[PATCH] block: Use div64_ul instead of do_div

[PATCH] block: Use div64_ul instead of do_div

[cgel.zte]

From: Changcheng Deng <deng.changcheng@zte.com.cn>

do_div() does a 64-by-32 division. If the divisor is unsigned long, using
div64_ul can avoid truncation to 32-bit.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
---
 block/blk-throttle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 39bb6e68a9a2..2db635d66617 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1903,7 +1903,7 @@ static void throtl_downgrade_check(struct throtl_grp *tg)
 
 	if (tg->bps[READ][LIMIT_LOW]) {
 		bps = tg->last_bytes_disp[READ] * HZ;
-		do_div(bps, elapsed_time);
+		div64_ul(bps, elapsed_time);
 		if (bps >= tg->bps[READ][LIMIT_LOW])
 			tg->last_low_overflow_time[READ] = now;
 	}
-- 
2.25.1

[PATCH v2] block: Use div64_ul instead of do_div

[cgel.zte]

From: Changcheng Deng <deng.changcheng@zte.com.cn>

do_div() does a 64-by-32 division. Here the divisor is an unsigned long
which on some platforms is 64 bit wide. So use div64_ul instead of do_div
to avoid a possible truncation.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
---
 block/blk-throttle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 39bb6e68a9a2..2db635d66617 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1903,7 +1903,7 @@ static void throtl_downgrade_check(struct throtl_grp *tg)
 
 	if (tg->bps[READ][LIMIT_LOW]) {
 		bps = tg->last_bytes_disp[READ] * HZ;
-		do_div(bps, elapsed_time);
+		bps = div64_ul(bps, elapsed_time);
 		if (bps >= tg->bps[READ][LIMIT_LOW])
 			tg->last_low_overflow_time[READ] = now;
 	}
-- 
2.25.1

Re: [PATCH v2] block: Use div64_ul instead of do_div

[Tejun Heo]

On Thu, Nov 18, 2021 at 03:40:33AM +0000, cgel.zte@gmail.com wrote:
> From: Changcheng Deng <deng.changcheng@zte.com.cn>
> 
> do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> to avoid a possible truncation.
> 
> Reported-by: Zeal Robot <zealci@zte.com.cn>
> Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
> ---
>  block/blk-throttle.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 39bb6e68a9a2..2db635d66617 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -1903,7 +1903,7 @@ static void throtl_downgrade_check(struct throtl_grp *tg)
>  
>  	if (tg->bps[READ][LIMIT_LOW]) {
>  		bps = tg->last_bytes_disp[READ] * HZ;
> -		do_div(bps, elapsed_time);
> +		bps = div64_ul(bps, elapsed_time);

Yeah, the divisor is ulong for whatever reason but the thing is in jiffies.
I have a hard time imagining it overflowing 32bit. I'm not sure how useful
this patch is.

-- 
tejun

Re: [PATCH v2] block: Use div64_ul instead of do_div

[Michal Koutný]

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Hello.

On Thu, Nov 18, 2021 at 03:40:33AM +0000, cgel.zte@gmail.com wrote:
> Reported-by: Zeal Robot <zealci@zte.com.cn>
> Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>

Just feedback to the bot or to you -- the same do_div() is in the same
function for writes too. I think the processing shouldn't stop at the
first occurence.

Michal


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