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 14:38:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.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

On 4/29/21 2:08 PM, Song Bao Hua (Barry Song) wrote:
>> 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?

>From what Greg said, it sounds like removing the BUILD_BUG_ON(), making
it a binary sysfs file, and making it support arbitrary lengths is the
way to go.