[PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Martijn Coenen]

Configuring a loop device for a filesystem that is located at an offset
currently requires calling LOOP_SET_FD and LOOP_SET_STATUS(64)
consecutively. This has some downsides.

The most important downside is that it can be slow. Here's setting
up ~70 regular loop devices on an x86 Android device:

vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r /dev/block/loop$i com.android.adbd.apex; done
    0m01.85s real     0m00.01s user     0m00.01s system

Here's configuring ~70 devices in the same way, but with an offset:

vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
    0m03.40s real     0m00.02s user     0m00.03s system

This is almost twice as slow; the main reason for this slowness is that
LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to freeze the associated
queue; this requires waiting for RCU synchronization, which I've
measured can take about 15-20ms on this device on average.

A more minor downside of having to do two ioctls is that on devices with
max_part > 0, the kernel will initiate a partition scan, which is
needless work if the image is at an offset.

This change introduces a new ioctl to combine setting the backing file
together with the offset, which avoids the above problems. Adding more
parameters could be a consideration, but offset appears to be the only
commonly used parameter that is required for accessing the device
safely.

Signed-off-by: Martijn Coenen <maco@android.com>
---
 drivers/block/loop.c      | 25 +++++++++++++++++++------
 include/uapi/linux/loop.h |  6 ++++++
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index a42c49e04954..517031e1d10c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -932,8 +932,8 @@ static void loop_update_rotational(struct loop_device *lo)
 		blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
 }
 
-static int loop_set_fd(struct loop_device *lo, fmode_t mode,
-		       struct block_device *bdev, unsigned int arg)
+static int loop_set_fd_with_offset(struct loop_device *lo, fmode_t mode,
+		struct block_device *bdev, unsigned int arg, loff_t offset)
 {
 	struct file	*file;
 	struct inode	*inode;
@@ -957,7 +957,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	 * here to avoid changing device under exclusive owner.
 	 */
 	if (!(mode & FMODE_EXCL)) {
-		claimed_bdev = bd_start_claiming(bdev, loop_set_fd);
+		claimed_bdev = bd_start_claiming(bdev, loop_set_fd_with_offset);
 		if (IS_ERR(claimed_bdev)) {
 			error = PTR_ERR(claimed_bdev);
 			goto out_putf;
@@ -1002,6 +1002,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	lo->transfer = NULL;
 	lo->ioctl = NULL;
 	lo->lo_sizelimit = 0;
+	lo->lo_offset = offset;
 	lo->old_gfp_mask = mapping_gfp_mask(mapping);
 	mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
 
@@ -1042,14 +1043,14 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	if (partscan)
 		loop_reread_partitions(lo, bdev);
 	if (claimed_bdev)
-		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
+		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
 	return 0;
 
 out_unlock:
 	mutex_unlock(&loop_ctl_mutex);
 out_bdev:
 	if (claimed_bdev)
-		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
+		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
 out_putf:
 	fput(file);
 out:
@@ -1601,7 +1602,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
 
 	switch (cmd) {
 	case LOOP_SET_FD:
-		return loop_set_fd(lo, mode, bdev, arg);
+		return loop_set_fd_with_offset(lo, mode, bdev, arg, 0);
 	case LOOP_CHANGE_FD:
 		return loop_change_fd(lo, bdev, arg);
 	case LOOP_CLR_FD:
@@ -1624,6 +1625,17 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
 		break;
 	case LOOP_GET_STATUS64:
 		return loop_get_status64(lo, (struct loop_info64 __user *) arg);
+	case LOOP_SET_FD_WITH_OFFSET: {
+		struct loop_fd_with_offset fdwo;
+
+		if (copy_from_user(&fdwo,
+				(struct loop_fd_with_offset __user *) arg,
+				sizeof(struct loop_fd_with_offset)))
+			return -EFAULT;
+
+		return loop_set_fd_with_offset(lo, mode, bdev, fdwo.fd,
+				fdwo.lo_offset);
+	}
 	case LOOP_SET_CAPACITY:
 	case LOOP_SET_DIRECT_IO:
 	case LOOP_SET_BLOCK_SIZE:
@@ -1774,6 +1786,7 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
 	case LOOP_SET_CAPACITY:
 	case LOOP_CLR_FD:
 	case LOOP_GET_STATUS64:
+	case LOOP_SET_FD_WITH_OFFSET:
 	case LOOP_SET_STATUS64:
 		arg = (unsigned long) compat_ptr(arg);
 		/* fall through */
diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
index 080a8df134ef..289829bc5abd 100644
--- a/include/uapi/linux/loop.h
+++ b/include/uapi/linux/loop.h
@@ -60,6 +60,11 @@ struct loop_info64 {
 	__u64		   lo_init[2];
 };
 
+struct loop_fd_with_offset {
+	__u64          lo_offset;
+	__u32          fd;
+};
+
 /*
  * Loop filter types
  */
@@ -90,6 +95,7 @@ struct loop_info64 {
 #define LOOP_SET_CAPACITY	0x4C07
 #define LOOP_SET_DIRECT_IO	0x4C08
 #define LOOP_SET_BLOCK_SIZE	0x4C09
+#define LOOP_SET_FD_WITH_OFFSET	0x4C0A
 
 /* /dev/loop-control interface */
 #define LOOP_CTL_ADD		0x4C80
-- 
2.26.0.rc2.310.g2932bb562d-goog

Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Chaitanya Kulkarni]

On 03/29/2020 07:05 AM, Martijn Coenen wrote:
> Configuring a loop device for a filesystem that is located at an offset
> currently requires calling LOOP_SET_FD and LOOP_SET_STATUS(64)
> consecutively. This has some downsides.
>
> The most important downside is that it can be slow. Here's setting
> up ~70 regular loop devices on an x86 Android device:
>
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r /dev/block/loop$i com.android.adbd.apex; done
>      0m01.85s real     0m00.01s user     0m00.01s system
>
> Here's configuring ~70 devices in the same way, but with an offset:
>
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
>      0m03.40s real     0m00.02s user     0m00.03s system
>
> This is almost twice as slow; the main reason for this slowness is that
> LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to freeze the associated
> queue; this requires waiting for RCU synchronization, which I've
> measured can take about 15-20ms on this device on average.
>
> A more minor downside of having to do two ioctls is that on devices with
> max_part > 0, the kernel will initiate a partition scan, which is
> needless work if the image is at an offset.
>
> This change introduces a new ioctl to combine setting the backing file
> together with the offset, which avoids the above problems. Adding more
> parameters could be a consideration, but offset appears to be the only
> commonly used parameter that is required for accessing the device
> safely.
>
> Signed-off-by: Martijn Coenen<maco@android.com>

This patch seems to solve problem, can you please make sure to add a
blktest [1] for the same since it is a new IOCTL ?

[1] https://github.com/osandov/blktests.

Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Bart Van Assche]

On 2020-03-29 07:04, Martijn Coenen wrote:
> -static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> -		       struct block_device *bdev, unsigned int arg)
> +static int loop_set_fd_with_offset(struct loop_device *lo, fmode_t mode,
> +		struct block_device *bdev, unsigned int arg, loff_t offset)

Since this function has to be modified, please add an additional patch
to rename 'arg' into 'fd'. Additionally, how about renaming
"loop_set_fd_with_offset" into "loop_set_fd_and_offset"? I think the
latter name reflects more clearly the purpose of this function.

> @@ -1624,6 +1625,17 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
>  		break;
>  	case LOOP_GET_STATUS64:
>  		return loop_get_status64(lo, (struct loop_info64 __user *) arg);
> +	case LOOP_SET_FD_WITH_OFFSET: {
> +		struct loop_fd_with_offset fdwo;
> +
> +		if (copy_from_user(&fdwo,
> +				(struct loop_fd_with_offset __user *) arg,
> +				sizeof(struct loop_fd_with_offset)))
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The kernel code that I'm familiar with uses sizeof(<variable name>)
instead of sizeof(<struct name>). That makes it less likely that
changing the type of the variable will introduce a mismatch between the
sizeof() expression and the size of the variable.

Thanks,

Bart.

Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Ming Lei]

On Sun, Mar 29, 2020 at 04:04:59PM +0200, Martijn Coenen wrote:
> Configuring a loop device for a filesystem that is located at an offset
> currently requires calling LOOP_SET_FD and LOOP_SET_STATUS(64)
> consecutively. This has some downsides.
> 
> The most important downside is that it can be slow. Here's setting
> up ~70 regular loop devices on an x86 Android device:
> 
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r /dev/block/loop$i com.android.adbd.apex; done
>     0m01.85s real     0m00.01s user     0m00.01s system
> 
> Here's configuring ~70 devices in the same way, but with an offset:
> 
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
>     0m03.40s real     0m00.02s user     0m00.03s system
> 
> This is almost twice as slow; the main reason for this slowness is that
> LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to freeze the associated
> queue; this requires waiting for RCU synchronization, which I've
> measured can take about 15-20ms on this device on average.
> 
> A more minor downside of having to do two ioctls is that on devices with
> max_part > 0, the kernel will initiate a partition scan, which is
> needless work if the image is at an offset.
> 
> This change introduces a new ioctl to combine setting the backing file
> together with the offset, which avoids the above problems. Adding more
> parameters could be a consideration, but offset appears to be the only
> commonly used parameter that is required for accessing the device
> safely.

The new ioctl LOOP_SET_FD_WITH_OFFSET looks not generic enough, could
you consider to add one ioctl LOOP_SET_FD_AND_STATUS to cover both
SET_FD and SET_STATUS so that using two ioctl() to setup loop can become
deprecated finally?


Thanks,
Ming

Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Martijn Coenen]

Hi Ming,

On Mon, Mar 30, 2020 at 3:00 AM Ming Lei <ming.lei@redhat.com> wrote:
> The new ioctl LOOP_SET_FD_WITH_OFFSET looks not generic enough, could
> you consider to add one ioctl LOOP_SET_FD_AND_STATUS to cover both
> SET_FD and SET_STATUS so that using two ioctl() to setup loop can become
> deprecated finally?

I originally started out doing that. However, it is a significantly
larger refactoring of the loop driver, and it makes things like error
handling more complex. I thought configuring loop with an offset is
the most common case. But if there's a preference to do an ioctl that
takes the full status, I can work on that.

Best,
Martijn

>
>
> Thanks,
> Ming
>

Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Christoph Hellwig]

On Mon, Mar 30, 2020 at 10:06:41AM +0200, Martijn Coenen wrote:
> Hi Ming,
> 
> On Mon, Mar 30, 2020 at 3:00 AM Ming Lei <ming.lei@redhat.com> wrote:
> > The new ioctl LOOP_SET_FD_WITH_OFFSET looks not generic enough, could
> > you consider to add one ioctl LOOP_SET_FD_AND_STATUS to cover both
> > SET_FD and SET_STATUS so that using two ioctl() to setup loop can become
> > deprecated finally?
> 
> I originally started out doing that. However, it is a significantly
> larger refactoring of the loop driver, and it makes things like error
> handling more complex. I thought configuring loop with an offset is
> the most common case. But if there's a preference to do an ioctl that
> takes the full status, I can work on that.

I think the full blown set fd an status would seem a lot more useful,
or even better a LOOP_CTL_ADD variant that sets up everything important
on the character device so that we avoid the half set up block devices
entirely.

Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Martijn Coenen]

On Tue, Mar 31, 2020 at 9:48 AM Christoph Hellwig <hch@lst.de> wrote:
> I think the full blown set fd an status would seem a lot more useful,
> or even better a LOOP_CTL_ADD variant that sets up everything important
> on the character device so that we avoid the half set up block devices
> entirely.

Thanks for the feedback, I will work on that then. I think I could do
both - LOOP_SET_FD_AND_STATUS and a new variant of LOOP_CTL_ADD that
calls it - the former could still be useful if the kernel pre-created
a large amount of loop devices.

Martijn

Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Martijn Coenen]

Apologies for the resend, will follow up with a new series.

Martijn

On Tue, Mar 31, 2020 at 1:40 PM Martijn Coenen <maco@android.com> wrote:
>
> Configuring a loop device for a filesystem that is located at an offset
> currently requires calling LOOP_SET_FD and LOOP_SET_STATUS(64)
> consecutively. This has some downsides.
>
> The most important downside is that it can be slow. Here's setting
> up ~70 regular loop devices on an x86 Android device:
>
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r /dev/block/loop$i com.android.adbd.apex; done
>     0m01.85s real     0m00.01s user     0m00.01s system
>
> Here's configuring ~70 devices in the same way, but with an offset:
>
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
>     0m03.40s real     0m00.02s user     0m00.03s system
>
> This is almost twice as slow; the main reason for this slowness is that
> LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to freeze the associated
> queue; this requires waiting for RCU synchronization, which I've
> measured can take about 15-20ms on this device on average.
>
> A more minor downside of having to do two ioctls is that on devices with
> max_part > 0, the kernel will initiate a partition scan, which is
> needless work if the image is at an offset.
>
> This change introduces a new ioctl to combine setting the backing file
> together with the offset, which avoids the above problems. Adding more
> parameters could be a consideration, but offset appears to be the only
> commonly used parameter that is required for accessing the device
> safely.
>
> Signed-off-by: Martijn Coenen <maco@android.com>
> ---
>  drivers/block/loop.c      | 25 +++++++++++++++++++------
>  include/uapi/linux/loop.h |  6 ++++++
>  2 files changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index a42c49e04954..517031e1d10c 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -932,8 +932,8 @@ static void loop_update_rotational(struct loop_device *lo)
>                 blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
>  }
>
> -static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> -                      struct block_device *bdev, unsigned int arg)
> +static int loop_set_fd_with_offset(struct loop_device *lo, fmode_t mode,
> +               struct block_device *bdev, unsigned int arg, loff_t offset)
>  {
>         struct file     *file;
>         struct inode    *inode;
> @@ -957,7 +957,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>          * here to avoid changing device under exclusive owner.
>          */
>         if (!(mode & FMODE_EXCL)) {
> -               claimed_bdev = bd_start_claiming(bdev, loop_set_fd);
> +               claimed_bdev = bd_start_claiming(bdev, loop_set_fd_with_offset);
>                 if (IS_ERR(claimed_bdev)) {
>                         error = PTR_ERR(claimed_bdev);
>                         goto out_putf;
> @@ -1002,6 +1002,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>         lo->transfer = NULL;
>         lo->ioctl = NULL;
>         lo->lo_sizelimit = 0;
> +       lo->lo_offset = offset;
>         lo->old_gfp_mask = mapping_gfp_mask(mapping);
>         mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
>
> @@ -1042,14 +1043,14 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>         if (partscan)
>                 loop_reread_partitions(lo, bdev);
>         if (claimed_bdev)
> -               bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
> +               bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
>         return 0;
>
>  out_unlock:
>         mutex_unlock(&loop_ctl_mutex);
>  out_bdev:
>         if (claimed_bdev)
> -               bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
> +               bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
>  out_putf:
>         fput(file);
>  out:
> @@ -1601,7 +1602,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
>
>         switch (cmd) {
>         case LOOP_SET_FD:
> -               return loop_set_fd(lo, mode, bdev, arg);
> +               return loop_set_fd_with_offset(lo, mode, bdev, arg, 0);
>         case LOOP_CHANGE_FD:
>                 return loop_change_fd(lo, bdev, arg);
>         case LOOP_CLR_FD:
> @@ -1624,6 +1625,17 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
>                 break;
>         case LOOP_GET_STATUS64:
>                 return loop_get_status64(lo, (struct loop_info64 __user *) arg);
> +       case LOOP_SET_FD_WITH_OFFSET: {
> +               struct loop_fd_with_offset fdwo;
> +
> +               if (copy_from_user(&fdwo,
> +                               (struct loop_fd_with_offset __user *) arg,
> +                               sizeof(struct loop_fd_with_offset)))
> +                       return -EFAULT;
> +
> +               return loop_set_fd_with_offset(lo, mode, bdev, fdwo.fd,
> +                               fdwo.lo_offset);
> +       }
>         case LOOP_SET_CAPACITY:
>         case LOOP_SET_DIRECT_IO:
>         case LOOP_SET_BLOCK_SIZE:
> @@ -1774,6 +1786,7 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
>         case LOOP_SET_CAPACITY:
>         case LOOP_CLR_FD:
>         case LOOP_GET_STATUS64:
> +       case LOOP_SET_FD_WITH_OFFSET:
>         case LOOP_SET_STATUS64:
>                 arg = (unsigned long) compat_ptr(arg);
>                 /* fall through */
> diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
> index 080a8df134ef..289829bc5abd 100644
> --- a/include/uapi/linux/loop.h
> +++ b/include/uapi/linux/loop.h
> @@ -60,6 +60,11 @@ struct loop_info64 {
>         __u64              lo_init[2];
>  };
>
> +struct loop_fd_with_offset {
> +       __u64          lo_offset;
> +       __u32          fd;
> +};
> +
>  /*
>   * Loop filter types
>   */
> @@ -90,6 +95,7 @@ struct loop_info64 {
>  #define LOOP_SET_CAPACITY      0x4C07
>  #define LOOP_SET_DIRECT_IO     0x4C08
>  #define LOOP_SET_BLOCK_SIZE    0x4C09
> +#define LOOP_SET_FD_WITH_OFFSET        0x4C0A
>
>  /* /dev/loop-control interface */
>  #define LOOP_CTL_ADD           0x4C80
> --
> 2.26.0.rc2.310.g2932bb562d-goog
>

[PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

[Martijn Coenen]

Configuring a loop device for a filesystem that is located at an offset
currently requires calling LOOP_SET_FD and LOOP_SET_STATUS(64)
consecutively. This has some downsides.

The most important downside is that it can be slow. Here's setting
up ~70 regular loop devices on an x86 Android device:

vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r /dev/block/loop$i com.android.adbd.apex; done
    0m01.85s real     0m00.01s user     0m00.01s system

Here's configuring ~70 devices in the same way, but with an offset:

vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
    0m03.40s real     0m00.02s user     0m00.03s system

This is almost twice as slow; the main reason for this slowness is that
LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to freeze the associated
queue; this requires waiting for RCU synchronization, which I've
measured can take about 15-20ms on this device on average.

A more minor downside of having to do two ioctls is that on devices with
max_part > 0, the kernel will initiate a partition scan, which is
needless work if the image is at an offset.

This change introduces a new ioctl to combine setting the backing file
together with the offset, which avoids the above problems. Adding more
parameters could be a consideration, but offset appears to be the only
commonly used parameter that is required for accessing the device
safely.

Signed-off-by: Martijn Coenen <maco@android.com>
---
 drivers/block/loop.c      | 25 +++++++++++++++++++------
 include/uapi/linux/loop.h |  6 ++++++
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index a42c49e04954..517031e1d10c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -932,8 +932,8 @@ static void loop_update_rotational(struct loop_device *lo)
 		blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
 }
 
-static int loop_set_fd(struct loop_device *lo, fmode_t mode,
-		       struct block_device *bdev, unsigned int arg)
+static int loop_set_fd_with_offset(struct loop_device *lo, fmode_t mode,
+		struct block_device *bdev, unsigned int arg, loff_t offset)
 {
 	struct file	*file;
 	struct inode	*inode;
@@ -957,7 +957,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	 * here to avoid changing device under exclusive owner.
 	 */
 	if (!(mode & FMODE_EXCL)) {
-		claimed_bdev = bd_start_claiming(bdev, loop_set_fd);
+		claimed_bdev = bd_start_claiming(bdev, loop_set_fd_with_offset);
 		if (IS_ERR(claimed_bdev)) {
 			error = PTR_ERR(claimed_bdev);
 			goto out_putf;
@@ -1002,6 +1002,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	lo->transfer = NULL;
 	lo->ioctl = NULL;
 	lo->lo_sizelimit = 0;
+	lo->lo_offset = offset;
 	lo->old_gfp_mask = mapping_gfp_mask(mapping);
 	mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
 
@@ -1042,14 +1043,14 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	if (partscan)
 		loop_reread_partitions(lo, bdev);
 	if (claimed_bdev)
-		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
+		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
 	return 0;
 
 out_unlock:
 	mutex_unlock(&loop_ctl_mutex);
 out_bdev:
 	if (claimed_bdev)
-		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
+		bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
 out_putf:
 	fput(file);
 out:
@@ -1601,7 +1602,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
 
 	switch (cmd) {
 	case LOOP_SET_FD:
-		return loop_set_fd(lo, mode, bdev, arg);
+		return loop_set_fd_with_offset(lo, mode, bdev, arg, 0);
 	case LOOP_CHANGE_FD:
 		return loop_change_fd(lo, bdev, arg);
 	case LOOP_CLR_FD:
@@ -1624,6 +1625,17 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
 		break;
 	case LOOP_GET_STATUS64:
 		return loop_get_status64(lo, (struct loop_info64 __user *) arg);
+	case LOOP_SET_FD_WITH_OFFSET: {
+		struct loop_fd_with_offset fdwo;
+
+		if (copy_from_user(&fdwo,
+				(struct loop_fd_with_offset __user *) arg,
+				sizeof(struct loop_fd_with_offset)))
+			return -EFAULT;
+
+		return loop_set_fd_with_offset(lo, mode, bdev, fdwo.fd,
+				fdwo.lo_offset);
+	}
 	case LOOP_SET_CAPACITY:
 	case LOOP_SET_DIRECT_IO:
 	case LOOP_SET_BLOCK_SIZE:
@@ -1774,6 +1786,7 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
 	case LOOP_SET_CAPACITY:
 	case LOOP_CLR_FD:
 	case LOOP_GET_STATUS64:
+	case LOOP_SET_FD_WITH_OFFSET:
 	case LOOP_SET_STATUS64:
 		arg = (unsigned long) compat_ptr(arg);
 		/* fall through */
diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
index 080a8df134ef..289829bc5abd 100644
--- a/include/uapi/linux/loop.h
+++ b/include/uapi/linux/loop.h
@@ -60,6 +60,11 @@ struct loop_info64 {
 	__u64		   lo_init[2];
 };
 
+struct loop_fd_with_offset {
+	__u64          lo_offset;
+	__u32          fd;
+};
+
 /*
  * Loop filter types
  */
@@ -90,6 +95,7 @@ struct loop_info64 {
 #define LOOP_SET_CAPACITY	0x4C07
 #define LOOP_SET_DIRECT_IO	0x4C08
 #define LOOP_SET_BLOCK_SIZE	0x4C09
+#define LOOP_SET_FD_WITH_OFFSET	0x4C0A
 
 /* /dev/loop-control interface */
 #define LOOP_CTL_ADD		0x4C80
-- 
2.26.0.rc2.310.g2932bb562d-goog