linux-kernel - Re: [PATCH v3] block: create ioctl to discard-or-zeroout a range of blocks

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Date:	Fri, 13 Nov 2015 15:23:25 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@...radead.org>,
	"Seymour\, Shane M" <shane.seymour@....com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-api\@vger.kernel.org" <linux-api@...r.kernel.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"martin.petersen\@oracle.com" <martin.petersen@...cle.com>
Subject: Re: [PATCH v3] block: create ioctl to discard-or-zeroout a range of blocks

"Darrick J. Wong" <darrick.wong@...cle.com> writes:

> Create a new ioctl to expose the block layer's newfound ability to
> issue either a zeroing discard, a WRITE SAME with a zero page, or a
> regular write with the zero page.  This BLKZEROOUT2 ioctl takes
> {start, length, flags} as parameters.  So far, the only flag available
> is to enable the zeroing discard part -- without it, the call invokes
> the old BLKZEROOUT behavior.  start and length have the same meaning
> as in BLKZEROOUT.
>
> Furthermore, because BLKZEROOUT2 issues commands directly to the
> storage device, we must invalidate the page cache (as a regular
> O_DIRECT write would do) to avoid returning stale cache contents at a
> later time.
>
> v3: Add extra padding for future expansion, and check the padding is zero.

Is there someplace we document ioctls?  This stuff really could use some
good documentation.

Cheers,
Jeff

>
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> ---
>  block/ioctl.c           |   48 ++++++++++++++++++++++++++++++++++++++++-------
>  include/uapi/linux/fs.h |    9 +++++++++
>  2 files changed, 50 insertions(+), 7 deletions(-)
>
> diff --git a/block/ioctl.c b/block/ioctl.c
> index 8061eba..8e67551 100644
> --- a/block/ioctl.c
> +++ b/block/ioctl.c
> @@ -213,19 +213,39 @@ static int blk_ioctl_discard(struct block_device *bdev, uint64_t start,
>  }
>  
>  static int blk_ioctl_zeroout(struct block_device *bdev, uint64_t start,
> -			     uint64_t len)
> +			     uint64_t len, uint32_t flags)
>  {
> +	int ret;
> +	struct address_space *mapping;
> +	uint64_t end = start + len - 1;
> +
> +	if (flags & ~BLKZEROOUT2_DISCARD_OK)
> +		return -EINVAL;
>  	if (start & 511)
>  		return -EINVAL;
>  	if (len & 511)
>  		return -EINVAL;
> -	start >>= 9;
> -	len >>= 9;
> -
> -	if (start + len > (i_size_read(bdev->bd_inode) >> 9))
> +	if (end >= i_size_read(bdev->bd_inode))
>  		return -EINVAL;
>  
> -	return blkdev_issue_zeroout(bdev, start, len, GFP_KERNEL, false);
> +	/* Invalidate the page cache, including dirty pages */
> +	mapping = bdev->bd_inode->i_mapping;
> +	truncate_inode_pages_range(mapping, start, end);
> +
> +	ret = blkdev_issue_zeroout(bdev, start >> 9, len >> 9, GFP_KERNEL,
> +				   flags & BLKZEROOUT2_DISCARD_OK);
> +	if (ret)
> +		goto out;
> +
> +	/*
> +	 * Invalidate again; if someone wandered in and dirtied a page,
> +	 * the caller will be given -EBUSY.
> +	 */
> +	ret = invalidate_inode_pages2_range(mapping,
> +					    start >> PAGE_CACHE_SHIFT,
> +					    end >> PAGE_CACHE_SHIFT);
> +out:
> +	return ret;
>  }
>  
>  static int put_ushort(unsigned long arg, unsigned short val)
> @@ -353,7 +373,21 @@ int blkdev_ioctl(struct block_device *bdev, fmode_t mode, unsigned cmd,
>  		if (copy_from_user(range, (void __user *)arg, sizeof(range)))
>  			return -EFAULT;
>  
> -		return blk_ioctl_zeroout(bdev, range[0], range[1]);
> +		return blk_ioctl_zeroout(bdev, range[0], range[1], 0);
> +	}
> +	case BLKZEROOUT2: {
> +		struct blkzeroout2 p;
> +
> +		if (!(mode & FMODE_WRITE))
> +			return -EBADF;
> +
> +		if (copy_from_user(&p, (void __user *)arg, sizeof(p)))
> +			return -EFAULT;
> +
> +		if (p.padding || p.padding2)
> +			return -EINVAL;
> +
> +		return blk_ioctl_zeroout(bdev, p.start, p.length, p.flags);
>  	}
>  
>  	case HDIO_GETGEO: {
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 9b964a5..b811fa4 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -152,6 +152,15 @@ struct inodes_stat_t {
>  #define BLKSECDISCARD _IO(0x12,125)
>  #define BLKROTATIONAL _IO(0x12,126)
>  #define BLKZEROOUT _IO(0x12,127)
> +struct blkzeroout2 {
> +	__u64 start;
> +	__u64 length;
> +	__u32 flags;
> +	__u32 padding;
> +	__u64 padding2;
> +};
> +#define BLKZEROOUT2_DISCARD_OK	1
> +#define BLKZEROOUT2 _IOR(0x12, 127, struct blkzeroout2)
>  
>  #define BMAP_IOCTL 1		/* obsolete - kept for compatibility */
>  #define FIBMAP	   _IO(0x00,1)	/* bmap access */
> --
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