Re: [PATCH] zram: propagate error to user

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

On (03/07/14 10:56), Minchan Kim wrote:
> When we initialized zcomp with single, we couldn't change
> max_comp_streams without zram reset but current interface doesn't
> show any error to user and even it changes max_comp_streams's value
> without any effect so it would make user very confusing.
> 
> This patch prevents max_comp_streams's change when zcomp was
> initialized as single zcomp and emit the error to user(ex, echo).
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  Documentation/blockdev/zram.txt |  9 +++++----
>  drivers/block/zram/zcomp.c      | 10 +++++-----
>  drivers/block/zram/zcomp.h      |  4 ++--
>  drivers/block/zram/zram_drv.c   | 15 +++++++++++----
>  4 files changed, 23 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> index 2604ffed51db..0595c3f56ccf 100644
> --- a/Documentation/blockdev/zram.txt
> +++ b/Documentation/blockdev/zram.txt
> @@ -37,10 +37,11 @@ Note:
>  In order to enable compression backend's multi stream support max_comp_streams
>  must be initially set to desired concurrency level before ZRAM device
>  initialisation. Once the device initialised as a single stream compression
> -backend (max_comp_streams equals to 0) changing the value of max_comp_streams
> -will not take any effect, because single stream compression backend implemented
> -as a special case and does not support dynamic max_comp_streams. Only multi
> -stream backend supports dynamic max_comp_streams adjustment.
> +backend (max_comp_streams equals to 1), you will see error if you try to change
> +the value of max_comp_streams because single stream compression backend
> +implemented as a special case by lock overhead issue and does not support
> +dynamic max_comp_streams. Only multi stream backend supports dynamic
> +max_comp_streams adjustment.
>  
>  3) Select compression algorithm
>  	Using comp_algorithm device attribute one can see available and
> diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> index 92a83df40a27..15fe6a27781b 100644
> --- a/drivers/block/zram/zcomp.c
> +++ b/drivers/block/zram/zcomp.c
> @@ -152,7 +152,7 @@ static void zcomp_strm_multi_release(struct zcomp *comp, struct zcomp_strm *zstr
>  }
>  
>  /* change max_strm limit */
> -static int zcomp_strm_multi_set_max_streams(struct zcomp *comp, int num_strm)
> +static bool zcomp_strm_multi_set_max_streams(struct zcomp *comp, int num_strm)
>  {
>  	struct zcomp_strm_multi *zs = comp->stream;
>  	struct zcomp_strm *zstrm;
> @@ -171,7 +171,7 @@ static int zcomp_strm_multi_set_max_streams(struct zcomp *comp, int num_strm)
>  		zs->avail_strm--;
>  	}
>  	spin_unlock(&zs->strm_lock);
> -	return 0;
> +	return true;
>  }
>  
>  static void zcomp_strm_multi_destroy(struct zcomp *comp)
> @@ -231,10 +231,10 @@ static void zcomp_strm_single_release(struct zcomp *comp,
>  	mutex_unlock(&zs->strm_lock);
>  }
>  
> -static int zcomp_strm_single_set_max_streams(struct zcomp *comp, int num_strm)
> +static bool zcomp_strm_single_set_max_streams(struct zcomp *comp, int num_strm)
>  {
>  	/* zcomp_strm_single support only max_comp_streams == 1 */
> -	return -ENOTSUPP;
> +	return 0;
>  }

IMHO, -ENOTSUPP for unsupported operation fits better than `false'.
yes, currently there are only two possible returns:
	0 -- success
	-ENOTSUPP - not supported operation

though, we can extend functions later and return additional codes, other
than `false' and `true'.

for example, -E2BIG if user requested extremly large number of streams,
like 5000 streams.

>  
>  static void zcomp_strm_single_destroy(struct zcomp *comp)
> @@ -283,7 +283,7 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
>  	return sz;
>  }
>  
> -int zcomp_set_max_streams(struct zcomp *comp, int num_strm)
> +bool zcomp_set_max_streams(struct zcomp *comp, int num_strm)
>  {
>  	return comp->set_max_streams(comp, num_strm);
>  }
> diff --git a/drivers/block/zram/zcomp.h b/drivers/block/zram/zcomp.h
> index 8b8997f8613b..c59d1fca72c0 100644
> --- a/drivers/block/zram/zcomp.h
> +++ b/drivers/block/zram/zcomp.h
> @@ -46,7 +46,7 @@ struct zcomp {
>  
>  	struct zcomp_strm *(*strm_find)(struct zcomp *comp);
>  	void (*strm_release)(struct zcomp *comp, struct zcomp_strm *zstrm);
> -	int (*set_max_streams)(struct zcomp *comp, int num_strm);
> +	bool (*set_max_streams)(struct zcomp *comp, int num_strm);
>  	void (*destroy)(struct zcomp *comp);
>  };
>  
> @@ -64,5 +64,5 @@ int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
>  int zcomp_decompress(struct zcomp *comp, const unsigned char *src,
>  		size_t src_len, unsigned char *dst);
>  
> -int zcomp_set_max_streams(struct zcomp *comp, int num_strm);
> +bool zcomp_set_max_streams(struct zcomp *comp, int num_strm);
>  #endif /* _ZCOMP_H_ */
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 7631ef0cbc41..eead9db9b100 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -128,17 +128,24 @@ static ssize_t max_comp_streams_store(struct device *dev,
>  	struct zram *zram = dev_to_zram(dev);
>  
>  	if (kstrtoint(buf, 0, &num))
> -		return -EINVAL;
> +		goto out;
>  	if (num < 1)
> -		return -EINVAL;
> +		goto out;
> +
>  	down_write(&zram->init_lock);
>  	if (init_done(zram)) {
> -		if (zcomp_set_max_streams(zram->comp, num))
> +		if (!zcomp_set_max_streams(zram->comp, num)) {
>  			pr_info("Cannot change max compression streams\n");
> +			goto out_unlock;
> +		}
>  	}
> +
>  	zram->max_comp_streams = num;

agree, better to avoid this for a single stream.

> -	up_write(&zram->init_lock);
>  	return len;
> +out_unlock:
> +	up_write(&zram->init_lock);
> +out:
> +	return -EINVAL;
>  }
>  
>  static ssize_t comp_algorithm_show(struct device *dev,
> -- 
> 1.9.0
>