Re: [PATCH v3 3/4] scatterlist: add sgl_compare_sgl() function

[Bodo Stroesser]

Am 19.10.20 um 21:19 schrieb Douglas Gilbert:
> After enabling copies between scatter gather lists (sgl_s),
> another storage related operation is to compare two sgl_s.
> This new function is modelled on NVMe's Compare command and
> the SCSI VERIFY(BYTCHK=1) command. Like memcmp() this function
> returns false on the first miscompare and stops comparing.
>
> Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
> ---
>   include/linux/scatterlist.h |  4 +++
>   lib/scatterlist.c           | 61 +++++++++++++++++++++++++++++++++++++
>   2 files changed, 65 insertions(+)
>
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 6649414c0749..ae260dc5fedb 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -325,6 +325,10 @@ size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_ski
>   		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
>   		    size_t n_bytes);
>   
> +bool sgl_compare_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
> +		     struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
> +		     size_t n_bytes);
> +
>   /*
>    * Maximum number of entries that will be allocated in one piece, if
>    * a list larger than this is required then chaining will be utilized.
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 1f9e093ad7da..49185536acba 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -1049,3 +1049,64 @@ size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_ski
>   }
>   EXPORT_SYMBOL(sgl_copy_sgl);
>   
> +/**
> + * sgl_compare_sgl - Compare x and y (both sgl_s)
> + * @x_sgl:		 x (left) sgl
> + * @x_nents:		 Number of SG entries in x (left) sgl
> + * @x_skip:		 Number of bytes to skip in x (left) before starting
> + * @y_sgl:		 y (right) sgl
> + * @y_nents:		 Number of SG entries in y (right) sgl
> + * @y_skip:		 Number of bytes to skip in y (right) before starting
> + * @n_bytes:		 The (maximum) number of bytes to compare
> + *
> + * Returns:
> + *   true if x and y compare equal before x, y or n_bytes is exhausted.
> + *   Otherwise on a miscompare, returns false (and stops comparing).
> + *
> + * Notes:
> + *   x and y are symmetrical: they can be swapped and the result is the same.
> + *
> + *   Implementation is based on memcmp(). x and y segments may overlap.
> + *
> + *   The notes in sgl_copy_sgl() about large sgl_s _applies here as well.
> + *
> + **/
> +bool sgl_compare_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
> +		     struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
> +		     size_t n_bytes)
> +{
> +	bool equ = true;
> +	size_t len;
> +	size_t offset = 0;
> +	struct sg_mapping_iter x_iter, y_iter;
> +
> +	if (n_bytes == 0)
> +		return true;
> +	sg_miter_start(&x_iter, x_sgl, x_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
> +	sg_miter_start(&y_iter, y_sgl, y_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
> +	if (!sg_miter_skip(&x_iter, x_skip))
> +		goto fini;
> +	if (!sg_miter_skip(&y_iter, y_skip))
> +		goto fini;
> +
> +	while (equ && offset < n_bytes) {
> +		if (!sg_miter_next(&x_iter))
> +			break;
> +		if (!sg_miter_next(&y_iter))
> +			break;
> +		len = min3(x_iter.length, y_iter.length, n_bytes - offset);
> +
> +		equ = !memcmp(x_iter.addr, y_iter.addr, len);
> +		offset += len;
> +		/* LIFO order is important when SG_MITER_ATOMIC is used */
> +		y_iter.consumed = len;
> +		sg_miter_stop(&y_iter);
> +		x_iter.consumed = len;
> +		sg_miter_stop(&x_iter);
> +	}
> +fini:
> +	sg_miter_stop(&y_iter);
> +	sg_miter_stop(&x_iter);
> +	return equ;
> +}
> +EXPORT_SYMBOL(sgl_compare_sgl);

Reviewed-by: Bodo Stroesser <bostroesser@xxxxxxxxx>