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

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Date:	Sun, 22 Dec 2013 22:51:27 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Chen Gang <gang.chen.5i5j@...il.com>
Cc:	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>,
	hare@...e.de, Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH] drivers: target: target_core_mod: use div64_u64_rem()
 instead of operator '%' for u64

On Sun, 2013-12-22 at 17:17 +0800, Chen Gang wrote:
> On 12/22/2013 10:56 AM, Nicholas A. Bellinger wrote:
> > Hi Chen,
> > 
> > On Sat, 2013-12-21 at 10:08 +0800, Chen Gang wrote:
> >> In kernel, need use div64_u64_rem() instead of operator '%' for u64, or
> >> can not pass compiling (with allmodconfig under metag):
> >>
> >>     MODPOST 2909 modules
> >>   ERROR: "__umoddi3" [drivers/target/target_core_mod.ko] undefined!
> >>
> >> Also need u64 type cast for u32 variable multiply u32 variable, or will
> >> cause type overflow issue.
> >>
> >>
> >> Signed-off-by: Chen Gang <gang.chen.5i5j@...il.com>
> >> ---
> >>  drivers/target/target_core_alua.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> > 
> > FYI, this unsigned long long division in core_alua_state_lba_dependent()
> > was fixed for 32-bit in linux-next >= 12192013 code.
> > 
> 
> OK, thanks.
> 
> 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.

Hannes, can you please double check this below..?

Thank you,

--nab

> > 
> >> diff --git a/drivers/target/target_core_alua.c b/drivers/target/target_core_alua.c
> >> index dc0d399..ff2aadc 100644
> >> --- a/drivers/target/target_core_alua.c
> >> +++ b/drivers/target/target_core_alua.c
> >> @@ -489,7 +489,8 @@ static inline int core_alua_state_lba_dependent(
> >>  			u64 first_lba = map->lba_map_first_lba;
> >>  
> >>  			if (segment_mult) {
> >> -				start_lba = lba % (segment_size * segment_mult);
> >> +				u64 tmp = (u64)segment_size * segment_mult;
> >> +				div64_u64_rem(lba, tmp, &start_lba);
> >>  				last_lba = first_lba + segment_size - 1;
> >>  				if (start_lba >= first_lba &&
> >>  				    start_lba <= last_lba) {
> > 
> > 
> 
> 


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