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

Douglas Gilbert
2021-01-19T01:05:23Z

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@gmail.com>
>> Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
>> ---
>> 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@suse.de>

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