Re: [PATCH v5 11/11] loop: Add LOOP_CONFIGURE ioctl

Kristian Klausen
2021-06-11T23:01:50Z

On 13.05.2020 15.38, Martijn Coenen wrote:
> This allows userspace to completely setup a loop device with a single
> ioctl, removing the in-between state where the device can be partially
> configured - eg the loop device has a backing file associated with it,
> but is reading from the wrong offset.
>
> Besides removing the intermediate state, another big benefit of this
> ioctl is that LOOP_SET_STATUS can be 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.
>
> In addition to doing what LOOP_SET_STATUS can do, LOOP_CONFIGURE can
> also be used to:
> - Set the correct block size immediately by setting
> loop_config.block_size (avoids LOOP_SET_BLOCK_SIZE)
> - Explicitly request direct I/O mode by setting LO_FLAGS_DIRECT_IO
> in loop_config.info.lo_flags (avoids LOOP_SET_DIRECT_IO)
> - Explicitly request read-only mode by setting LO_FLAGS_READ_ONLY
> in loop_config.info.lo_flags
>
> Here's setting up ~70 regular loop devices with an offset on an x86
> Android device, using LOOP_SET_FD and LOOP_SET_STATUS:
>
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
> 0m03.40s real 0m00.02s user 0m00.03s system
>
> Here's configuring ~70 devices in the same way, but using a modified
> losetup that uses the new LOOP_CONFIGURE ioctl:
>
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
> 0m01.94s real 0m00.01s user 0m00.01s system
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Martijn Coenen <maco@android.com>
> ---
> drivers/block/loop.c | 104 ++++++++++++++++++++++++++++----------
> include/uapi/linux/loop.h | 21 ++++++++
> 2 files changed, 97 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 13518ba191f5..a565c5aafa52 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -228,6 +228,19 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
> blk_mq_unfreeze_queue(lo->lo_queue);
> }
>
> +/**
> + * loop_validate_block_size() - validates the passed in block size
> + * @bsize: size to validate
> + */
> +static int
> +loop_validate_block_size(unsigned short bsize)
> +{
> + if (bsize < 512 || bsize > PAGE_SIZE || !is_power_of_2(bsize))
> + return -EINVAL;
> +
> + return 0;
> +}
> +
> /**
> * loop_set_size() - sets device size and notifies userspace
> * @lo: struct loop_device to set the size for
> @@ -1050,23 +1063,24 @@ loop_set_status_from_info(struct loop_device *lo,
> return 0;
> }
>
> -static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> - struct block_device *bdev, unsigned int arg)
> +static int loop_configure(struct loop_device *lo, fmode_t mode,
> + struct block_device *bdev,
> + const struct loop_config *config)
> {
> struct file *file;
> struct inode *inode;
> struct address_space *mapping;
> struct block_device *claimed_bdev = NULL;
> - int lo_flags = 0;
> int error;
> loff_t size;
> bool partscan;
> + unsigned short bsize;
>
> /* This is safe, since we have a reference from open(). */
> __module_get(THIS_MODULE);
>
> error = -EBADF;
> - file = fget(arg);
> + file = fget(config->fd);
> if (!file)
> goto out;
>
> @@ -1075,7 +1089,7 @@ 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_configure);
> if (IS_ERR(claimed_bdev)) {
> error = PTR_ERR(claimed_bdev);
> goto out_putf;
> @@ -1097,11 +1111,26 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> mapping = file->f_mapping;
> inode = mapping->host;
>
> + size = get_loop_size(lo, file);
> +
> + if ((config->info.lo_flags & ~LOOP_CONFIGURE_SETTABLE_FLAGS) != 0) {
> + error = -EINVAL;
> + goto out_unlock;
> + }
> +
> + if (config->block_size) {
> + error = loop_validate_block_size(config->block_size);
> + if (error)
> + goto out_unlock;
> + }
> +
> + error = loop_set_status_from_info(lo, &config->info);

loop_set_status_from_info() doesn't call loop_config_discard() nor does
loop_configure(), I assume it should be called?

I'm asking because I upgraded to util-linux 2.37 which use the new
LOOP_CONFIGURE ioctl and fstrim is complaining that "the discard
operation is not supported" and the missing call of
loop_config_discard() stood out. Sadly I haven't been able to reproduce
the issue reliably outside CI (yet).

> + if (error)
> + goto out_unlock;
> +
> if (!(file->f_mode & FMODE_WRITE) || !(mode & FMODE_WRITE) ||
> !file->f_op->write_iter)
> - lo_flags |= LO_FLAGS_READ_ONLY;
> -
> - size = get_loop_size(lo, file);
> + lo->lo_flags |= LO_FLAGS_READ_ONLY;
>
> error = loop_prepare_queue(lo);
> if (error)
>