Re: [PATCH v2 5/6] state: backend_storage_direct: also use cached data on write

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, Jun 24, 2016 at 12:06:01PM +0200, Markus Pargmann wrote:
> From: Michael Grzeschik <m.grzeschik@pengutronix.de>
> 
> Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
> Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
> ---
>  common/state/backend_bucket_direct.c | 55 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 55 insertions(+)

What's the problem this patch tries to solve?

Sascha

> 
> diff --git a/common/state/backend_bucket_direct.c b/common/state/backend_bucket_direct.c
> index 08892f001e25..772267f6dc1d 100644
> --- a/common/state/backend_bucket_direct.c
> +++ b/common/state/backend_bucket_direct.c
> @@ -29,6 +29,10 @@ struct state_backend_storage_bucket_direct {
>  
>  	int fd;
>  
> +	/* Cached data of the last read/write */
> +	u8 *current_data;
> +	ssize_t current_data_len;
> +
>  	struct device_d *dev;
>  };
>  
> @@ -99,6 +103,29 @@ static int state_backend_bucket_direct_read(struct state_backend_storage_bucket
>  	return 0;
>  }
>  
> +
> +static int state_backend_bucket_direct_fill_cache(
> +		struct state_backend_storage_bucket *bucket)
> +{
> +	struct state_backend_storage_bucket_direct *direct =
> +	    get_bucket_direct(bucket);
> +	ssize_t read_len;
> +	uint8_t *buf;
> +	int ret;
> +
> +	ret = state_backend_bucket_direct_read(bucket, &buf, &read_len);
> +	if (ret < 0) {
> +		dev_err(direct->dev, "Failed to read from file, %d\n", ret);
> +		free(buf);
> +		return ret;
> +	}
> +
> +	direct->current_data = buf;
> +	direct->current_data_len = read_len;
> +
> +	return 0;
> +}
> +
>  static int state_backend_bucket_direct_write(struct state_backend_storage_bucket
>  					     *bucket, const uint8_t * buf,
>  					     ssize_t len)
> @@ -111,6 +138,11 @@ static int state_backend_bucket_direct_write(struct state_backend_storage_bucket
>  	if (direct->max_size && len > direct->max_size)
>  		return -E2BIG;
>  
> +	/* Nothing in cache? Then read to see what is on the device currently */
> +	if (!direct->current_data) {
> +		state_backend_bucket_direct_fill_cache(bucket);
> +	}
> +
>  	ret = lseek(direct->fd, direct->offset, SEEK_SET);
>  	if (ret < 0) {
>  		dev_err(direct->dev, "Failed to seek file, %d\n", ret);
> @@ -119,6 +151,26 @@ static int state_backend_bucket_direct_write(struct state_backend_storage_bucket
>  
>  	meta.magic = direct_magic;
>  	meta.written_length = len;
> +
> +	/*
> +	 * If we would write the same data that is currently on the device, we
> +	 * can return successfully without writing.
> +	 * Note that the cache may still be empty if the storage is empty or
> +	 * errors occured.
> +	 */
> +	if (direct->current_data) {
> +		dev_dbg(direct->dev, "Comparing cached data, writing %zd bytes, cached %zd bytes\n",
> +			len, direct->current_data_len);
> +		if (len == direct->current_data_len &&
> +		    !memcmp(direct->current_data, buf, len)) {
> +			dev_dbg(direct->dev, "Data already on device, not writing again\n");
> +			return ret;
> +		} else {
> +			free(direct->current_data);
> +			direct->current_data = NULL;
> +		}
> +	}
> +
>  	ret = write_full(direct->fd, &meta, sizeof(meta));
>  	if (ret < 0) {
>  		dev_err(direct->dev, "Failed to write metadata to file, %d\n", ret);
> @@ -147,6 +199,9 @@ static void state_backend_bucket_direct_free(struct
>  	struct state_backend_storage_bucket_direct *direct =
>  	    get_bucket_direct(bucket);
>  
> +	if (direct->current_data)
> +		free(direct->current_data);
> +
>  	close(direct->fd);
>  	free(direct);
>  }
> -- 
> 2.8.1
> 
> 
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