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 <[email protected]>
---
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
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<[email protected]>
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.
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.
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
Hi Ming,
On Mon, Mar 30, 2020 at 3:00 AM Ming Lei <[email protected]> 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
>
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 <[email protected]> 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.
On Tue, Mar 31, 2020 at 9:48 AM Christoph Hellwig <[email protected]> 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