Re: [PATCH] zram: support REQ_DISCARD

[Jerome Marchand <jmarchan@redhat.com>]

On 02/24/2014 06:51 AM, Joonsoo Kim wrote:
> zram is ram based block device and can be used by backend of filesystem.
> When filesystem deletes a file, it normally doesn't do anything on data
> block of that file. It just marks on metadata of that file. This behavior
> has no problem on disk based block device, but has problems on ram based
> block device, since we can't free memory used for data block. To overcome
> this disadvantage, there is REQ_DISCARD functionality. If block device
> support REQ_DISCARD and filesystem is mounted with discard option,
> filesystem sends REQ_DISCARD to block device whenever some data blocks are
> discarded. All we have to do is to handle this request.
> 
> This patch implements to flag up QUEUE_FLAG_DISCARD and handle this
> REQ_DISCARD request. With it, we can free memory used by zram if it isn't
> used.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> ---
> This patch is based on master branch of linux-next tree.
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 5ec61be..cff2c0e 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -501,6 +501,20 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
>  	return ret;
>  }
>  
> +static void zram_bio_discard(struct zram *zram, struct bio *bio)
> +{
> +	u32 index = bio->bi_iter.bi_sector >> SECTORS_PER_PAGE_SHIFT;

Hi Joonsoo,

If bi_sector is not aligned on a page size, we might end up discarding
a page that still contain valid data.


> +	size_t n = bio->bi_iter.bi_size;
> +
> +	while (n >= PAGE_SIZE) {
> +		write_lock(&zram->meta->tb_lock);
> +		zram_free_page(zram, index);
> +		write_unlock(&zram->meta->tb_lock);
> +		index++;
> +		n -= PAGE_SIZE;
> +	}
> +}
> +
>  static void zram_reset_device(struct zram *zram, bool reset_capacity)
>  {
>  	size_t index;
> @@ -618,6 +632,12 @@ static void __zram_make_request(struct zram *zram, struct bio *bio)
>  	struct bio_vec bvec;
>  	struct bvec_iter iter;
>  
> +	if (unlikely(bio->bi_rw & REQ_DISCARD)) {
> +		zram_bio_discard(zram, bio);
> +		bio_endio(bio, 0);
> +		return;
> +	}
> +
>  	index = bio->bi_iter.bi_sector >> SECTORS_PER_PAGE_SHIFT;
>  	offset = (bio->bi_iter.bi_sector &
>  		  (SECTORS_PER_PAGE - 1)) << SECTOR_SHIFT;
> @@ -784,6 +804,10 @@ static int create_device(struct zram *zram, int device_id)
>  					ZRAM_LOGICAL_BLOCK_SIZE);
>  	blk_queue_io_min(zram->disk->queue, PAGE_SIZE);
>  	blk_queue_io_opt(zram->disk->queue, PAGE_SIZE);
> +	zram->disk->queue->limits.discard_granularity = PAGE_SIZE;
> +	zram->disk->queue->limits.max_discard_sectors = UINT_MAX;
> +	zram->disk->queue->limits.discard_zeroes_data = 1;
> +	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, zram->disk->queue);
>  
>  	add_disk(zram->disk);
>  
>