Re: [PATCH v6 3/4] scatterlist: add sgl_compare_sgl() function

[David Disseldorp]

On Mon, 18 Jan 2021 11:30:05 -0500, Douglas Gilbert wrote:

> 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.
> 
> A helper function called sgl_compare_sgl_idx() is added. It takes an
> additional parameter (miscompare_idx) which is a pointer. If that
> pointer is non-NULL and a miscompare is detected (i.e. the function
> returns false) then the byte index of the first miscompare is written
> to *miscomapre_idx. Knowing the location of the first miscompare is
> needed to implement the SCSI COMPARE AND WRITE command properly.
> 
> Reviewed-by: Bodo Stroesser <bostroesser@xxxxxxxxx>
> Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
> ---
>  include/linux/scatterlist.h |   8 +++
>  lib/scatterlist.c           | 109 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 117 insertions(+)
> 
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 3f836a3246aa..71be65f9ebb5 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -325,6 +325,14 @@ 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);
> +
> +bool sgl_compare_sgl_idx(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, size_t *miscompare_idx);


This patch looks good and works fine as a replacement for
compare_and_write_do_cmp(). One minor suggestion would be to name it
sgl_equal() or similar, to perhaps better reflect the bool return and
avoid memcmp() confusion. Either way:
Reviewed-by: David Disseldorp <ddiss@xxxxxxx>

Cheers, David