[PATCH v8 0/2] Better discard support for block devices

[PATCH v8 0/2] Better discard support for block devices

[Andrzej Pietrasiewicz]

The series is a respin of v7:

https://lore.kernel.org/lkml/20191114235008.185111-1-evgreen@chromium.org/

with "Reviewed-by" from Gwendal and Bart for PATCH 1/2
and an improved comment in loop_config_discard() in PATCH 2/2.

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 v8:
- Improved comment in loop_config_discard() (Darrick/Evan)

Changes in v7:
- Use errno_to_blk_status() (Christoph)
- Rebase on top of Darrick's patch
- Tweak opening line of commit description (Darrick)

Changes in v6:
- Updated tags

Changes in v5:
- Don't mirror discard if lo_encrypt_key_size is non-zero (Gwendal)

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



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

 drivers/block/loop.c | 48 ++++++++++++++++++++++++++++++++------------
 1 file changed, 35 insertions(+), 13 deletions(-)

-- 
2.17.1

[PATCH v8 1/2] loop: Report EOPNOTSUPP properly

[Andrzej Pietrasiewicz]

From: Evan Green <evgreen@chromium.org>

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: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
---
 drivers/block/loop.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 739b372a5112..6969be9a855a 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -461,7 +461,7 @@ 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)
-			ret = BLK_STS_IOERR;
+			ret = errno_to_blk_status(cmd->ret);
 		goto end_io;
 	}
 
@@ -1953,7 +1953,10 @@ 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);
 	}
 }
-- 
2.17.1

[PATCH v8 2/2] loop: Better discard support for block devices

[Andrzej Pietrasiewicz]

From: Evan Green <evgreen@chromium.org>

If the backing device for a loop device is itself 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.

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>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
[used updated version of Evan's comment in loop_config_discard()]
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
---
 drivers/block/loop.c | 41 ++++++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 11 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 6969be9a855a..d7f30338b8ec 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -427,11 +427,12 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
 	 * information.
 	 */
 	struct file *file = lo->lo_backing_file;
+	struct request_queue *q = lo->lo_queue;
 	int ret;
 
 	mode |= FALLOC_FL_KEEP_SIZE;
 
-	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
+	if (!blk_queue_discard(q)) {
 		ret = -EOPNOTSUPP;
 		goto out;
 	}
@@ -864,6 +865,22 @@ 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. Set the discard sectors to the block device's zeroing
+	 * capabilities because loop discards result in blkdev_issue_zeroout(),
+	 * not blkdev_issue_discard(). This maintains consistent behavior with
+	 * file-backed loop devices: discarded regions read back as zero.
+	 */
+	if (S_ISBLK(inode->i_mode) && !lo->lo_encrypt_key_size) {
+		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
@@ -871,22 +888,24 @@ 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_flag_set(QUEUE_FLAG_DISCARD, q);
+		blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
+		blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
+	}
+
+	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)
-- 
2.17.1

Re: [PATCH v8 1/2] loop: Report EOPNOTSUPP properly

[Christoph Hellwig]

On Thu, Apr 02, 2020 at 07:06:02PM +0200, Andrzej Pietrasiewicz wrote:
> From: Evan Green <evgreen@chromium.org>
> 
> 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: Gwendal Grignou <gwendal@chromium.org>
> Reviewed-by: Bart Van Assche <bvanassche@acm.org>
> Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v8 2/2] loop: Better discard support for block devices

[Christoph Hellwig]

On Thu, Apr 02, 2020 at 07:06:03PM +0200, Andrzej Pietrasiewicz wrote:
> From: Evan Green <evgreen@chromium.org>
> 
> If the backing device for a loop device is itself 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.
> 
> 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>
> Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
> Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
> [used updated version of Evan's comment in loop_config_discard()]
> Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
> ---
>  drivers/block/loop.c | 41 ++++++++++++++++++++++++++++++-----------
>  1 file changed, 30 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 6969be9a855a..d7f30338b8ec 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -427,11 +427,12 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
>  	 * information.
>  	 */
>  	struct file *file = lo->lo_backing_file;
> +	struct request_queue *q = lo->lo_queue;
>  	int ret;
>  
>  	mode |= FALLOC_FL_KEEP_SIZE;
>  
> -	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> +	if (!blk_queue_discard(q)) {
>  		ret = -EOPNOTSUPP;
>  		goto out;
>  	}
> @@ -864,6 +865,22 @@ 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. Set the discard sectors to the block device's zeroing
> +	 * capabilities because loop discards result in blkdev_issue_zeroout(),
> +	 * not blkdev_issue_discard(). This maintains consistent behavior with
> +	 * file-backed loop devices: discarded regions read back as zero.
> +	 */
> +	if (S_ISBLK(inode->i_mode) && !lo->lo_encrypt_key_size) {
> +		backingq = bdev_get_queue(inode->i_bdev);

The backingq could move into this local scope.

> +	} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {

No need for the inner braces.

But the actual functionality looks good to me.

Re: [PATCH v8 2/2] loop: Better discard support for block devices

[Andrzej Pietrasiewicz]

Hi Christoph,

W dniu 03.04.2020 o 08:39, Christoph Hellwig pisze:
> On Thu, Apr 02, 2020 at 07:06:03PM +0200, Andrzej Pietrasiewicz wrote:
>> From: Evan Green <evgreen@chromium.org>
>>

<snip>

>> +	struct request_queue *backingq;
>> +
>> +	/*
>> +	 * If the backing device is a block device, mirror its zeroing
>> +	 * capability. Set the discard sectors to the block device's zeroing
>> +	 * capabilities because loop discards result in blkdev_issue_zeroout(),
>> +	 * not blkdev_issue_discard(). This maintains consistent behavior with
>> +	 * file-backed loop devices: discarded regions read back as zero.
>> +	 */
>> +	if (S_ISBLK(inode->i_mode) && !lo->lo_encrypt_key_size) {
>> +		backingq = bdev_get_queue(inode->i_bdev);
> 
> The backingq could move into this local scope.
> 
>> +	} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> 
> No need for the inner braces.
> 
> But the actual functionality looks good to me.
> 

Would you A-b or R-b if I corrected the two small issues which you found?

Andrzej

Re: [PATCH v8 2/2] loop: Better discard support for block devices

[Christoph Hellwig]

On Fri, Apr 03, 2020 at 12:27:26PM +0200, Andrzej Pietrasiewicz wrote:
> > The backingq could move into this local scope.
> > 
> > > +	} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> > 
> > No need for the inner braces.
> > 
> > But the actual functionality looks good to me.
> > 
> 
> Would you A-b or R-b if I corrected the two small issues which you found?

Sure:

Reviewed-by: Christoph Hellwig <hch@lst.de>