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Subject: Re: [PATCH v3 2/4] scatterlist: add sgl_copy_sgl() function
To:     Douglas Gilbert <dgilbert@interlog.com>,
        linux-scsi@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     martin.petersen@oracle.com, axboe@kernel.dk, bvanassche@acm.org
References: <20201019191928.77845-1-dgilbert@interlog.com>
 <20201019191928.77845-3-dgilbert@interlog.com>
From:   Bodo Stroesser <bostroesser@gmail.com>
Message-ID: <b550e74f-3a7c-f6da-5172-fb38c641436e@gmail.com>
Date:   Tue, 3 Nov 2020 14:22:31 +0100
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Am 19.10.20 um 21:19 schrieb Douglas Gilbert:
> Both the SCSI and NVMe subsystems receive user data from the block
> layer in scatterlist_s (aka scatter gather lists (sgl) which are
> often arrays). If drivers in those subsystems represent storage
> (e.g. a ramdisk) or cache "hot" user data then they may also
> choose to use scatterlist_s. Currently there are no sgl to sgl
> operations in the kernel. Start with a sgl to sgl copy. Stops
> when the first of the number of requested bytes to copy, or the
> source sgl, or the destination sgl is exhausted. So the
> destination sgl will _not_ grow.
> 
> Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
> ---
>   include/linux/scatterlist.h |  4 ++
>   lib/scatterlist.c           | 75 +++++++++++++++++++++++++++++++++++++
>   2 files changed, 79 insertions(+)
> 
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 80178afc2a4a..6649414c0749 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -321,6 +321,10 @@ size_t sg_pcopy_to_buffer(struct scatterlist *sgl, unsigned int nents,
>   size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
>   		       size_t buflen, off_t skip);
>   
> +size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_skip,
> +		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_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 d5770e7f1030..1f9e093ad7da 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -974,3 +974,78 @@ size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
>   	return offset;
>   }
>   EXPORT_SYMBOL(sg_zero_buffer);
> +
> +/**
> + * sgl_copy_sgl - Copy over a destination sgl from a source sgl
> + * @d_sgl:		 Destination sgl
> + * @d_nents:		 Number of SG entries in destination sgl
> + * @d_skip:		 Number of bytes to skip in destination before starting
> + * @s_sgl:		 Source sgl
> + * @s_nents:		 Number of SG entries in source sgl
> + * @s_skip:		 Number of bytes to skip in source before starting
> + * @n_bytes:		 The (maximum) number of bytes to copy
> + *
> + * Returns:
> + *   The number of copied bytes.
> + *
> + * Notes:
> + *   Destination arguments appear before the source arguments, as with memcpy().
> + *
> + *   Stops copying if either d_sgl, s_sgl or n_bytes is exhausted.
> + *
> + *   Since memcpy() is used, overlapping copies (where d_sgl and s_sgl belong
> + *   to the same sgl and the copy regions overlap) are not supported.
> + *
> + *   Large copies are broken into copy segments whose sizes may vary. Those
> + *   copy segment sizes are chosen by the min3() statement in the code below.
> + *   Since SG_MITER_ATOMIC is used for both sides, each copy segment is started
> + *   with kmap_atomic() [in sg_miter_next()] and completed with kunmap_atomic()
> + *   [in sg_miter_stop()]. This means pre-emption is inhibited for relatively
> + *   short periods even in very large copies.
> + *
> + *   If d_skip is large, potentially spanning multiple d_nents then some
> + *   integer arithmetic to adjust d_sgl may improve performance. For example
> + *   if d_sgl is built using sgl_alloc_order(chainable=false) then the sgl
> + *   will be an array with equally sized segments facilitating that
> + *   arithmetic. The suggestion applies to s_skip, s_sgl and s_nents as well.
> + *
> + **/
> +size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_skip,
> +		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
> +		    size_t n_bytes)
> +{
> +	size_t len;
> +	size_t offset = 0;
> +	struct sg_mapping_iter d_iter, s_iter;
> +
> +	if (n_bytes == 0)
> +		return 0;
> +	sg_miter_start(&s_iter, s_sgl, s_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
> +	sg_miter_start(&d_iter, d_sgl, d_nents, SG_MITER_ATOMIC | SG_MITER_TO_SG);
> +	if (!sg_miter_skip(&s_iter, s_skip))
> +		goto fini;
> +	if (!sg_miter_skip(&d_iter, d_skip))
> +		goto fini;
> +
> +	while (offset < n_bytes) {
> +		if (!sg_miter_next(&s_iter))
> +			break;
> +		if (!sg_miter_next(&d_iter))
> +			break;
> +		len = min3(d_iter.length, s_iter.length, n_bytes - offset);
> +
> +		memcpy(d_iter.addr, s_iter.addr, len);
> +		offset += len;
> +		/* LIFO order (stop d_iter before s_iter) needed with SG_MITER_ATOMIC */
> +		d_iter.consumed = len;
> +		sg_miter_stop(&d_iter);
> +		s_iter.consumed = len;
> +		sg_miter_stop(&s_iter);
> +	}
> +fini:
> +	sg_miter_stop(&d_iter);
> +	sg_miter_stop(&s_iter);
> +	return offset;
> +}
> +EXPORT_SYMBOL(sgl_copy_sgl);
> +
> 

Reviewed-by: Bodo Stroesser <bostroesser@gmail.com