linux-kernel - Re: [PATCH] drivers: target: target_core_mod: use div64_u64

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Date:	Thu, 09 Jan 2014 11:17:04 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
CC:	Chen Gang <gang.chen.5i5j@...il.com>,
	James Hogan <james.hogan@...tec.com>,
	linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH] drivers: target: target_core_mod: use div64_u64_rem()
 instead of operator '%' for u64

On 01/09/2014 12:18 AM, Nicholas A. Bellinger wrote:
> On Wed, 2014-01-08 at 08:32 +0100, Hannes Reinecke wrote:
>> On 12/24/2013 04:35 AM, Chen Gang wrote:
>>> On 12/23/2013 02:51 PM, Nicholas A. Bellinger wrote:
>>>> On Sun, 2013-12-22 at 17:17 +0800, Chen Gang wrote:
>
> <SNIP>
>
>>>>> The related fix patch changed "start_lba = lba % ..." to "start_lba =
>>>>> lba / ...", and also assumed "segment_size * segment_mult" is still
>>>>> within u32 (can not cause type over flow).
>>>>>
>>>>> I guess the original author already knew about them, and intended to do
>>>>> like that (if not, please let me know, thanks).
>>>>>
>>>>
>>>> Sorry, your correct that the original code is using modulo division to
>>>> calculate start_lba.
>>>>
>>>
>>> Oh, that's all right, (in fact, don't need sorry), I am not quite
>>> familiar with the details, so need related member help check it.  :-)
>>>
>>>> Hannes, can you please double check this below..?
>>>>
>>>
>>> Please help check when have time, thanks.
>>>
>> I would even convert segment_size and segment_mult to u64,
>> to ensure no overflows occur:
>>
>> diff --git a/drivers/target/target_core_alua.c
>> b/drivers/target/target_core_alua
>> .c
>> index 9b1856d..54b1e52 100644
>> --- a/drivers/target/target_core_alua.c
>> +++ b/drivers/target/target_core_alua.c
>> @@ -477,8 +477,7 @@ static inline int core_alua_state_lba_dependent(
>>          u8 *alua_ascq)
>>   {
>>          struct se_device *dev = cmd->se_dev;
>> -       u32 segment_size, segment_mult, sectors;
>> -       u64 lba;
>> +       u64 segment_size, segment_mult, sectors, lba;
>>
>>          /* Only need to check for cdb actually containing LBAs */
>>          if (!(cmd->se_cmd_flags & SCF_SCSI_DATA_CDB))
>>
>>
>
> Will squash the above into the original patch shortly in for-next..
>
>> Other than that the sector_div() patch is correct.
>>
>
> <nod> Thanks for confirming that sector_div() is correct here vs. the
> original code using modulo that Chen had pointed out.
>
Ah, _that_ was the issue.
I was wondering why you kept on poking me ...

Well.
No, that's actually _not_ correct.
The correct fix would be

diff --git a/drivers/target/target_core_alua.c 
b/drivers/target/target_core_alua.c
index 54b1e52..12da9b3 100644
--- a/drivers/target/target_core_alua.c
+++ b/drivers/target/target_core_alua.c
@@ -500,8 +500,7 @@ static inline int core_alua_state_lba_dependent(

                         if (segment_mult) {
                                 u64 tmp = lba;
-                               sector_div(tmp, segment_size * 
segment_mult);
-                               start_lba = tmp;
+                               start_lba = sector_div(tmp, segment_size 
* segment_mult);

                                 last_lba = first_lba + segment_size - 1;
                                 if (start_lba >= first_lba &&
(beware of line breaks ...)
Thing is, we need to calculate the offset into the segment to figure out 
which map entry to use.
The actual number of the segment (as had been calculated with the 
original fix) is immaterial here.

Sorry for this. The email thread just flew past me during Xmas
with me not paying real attention.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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