Re: [PATCH v8 4/6] block: loop: prepare for supporing direct IO

[Dave Kleikamp <dave.kleikamp-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On 07/30/2015 06:36 AM, Ming Lei wrote:
> This patches provides one interface for enabling direct IO
> from user space:
> 
> 	- userspace(such as losetup) can pass 'file' which is
> 	opened/fcntl as O_DIRECT
> 
> Also __loop_update_dio() is introduced to check if direct I/O
> can be used on current loop setting.
> 
> The last big change is to introduce LO_FLAGS_DIRECT_IO flag
> for userspace to know if direct IO is used to access backing
> file.

lo->use_dio and LO_FLAGS_DIRECT_IO seem redundant. Wouldn't it be
simpler to use one or the other?

> 
> Cc: linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Signed-off-by: Ming Lei <ming.lei-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>
> ---
>  drivers/block/loop.c      | 63 ++++++++++++++++++++++++++++++++++++++++++++++-
>  drivers/block/loop.h      |  2 ++
>  include/uapi/linux/loop.h |  1 +
>  3 files changed, 65 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 1875aad..799cc23 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -164,6 +164,47 @@ static loff_t get_loop_size(struct loop_device *lo, struct file *file)
>  	return get_size(lo->lo_offset, lo->lo_sizelimit, file);
>  }
>  
> +static void __loop_update_dio(struct loop_device *lo, bool dio)
> +{
> +	struct file *file = lo->lo_backing_file;
> +	struct inode *inode = file->f_mapping->host;
> +	bool use_dio;
> +	unsigned dio_align = inode->i_sb->s_bdev ?
> +		(bdev_io_min(inode->i_sb->s_bdev) - 1) : 0;
> +
> +	/*
> +	 * We support direct I/O only if lo_offset is aligned
> +	 * with the min I/O size of backing device.
> +	 *
> +	 * Request's offset and size will be checked in I/O path.
> +	 */
> +	if (dio) {
> +		if (!dio_align || (lo->lo_offset & dio_align))
> +			use_dio = false;
> +		else
> +			use_dio = true;
> +	} else {
> +		use_dio = false;
> +	}
> +
> +	/* flush dirty pages before changing direct IO */
> +	vfs_fsync(file, 0);
> +
> +	/*
> +	 * The flag of LO_FLAGS_DIRECT_IO is handled similarly with
> +	 * LO_FLAGS_READ_ONLY, both are set from kernel, and losetup
> +	 * will get updated by ioctl(LOOP_GET_STATUS)
> +	 */
> +	blk_mq_freeze_queue(lo->lo_queue);
> +	lo->use_dio = use_dio;
> +	lo->dio_align = dio_align;
> +	if (use_dio)
> +		lo->lo_flags |= LO_FLAGS_DIRECT_IO;
> +	else
> +		lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
> +	blk_mq_unfreeze_queue(lo->lo_queue);
> +}
> +
>  static int
>  figure_loop_size(struct loop_device *lo, loff_t offset, loff_t sizelimit)
>  {
> @@ -173,8 +214,12 @@ figure_loop_size(struct loop_device *lo, loff_t offset, loff_t sizelimit)
>  
>  	if (unlikely((loff_t)x != size))
>  		return -EFBIG;
> -	if (lo->lo_offset != offset)
> +	if (lo->lo_offset != offset) {
>  		lo->lo_offset = offset;
> +
> +		/* update dio if lo_offset is changed*/
> +		__loop_update_dio(lo, lo->use_dio);
> +	}
>  	if (lo->lo_sizelimit != sizelimit)
>  		lo->lo_sizelimit = sizelimit;
>  	set_capacity(lo->lo_disk, x);
> @@ -421,6 +466,11 @@ struct switch_request {
>  	struct completion wait;
>  };
>  
> +static inline void loop_update_dio(struct loop_device *lo)
> +{
> +	__loop_update_dio(lo, io_is_direct(lo->lo_backing_file));
> +}
> +
>  /*
>   * Do the actual switch; called from the BIO completion routine
>   */
> @@ -441,6 +491,7 @@ static void do_loop_switch(struct loop_device *lo, struct switch_request *p)
>  		mapping->host->i_bdev->bd_block_size : PAGE_SIZE;
>  	lo->old_gfp_mask = mapping_gfp_mask(mapping);
>  	mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
> +	loop_update_dio(lo);
>  }
>  
>  /*
> @@ -627,11 +678,19 @@ static ssize_t loop_attr_partscan_show(struct loop_device *lo, char *buf)
>  	return sprintf(buf, "%s\n", partscan ? "1" : "0");
>  }
>  
> +static ssize_t loop_attr_dio_show(struct loop_device *lo, char *buf)
> +{
> +	int dio = (lo->lo_flags & LO_FLAGS_DIRECT_IO);
> +
> +	return sprintf(buf, "%s\n", dio ? "1" : "0");
> +}
> +
>  LOOP_ATTR_RO(backing_file);
>  LOOP_ATTR_RO(offset);
>  LOOP_ATTR_RO(sizelimit);
>  LOOP_ATTR_RO(autoclear);
>  LOOP_ATTR_RO(partscan);
> +LOOP_ATTR_RO(dio);
>  
>  static struct attribute *loop_attrs[] = {
>  	&loop_attr_backing_file.attr,
> @@ -639,6 +698,7 @@ static struct attribute *loop_attrs[] = {
>  	&loop_attr_sizelimit.attr,
>  	&loop_attr_autoclear.attr,
>  	&loop_attr_partscan.attr,
> +	&loop_attr_dio.attr,
>  	NULL,
>  };
>  
> @@ -783,6 +843,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>  	if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
>  		blk_queue_flush(lo->lo_queue, REQ_FLUSH);
>  
> +	loop_update_dio(lo);
>  	set_capacity(lo->lo_disk, size);
>  	bd_set_size(bdev, size << 9);
>  	loop_sysfs_init(lo);
> diff --git a/drivers/block/loop.h b/drivers/block/loop.h
> index b6c7d21..63f8e14 100644
> --- a/drivers/block/loop.h
> +++ b/drivers/block/loop.h
> @@ -58,6 +58,8 @@ struct loop_device {
>  	struct mutex		lo_ctl_mutex;
>  	struct kthread_worker	worker;
>  	struct task_struct	*worker_task;
> +	unsigned		dio_align;
> +	bool			use_dio;
>  
>  	struct request_queue	*lo_queue;
>  	struct blk_mq_tag_set	tag_set;
> diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
> index e0cecd2..949851c 100644
> --- a/include/uapi/linux/loop.h
> +++ b/include/uapi/linux/loop.h
> @@ -21,6 +21,7 @@ enum {
>  	LO_FLAGS_READ_ONLY	= 1,
>  	LO_FLAGS_AUTOCLEAR	= 4,
>  	LO_FLAGS_PARTSCAN	= 8,
> +	LO_FLAGS_DIRECT_IO	= 16,
>  };
>  
>  #include <asm/posix_types.h>	/* for __kernel_old_dev_t */
>