Re: [PATCH V4 2/3] xen-blkfront: teach blkfront driver to handle discard requests

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Thu, Sep 01, 2011 at 06:39:09PM +0800, Li Dongyang wrote:
> The blkfront driver now will read discard related nodes from xenstore,
> and set up the request queue, then we can forward the
> discard requests to backend driver.


A better description is as follow:

xen-blkfront: Handle discard requests.
    
    If the backend advertises 'feature-discard', then interrogate
    the backend for alignment, granularity, and max discard block size.
    Setup the request queue with the appropiate values and send the
    discard operation as required.
    

> 
> Signed-off-by: Li Dongyang <lidongyang@novell.com>
> ---
>  drivers/block/xen-blkfront.c |  111 +++++++++++++++++++++++++++++++++---------
>  1 files changed, 88 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 9ea8c25..86e2c63 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -98,6 +98,9 @@ struct blkfront_info
>  	unsigned long shadow_free;
>  	unsigned int feature_flush;
>  	unsigned int flush_op;
> +	unsigned int feature_discard;
> +	unsigned int discard_granularity;
> +	unsigned int discard_alignment;
>  	int is_ready;
>  };
>  
> @@ -302,29 +305,36 @@ static int blkif_queue_request(struct request *req)
>  		ring_req->operation = info->flush_op;
>  	}
>  
> -	ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
> -	BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
> +	if (unlikely(req->cmd_flags & REQ_DISCARD)) {
> +		/* id, sector_number and handle are set above. */
> +		ring_req->operation = BLKIF_OP_DISCARD;
> +		ring_req->nr_segments = 0;
> +		ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
> +	} else {
> +		ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
> +		BUG_ON(ring_req->nr_segments > BLKIF_MAX_SEGMENTS_PER_REQUEST);
>  
> -	for_each_sg(info->sg, sg, ring_req->nr_segments, i) {
> -		buffer_mfn = pfn_to_mfn(page_to_pfn(sg_page(sg)));
> -		fsect = sg->offset >> 9;
> -		lsect = fsect + (sg->length >> 9) - 1;
> -		/* install a grant reference. */
> -		ref = gnttab_claim_grant_reference(&gref_head);
> -		BUG_ON(ref == -ENOSPC);
> +		for_each_sg(info->sg, sg, ring_req->nr_segments, i) {
> +			buffer_mfn = pfn_to_mfn(page_to_pfn(sg_page(sg)));
> +			fsect = sg->offset >> 9;
> +			lsect = fsect + (sg->length >> 9) - 1;
> +			/* install a grant reference. */
> +			ref = gnttab_claim_grant_reference(&gref_head);
> +			BUG_ON(ref == -ENOSPC);
>  
> -		gnttab_grant_foreign_access_ref(
> -				ref,
> -				info->xbdev->otherend_id,
> -				buffer_mfn,
> -				rq_data_dir(req) );
> -
> -		info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
> -		ring_req->u.rw.seg[i] =
> -				(struct blkif_request_segment) {
> -					.gref       = ref,
> -					.first_sect = fsect,
> -					.last_sect  = lsect };
> +			gnttab_grant_foreign_access_ref(
> +					ref,
> +					info->xbdev->otherend_id,
> +					buffer_mfn,
> +					rq_data_dir(req));
> +
> +			info->shadow[id].frame[i] = mfn_to_pfn(buffer_mfn);
> +			ring_req->u.rw.seg[i] =
> +					(struct blkif_request_segment) {
> +						.gref       = ref,
> +						.first_sect = fsect,
> +						.last_sect  = lsect };
> +		}
>  	}
>  
>  	info->ring.req_prod_pvt++;
> @@ -399,6 +409,7 @@ wait:
>  static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
>  {
>  	struct request_queue *rq;
> +	struct blkfront_info *info = gd->private_data;
>  
>  	rq = blk_init_queue(do_blkif_request, &blkif_io_lock);
>  	if (rq == NULL)
> @@ -406,6 +417,13 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
>  
>  	queue_flag_set_unlocked(QUEUE_FLAG_VIRT, rq);
>  
> +	if (info->feature_discard) {
> +		queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, rq);
> +		blk_queue_max_discard_sectors(rq, get_capacity(gd));

This is not correct. I took a look at the SCSI support for this
('sd_config_discard') and if you look there carefully you will see that
the value can be 64KB for example.

You need to provide a mechanism similar to 'discard-*' to fetch that data
from the backend.

> +		rq->limits.discard_granularity = info->discard_granularity;
> +		rq->limits.discard_alignment = info->discard_alignment;
> +	}
> +
>  	/* Hard sector size and max sectors impersonate the equiv. hardware. */
>  	blk_queue_logical_block_size(rq, sector_size);
>  	blk_queue_max_hw_sectors(rq, 512);
> @@ -722,6 +740,19 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
>  
>  		error = (bret->status == BLKIF_RSP_OKAY) ? 0 : -EIO;
>  		switch (bret->operation) {
> +		case BLKIF_OP_DISCARD:
> +			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
> +				struct request_queue *rq = info->rq;
> +				printk(KERN_WARNING "blkfront: %s: discard op failed\n",
> +					   info->gd->disk_name);
> +				error = -EOPNOTSUPP;
> +				info->feature_discard = 0;
> +				spin_lock(rq->queue_lock);
> +				queue_flag_clear(QUEUE_FLAG_DISCARD, rq);
> +				spin_unlock(rq->queue_lock);
> +			}
> +			__blk_end_request_all(req, error);
> +			break;
>  		case BLKIF_OP_FLUSH_DISKCACHE:
>  		case BLKIF_OP_WRITE_BARRIER:
>  			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
> @@ -1098,6 +1129,33 @@ blkfront_closing(struct blkfront_info *info)
>  	bdput(bdev);
>  }
>  
> +static void blkfront_setup_discard(struct blkfront_info *info)
> +{
> +	int err;
> +	char *type;
> +	unsigned int discard_granularity;
> +	unsigned int discard_alignment;
> +
> +	type = xenbus_read(XBT_NIL, info->xbdev->otherend, "type", NULL);
> +	if (IS_ERR(type))
> +		return;
> +
> +	if (strncmp(type, "phy", 3) == 0) {
> +		err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
> +			"discard-granularity", "%u", &discard_granularity,
> +			"discard-alignment", "%u", &discard_alignment,
> +			NULL);
> +		if (!err) {
> +			info->feature_discard = 1;
> +			info->discard_granularity = discard_granularity;
> +			info->discard_alignment = discard_alignment;
> +		}
> +	} else if (strncmp(type, "file", 4) == 0)
> +		info->feature_discard = 1;
> +
> +	kfree(type);
> +}
> +
>  /*
>   * Invoked when the backend is finally 'ready' (and has told produced
>   * the details about the physical device - #sectors, size, etc).
> @@ -1108,7 +1166,7 @@ static void blkfront_connect(struct blkfront_info *info)
>  	unsigned long sector_size;
>  	unsigned int binfo;
>  	int err;
> -	int barrier, flush;
> +	int barrier, flush, discard;
>  
>  	switch (info->connected) {
>  	case BLKIF_STATE_CONNECTED:
> @@ -1178,7 +1236,14 @@ static void blkfront_connect(struct blkfront_info *info)
>  		info->feature_flush = REQ_FLUSH;
>  		info->flush_op = BLKIF_OP_FLUSH_DISKCACHE;
>  	}
> -		
> +
> +	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
> +			    "feature-discard", "%d", &discard,
> +			    NULL);
> +
> +	if (!err && discard)
> +		blkfront_setup_discard(info);
> +
>  	err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
>  	if (err) {
>  		xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
> -- 
> 1.7.6