loop_config_discard(struct loop_device *lo)
>         struct file *file = lo->lo_backing_file;
>         struct inode *inode = file->f_mapping->host;
>         struct request_queue *q = lo->lo_queue;
> +       struct request_queue *backingq;
> +
> +       /*
> +        * If the backing device is a block device, mirror its zeroing
> +        * capability. REQ_OP_DISCARD translates to a zero-out even when backed
> +        * by block devices to keep consistent behavior with file-backed loop
> +        * devices.
> +        */
> +       if (S_ISBLK(inode->i_mode)) {

Gwendal pointed out elsewhere that this should be if
(S_ISBLK(inode->i_mode) && (lo->lo_encrypt_key_size == 0)). I think
that's correct because like the file-backed device, we want to fail
discard, forcing the user to manually zero out regions and write out
the encrypted zeroes. I'll plan to send a v5 soon.