linux-kernel - RE: [PATCH 1/2] CPU, NUMA topology ABIs: clarify the overflow issue of sysfs pagebuf

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Date:   Thu, 29 Apr 2021 21:08:50 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "tiantao (H)" <tiantao6@...ilicon.com>,
        "corbet@....net" <corbet@....net>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Valentin Schneider" <valentin.schneider@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: RE: [PATCH 1/2] CPU, NUMA topology ABIs: clarify the overflow issue
 of sysfs pagebuf



> -----Original Message-----
> From: Dave Hansen [mailto:dave.hansen@...el.com]
> Sent: Friday, April 30, 2021 2:21 AM
> To: tiantao (H) <tiantao6@...ilicon.com>; corbet@....net;
> gregkh@...uxfoundation.org; Song Bao Hua (Barry Song)
> <song.bao.hua@...ilicon.com>
> Cc: linux-doc@...r.kernel.org; linux-kernel@...r.kernel.org; Rafael J.
> Wysocki <rafael@...nel.org>; Peter Zijlstra <peterz@...radead.org>; Valentin
> Schneider <valentin.schneider@....com>; Dave Hansen
> <dave.hansen@...ux.intel.com>; Daniel Bristot de Oliveira <bristot@...hat.com>
> Subject: Re: [PATCH 1/2] CPU, NUMA topology ABIs: clarify the overflow issue
> of sysfs pagebuf
> 
> On 4/29/21 12:03 AM, Tian Tao wrote:
> > diff --git a/Documentation/ABI/stable/sysfs-devices-node
> b/Documentation/ABI/stable/sysfs-devices-node
> > index 484fc04..82dfe64 100644
> > --- a/Documentation/ABI/stable/sysfs-devices-node
> > +++ b/Documentation/ABI/stable/sysfs-devices-node
> > @@ -47,7 +47,10 @@ What:		/sys/devices/system/node/nodeX/cpulist
> >  Date:		October 2002
> >  Contact:	Linux Memory Management list <linux-mm@...ck.org>
> >  Description:
> > -		The CPUs associated to the node.
> > +		The CPUs associated to the node. The format is like 0-3,
> > +		8-11, 14,17. maximum size is PAGE_SIZE, so the tail
> > +		of the string will be trimmed while its size is larger
> > +		than PAGE_SIZE.
> 
> I think it's pretty arguable that truncating output on a real system is
> an ABI break.  Doing this would make the interface rather useless.
> 
> Don't we need a real solution rather than throwing up our hands?
> 
> Do we think >PAGE_SIZE data out of a sysfs file is a worse ABI break or
> something?

This kind of cpu list ABIs have been there for many years but have 
never been documented well.

We have two ABIs:
xxx_cpus - in format like 3333333333
xxx_cpus_list - in format like 0,3,5,7,9,11,13....

xxx_cpus_list is another human-readable version of xxx_cpus. It doesn't
include any more useful information than xxx_cpus.

xxx_cpus won't overflow based on BUILD_BUG_ON and maximum NR_CPUS
in kconfig nowadays.

if people all agree the trimmed list is a break of ABI, I think we may
totally remove this list. For these days, this list probably has never
overflowed but literally this could happen.

thoughts?

Thanks
Barry