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SubjectRe: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.
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
>

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