linux-kernel - Re: [PATCH v6 3/4] scatterlist: add sgl_compare

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Date:   Mon, 18 Jan 2021 20:04:20 -0500
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     David Disseldorp <ddiss@...e.de>
Cc:     linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
        target-devel@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, martin.petersen@...cle.com,
        jejb@...ux.vnet.ibm.com, bostroesser@...il.com, bvanassche@....org,
        jgg@...pe.ca
Subject: Re: [PATCH v6 3/4] scatterlist: add sgl_compare_sgl() function

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@...il.com>
>> Signed-off-by: Douglas Gilbert <dgilbert@...erlog.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@...e.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