Re: [RFC] 'discard sectors' block request.

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 05 Aug 2008 02:45:16 +0100 David Woodhouse <dwmw2@infradead.org> wrote:

> On Sat, 2008-07-26 at 13:02 -0700, Andrew Morton wrote:
> > I seem to be hearing a lot of silence over support for SSD devices.  I
> > have this vague worry that there will be a large rollout of SSD
> > hardware and Linux will be found to have pants-around-ankles.
> > 
> > For example, support for the T13 Trim command.  There will be others,
> > but I surely don't know what they are.
> 
> Here's a first attempt at that. It implements basic support for
> 'discard' requests in the block layer, implements that in FTL (the flash
> translation layer used on PCMCIA flash cards), and in FAT.

Looks nice and simple.

>
> ...
>
> +int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> +			 unsigned nr_sects, sector_t *error_sector)
> +{
> +	DECLARE_COMPLETION_ONSTACK(wait);
> +	struct request_queue *q;
> +	struct bio *bio;
> +	int ret;
> +
> +	if (bdev->bd_disk == NULL)
> +		return -ENXIO;
> +
> +	q = bdev_get_queue(bdev);
> +	if (!q)
> +		return -ENXIO;
> +
> +	bio = bio_alloc(GFP_KERNEL, 0);
> +	if (!bio)
> +		return -ENOMEM;
> +
> +	bio->bi_end_io = bio_end_empty_barrier;
> +	bio->bi_private = &wait;
> +	bio->bi_bdev = bdev;
> +	bio->bi_sector = sector;
> +	bio->bi_size = nr_sects << 9;
> +	submit_bio(1 << BIO_RW_DISCARD, bio);
> +
> +	wait_for_completion(&wait);

The synchronous nature might get pretty painful, even after merging is
working?

err, actually, it'll have to become async to be able to merge, won't it?

> +	/*
> +	 * The driver must store the error location in ->bi_sector, if
> +	 * it supports it. For non-stacked drivers, this should be copied
> +	 * from rq->sector.
> +	 */
> +	if (error_sector)
> +		*error_sector = bio->bi_sector;
> +
> +	ret = 0;
> +	if (bio_flagged(bio, BIO_EOPNOTSUPP))
> +		ret = -EOPNOTSUPP;
> +	else if (!bio_flagged(bio, BIO_UPTODATE))
> +		ret = -EIO;
> +
> +	bio_put(bio);
> +	return ret;
> +}
> +EXPORT_SYMBOL(blkdev_issue_discard);
>
> ...
>
> @@ -1411,6 +1421,10 @@ end_io:
>  			err = -EOPNOTSUPP;
>  			goto end_io;
>  		}
> +		if (bio_discard(bio) && !q->discard_fn) {
> +			err = -EOPNOTSUPP;
> +			goto end_io;
> +		}
>  
>  		ret = q->make_request_fn(q, bio);
>  	} while (ret);
> @@ -1488,7 +1502,7 @@ void submit_bio(int rw, struct bio *bio)
>  	 * If it's a regular read/write or a barrier with data attached,
>  	 * go through the normal accounting stuff before submission.
>  	 */
> -	if (!bio_empty_barrier(bio)) {
> +	if (!bio_empty_barrier(bio) && !bio_discard(bio)) {

Perhaps we should have a new bio_doesnt_have_any_data_in_it() helper,
rather than opn-coding it here?  Assuming that new non-data-containing
bios will turn up in the future.

>  
>  		BIO_BUG_ON(!bio->bi_size);
>  		BIO_BUG_ON(!bio->bi_io_vec);
> @@ -1562,13 +1576,12 @@ static int __end_that_request_first(struct request *req, int error,
>  		int nbytes;
>  
>  		/*
> -		 * For an empty barrier request, the low level driver must
> -		 * store a potential error location in ->sector. We pass
> -		 * that back up in ->bi_sector.
> +		 * For an empty barrier request or sector discard request, the
> +		 * low level driver must store a potential error location in 
> +		 * ->sector. We pass that back up in ->bi_sector.
>  		 */
> -		if (blk_empty_barrier(req))
> +		if (blk_empty_barrier(req) || blk_discard_rq(req))

Maybe ditto.

>  			bio->bi_sector = req->sector;
> -
>  		if (nr_bytes >= bio->bi_size) {
>  			req->bio = bio->bi_next;
>  			nbytes = bio->bi_size;
>
> ...
>
>  
> +static int ftl_discardsect(struct mtd_blktrans_dev *dev,
> +			   unsigned long sector, unsigned nr_sects)
> +{
> +	partition_t *part = (void *)dev;

wuh?  We're casting a `struct mtd_blktrans_ops *' into a partition_t?