linux-kernel - Re: NUMA x86: add constraints check for nid parameters

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Date:	Wed, 7 Dec 2011 20:40:22 +0100
From:	Petr Holasek <pholasek@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	Anton Arapov <anton@...hat.com>
Subject: Re: NUMA x86: add constraints check for nid parameters

On Tue, 06 Dec 2011, David Rientjes wrote:

> 
> On Fri, 2 Dec 2011, Petr Holasek wrote:
> 
> > > > > > This patch adds constraints checks into __node_distance() and
> > > > > > numa_set_distance() functions. If from or to parameters are
> > > > > > lower than zero, it results into oops now.
> > > > > 
> > > > > Passing negative numbers into __node_distance() sounds like a bug in
> > > > > the caller, and this patch will remove our means of detecting that bug.
> > > > 
> > > > That's true, but upper boundary is checked now, so why not to check lower?
> > > 
> > > Because it adds more code to the kernel and can hide bugs?
> > > 
> 
> The upper bound is checked to ensure that we don't dereference past end of 
> the array that stores the distance table, so it will catch errors for 
> things like memory hotplug when additional nodes are onlined and the data 
> structure isn't updated accordingly.

Thanks for clarification, I missed this case.

> 
> > > If what we're doing here is to be defensive against buggy BIOS tables
> > > (a good idea) then we should validate the BIOS table values as close as
> > > possible to the point where they were read frmo the BIOS.  And we should
> > > (probably) emit a warning if a bad table entry is detected, rather than
> > > silently fixing it up.
> > 
> > numa_set_distance() does exactly what you described above, only emits a
> > warning. I agree with your objections with __node_distance() checks, it
> > really can hide bugs in caller. So silent fix-up is the main problem and
> > we shouldn't check anything so the caller will be advised when using 
> > wrong nid by oops with a benefit of less code for us. Do I understand your
> > opinion on this type of code?
> 
> I'd have no objection to adding a check to numa_set_distance() to ensure 
> the node ids are non-negative in the same way we check that the distances 
> themselves are non-negative; that can catch errors when pxms are used 
> uninitialized when parsing the SRAT.  However, I think adding the check to 
> __node_distance() is unnecessary.

Ok, I'll try v2 without __node_distance()
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