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Date:   Mon, 11 Nov 2019 17:36:39 -0800
From:   "Darrick J. Wong" <darrick.wong@oracle.com>
To:     Evan Green <evgreen@chromium.org>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Martin K Petersen <martin.petersen@oracle.com>,
        Gwendal Grignou <gwendal@chromium.org>,
        Ming Lei <ming.lei@redhat.com>,
        Alexis Savery <asavery@chromium.org>,
        Douglas Anderson <dianders@chromium.org>,
        Bart Van Assche <bvanassche@acm.org>,
        Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 2/2] loop: Better discard support for block devices
Message-ID: <20191112013639.GE6235@magnolia>
References: <20191111185030.215451-1-evgreen@chromium.org>
 <20191111185030.215451-3-evgreen@chromium.org>
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On Mon, Nov 11, 2019 at 10:50:30AM -0800, Evan Green wrote:
> If the backing device for a loop device is a block device,
> then mirror the "write zeroes" capabilities of the underlying
> block device into the loop device. Copy this capability into both
> max_write_zeroes_sectors and max_discard_sectors of the loop device.
> 
> The reason for this is that REQ_OP_DISCARD on a loop device translates
> into blkdev_issue_zeroout(), rather than blkdev_issue_discard(). This
> presents a consistent interface for loop devices (that discarded data
> is zeroed), regardless of the backing device type of the loop device.
> There should be no behavior change for loop devices backed by regular
> files.
> 
> While in there, differentiate between REQ_OP_DISCARD and
> REQ_OP_WRITE_ZEROES, which are different for block devices,
> but which the loop device had just been lumping together, since
> they're largely the same for files.
> 
> This change fixes blktest block/003, and removes an extraneous
> error print in block/013 when testing on a loop device backed
> by a block device that does not support discard.
> 
> Signed-off-by: Evan Green <evgreen@chromium.org>
> Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
> Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
> ---
> 
> Changes in v6: None
> Changes in v5:
> - Don't mirror discard if lo_encrypt_key_size is non-zero (Gwendal)
> 
> Changes in v4:
> - Mirror blkdev's write_zeroes into loopdev's discard_sectors.
> 
> Changes in v3:
> - Updated commit description
> 
> Changes in v2: None
> 
>  drivers/block/loop.c | 57 ++++++++++++++++++++++++++++----------------
>  1 file changed, 37 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index d749156a3d88..236f6deb0772 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -417,19 +417,14 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
>  	return ret;
>  }
>  
> -static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
> +static int lo_discard(struct loop_device *lo, struct request *rq,
> +		int mode, loff_t pos)
>  {
> -	/*
> -	 * We use punch hole to reclaim the free space used by the
> -	 * image a.k.a. discard. However we do not support discard if
> -	 * encryption is enabled, because it may give an attacker
> -	 * useful information.
> -	 */
>  	struct file *file = lo->lo_backing_file;
> -	int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
> +	struct request_queue *q = lo->lo_queue;
>  	int ret;
>  
> -	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> +	if (!blk_queue_discard(q)) {
>  		ret = -EOPNOTSUPP;
>  		goto out;
>  	}
> @@ -599,8 +594,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
>  	case REQ_OP_FLUSH:
>  		return lo_req_flush(lo, rq);
>  	case REQ_OP_DISCARD:
> +		return lo_discard(lo, rq,
> +			FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pos);
> +
>  	case REQ_OP_WRITE_ZEROES:
> -		return lo_discard(lo, rq, pos);
> +		return lo_discard(lo, rq,
> +			FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE, pos);

Yes, this more or less reimplements what's already in -next...

> +
>  	case REQ_OP_WRITE:
>  		if (lo->transfer)
>  			return lo_write_transfer(lo, rq, pos);
> @@ -854,6 +854,21 @@ static void loop_config_discard(struct loop_device *lo)
>  	struct file *file = lo->lo_backing_file;
>  	struct inode *inode = file->f_mapping->host;
>  	struct request_queue *q = lo->lo_queue;
> +	struct request_queue *backingq;
> +
> +	/*
> +	 * If the backing device is a block device, mirror its zeroing
> +	 * capability. REQ_OP_DISCARD translates to a zero-out even when backed
> +	 * by block devices to keep consistent behavior with file-backed loop
> +	 * devices.
> +	 */
> +	if (S_ISBLK(inode->i_mode) && !lo->lo_encrypt_key_size) {
> +		backingq = bdev_get_queue(inode->i_bdev);

What happens if the inode is from a filesystem that can have multiple
backing devices (like btrfs)?

> +		blk_queue_max_discard_sectors(q,
> +			backingq->limits.max_write_zeroes_sectors);
> +
> +		blk_queue_max_write_zeroes_sectors(q,
> +			backingq->limits.max_write_zeroes_sectors);

Also, seeing as filesystems tend to implement PUNCH_HOLE and ZERO_RANGE
on their own independent of the hardware capabilities of the underlying
device, it doesn't make much sense to forward the blockdev limits to the
loop device.

(Put another way, XFS's ZERO_RANGE implementation can zero hundreds of
gigabytes at a time even if the underlying device is a spinning rust.)

--D

>  
>  	/*
>  	 * We use punch hole to reclaim the free space used by the
> @@ -861,22 +876,24 @@ static void loop_config_discard(struct loop_device *lo)
>  	 * encryption is enabled, because it may give an attacker
>  	 * useful information.
>  	 */
> -	if ((!file->f_op->fallocate) ||
> -	    lo->lo_encrypt_key_size) {
> +	} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
>  		q->limits.discard_granularity = 0;
>  		q->limits.discard_alignment = 0;
>  		blk_queue_max_discard_sectors(q, 0);
>  		blk_queue_max_write_zeroes_sectors(q, 0);
> -		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
> -		return;
> -	}
>  
> -	q->limits.discard_granularity = inode->i_sb->s_blocksize;
> -	q->limits.discard_alignment = 0;
> +	} else {
> +		q->limits.discard_granularity = inode->i_sb->s_blocksize;
> +		q->limits.discard_alignment = 0;
> +
> +		blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
> +		blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
> +	}
>  
> -	blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
> -	blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
> -	blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
> +	if (q->limits.max_write_zeroes_sectors)
> +		blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
> +	else
> +		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
>  }
>  
>  static void loop_unprepare_queue(struct loop_device *lo)
> -- 
> 2.21.0
>