linux-kernel - Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator

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Date:   Tue, 4 Apr 2017 15:28:59 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "paolo.valente@...aro.org" <paolo.valente@...aro.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "fchecconi@...il.com" <fchecconi@...il.com>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "avanzini.arianna@...il.com" <avanzini.arianna@...il.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "tj@...nel.org" <tj@...nel.org>,
        "ulf.hansson@...aro.org" <ulf.hansson@...aro.org>
Subject: Re: [PATCH V2 04/16] block, bfq: modify the peak-rate estimator

On Tue, 2017-04-04 at 12:42 +0200, Paolo Valente wrote:
> > Il giorno 31 mar 2017, alle ore 17:31, Bart Van Assche <bart.vanassche@...disk.com> ha scritto:
> > 
> > On Fri, 2017-03-31 at 14:47 +0200, Paolo Valente wrote:
> > > +               delta_ktime = ktime_get();
> > > +       delta_ktime = ktime_sub(delta_ktime, bfqd->last_budget_start);
> > > +       delta_usecs = ktime_to_us(delta_ktime);
> > 
> > This patch changes the type of the variable in which the result of ktime_to_us()
> > is stored from u64 into u32 and next compares that result with LONG_MAX. Since
> > ktime_to_us() returns a signed 64-bit number, are you sure you want to store that
> > result in a 32-bit variable? If ktime_to_us() would e.g. return 0xffffffff00000100
> > or 0x100000100 then the assignment will truncate these numbers to 0x100.
> 
> The instruction above the assignment you highlight stores in
> delta_ktime the difference between 'now' and the last budget start.
> The latter may have happened at most about 100 ms before 'now'.  So
> there should be no overflow issue.

Hello Paolo,

Please double check the following code: if (delta_usecs < 1000 || delta_usecs >= LONG_MAX)
Since delta_usecs is a 32-bit variable and LONG_MAX a 64-bit constant on 64-bit systems
I'm not sure that code will do what it is intended to do.

Thanks,

Bart.