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

[Douglas Gilbert]

On 2021-01-18 6:27 p.m., David Disseldorp wrote:
> 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>

Thanks. NVMe calls the command that does this Compare and SCSI uses
COMPARE AND WRITE (and VERIFY(BYTCHK=1) ) but "equal" is fine with me.
There will be another patchset version (at least) so there is time
to change.

Do you want:
  - sgl_equal(...), or
  - sgl_equal_sgl(...) ?

Doug Gilbert