Re: [PATCH v2] zram: support REQ_DISCARD

[Jerome Marchand]

On 02/26/2014 02:16 PM, Sergey Senozhatsky wrote:
> Hello,
> 
> On (02/26/14 14:23), 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@xxxxxxx>
>>
>> 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;
>> +	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;
>> +	}
>> +}
>> +
> 
> a side note, do we need zram_bio_discard() function? I mean, can we handle
> discard request in zram_bvec_rw(), where we already know index, etc. (passed
> from __zram_make_request())?
> 

We'd still have to make sure not to discard pages that are still partially
used, but it might simplify the code: __zram_make_request() already takes
care of splitting the request.

> for example:
> 
> @@ -510,6 +510,11 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
>                 ret = zram_bvec_write(zram, bvec, index, offset);
>         }
>  
> +       if (unlikely(bio->bi_rw & REQ_DISCARD)) {

+		if (!is_partial_io(bvec) {

> +               write_lock(&zram->meta->tb_lock);
> +               zram_free_page(zram, index);
> +               write_unlock(&zram->meta->tb_lock);

+		}

Also this code might still call zram_bvec_read() and increase num_reads
for discard request: I guess bio_data_dir(bio) == READ == 0 in this case.

Btw, why __zram_make_request() has an that rw argument? All the information
it needs is passed by the bio argument already. I kind of recollect to have
seen a cleanup patch that get rid of it or is it just my imagination?

Jerome

> +       }
>         return ret;
>  }
> 
> 	-ss
> 
>>  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;
>> +	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, zram->disk->queue);
>>  
>>  	add_disk(zram->disk);
>>  
>> -- 
>> 1.7.9.5
>>

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