linux-kernel - Re: [PATCH v2] zram: support REQ

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Date:	Wed, 26 Feb 2014 17:07:32 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH v2] zram: support REQ_DISCARD

Hi Joonsoo,

On Wed, Feb 26, 2014 at 02:23:15PM +0900, 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.
> 
> v2: handle unaligned case commented by Jerome
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 5ec61be..5364c1e 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -501,6 +501,36 @@ 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;
> +	size_t n = bio->bi_iter.bi_size;

Nitpick:
Please use more meaningful name(ex, len) rather than 'n'.

> +	size_t misalign;
> +
> +	 * On some arch, logical block (4096) aligned request couldn't be
> +	 * aligned to PAGE_SIZE, since their PAGE_SIZE aren't 4096.
> +	 * Therefore we should handle this misaligned case here.
> +	 */
> +	misalign = (bio->bi_iter.bi_sector &
> +			(SECTORS_PER_PAGE - 1)) << SECTOR_SHIFT;
> +	if (misalign) {
> +		if (n < misalign)
> +			return;
> +
> +		n -= misalign;
> +		index++;
> +	}
> +
> +	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 +648,12 @@ 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 +820,10 @@ 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;

I don't know what discard_zeroes_data does mean. It seems we should
make sure zram should return zero pages for discarded block on next
time but prolblem could happen if you bail out in discard logic
due to misalign but caller seem to know it was successful?

What happens in this case?

> +	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, zram->disk->queue);
>  
>  	add_disk(zram->disk);
>  
> -- 
> 1.7.9.5
> 
> --
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-- 
Kind regards,
Minchan Kim
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