linux-kernel - Re: [????] Re: divide error in x86 and cputime

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Message-ID: <20260104220325.0c9b9097@pumpkin>
Date: Sun, 4 Jan 2026 22:03:25 +0000
From: David Laight <david.laight.linux@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Xia Fukun <xiafukun@...wei.com>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...nel.org>, "Li,Rongqing" <lirongqing@...du.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "vschneid@...hat.com" <vschneid@...hat.com>, "mgorman@...e.de"
 <mgorman@...e.de>, "bsegall@...gle.com" <bsegall@...gle.com>,
 "rostedt@...dmis.org" <rostedt@...dmis.org>, "dietmar.eggemann@....com"
 <dietmar.eggemann@....com>, "vincent.guittot@...aro.org"
 <vincent.guittot@...aro.org>, "juri.lelli@...hat.com"
 <juri.lelli@...hat.com>, "Zhangqiao (2012 lab)" <zhangqiao22@...wei.com>
Subject: Re: [????] Re: divide error in x86 and cputime

On Sun, 4 Jan 2026 21:30:39 +0100
Oleg Nesterov <oleg@...hat.com> wrote:

> On 01/04, David Laight wrote:
> >
> > On Sun, 4 Jan 2026 15:23:19 +0100
> > Oleg Nesterov <oleg@...hat.com> wrote:
> >  
> > > Peter, Ingo,
> > >
> > > can you take
> > >
> > > 	[PATCH v3 0/2] x86/math64: handle #DE in mul_u64_u64_div_u64()
> > > 	https://lore.kernel.org/all/20250815164009.GA11676@redhat.com/
> > >
> > > ? at least 1/2 which fixes the problem with #DE ...  
> >  
> 
> ...
> 
> > But this code needs a completely different fix.  
> 
> Of course, this is clear. The fix above doesn't even try to address the
> problems in cputime_adjust() paths.
> 
> But. To me the fix above makes sense regardless. mul_u64_u64_div_u64()
> should nevet trigger #DE, and I thought that we already discussed this
> before.
> 
> Do you agree?

Yes, I think both the generic and x86 versions should just return ~0ull
for both overflow and divide by zero.

	David

> 
> Oleg.
>