linux-kernel - Re: [PATCH] x86/resctrl: Fix rdt_find

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Message-ID: <3b097a0a-bcc5-1929-eb50-0e251ebc1152@intel.com>
Date:   Mon, 10 Dec 2018 13:42:10 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, tony.luck@...el.com,
        jithu.joseph@...el.com, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: Fix rdt_find_domain() return value checks

Hi Boris,

On 12/10/2018 1:04 PM, Borislav Petkov wrote:
> On Mon, Dec 10, 2018 at 12:42:14PM -0800, Reinette Chatre wrote:
>> Would you be ok if the above is changed to
>>
>> if (id < 0)
>>         return ERR_PTR(-ENOENT);
>>
>> as part of this patch?
> 
> Yap.
> 
>> Looking at rdtgroup_mondata_show() is does seem as though ENOENT is the
>> actual intended error value,
> 
> #define ENOENT           2      /* No such file or directory */
> 
>> although ENODEV could perhaps also be considered since such a result
>> reflects that a particular cache instance could not be found.
> 
> #define ENODEV          19      /* No such device */
> 
> Yeah, they both kinda look ok to me and in the end of the day, the thing
> that matters most is pinpointing the place in the code which causes the
> error.

Thank you for the sanity check. I think ENODEV may reflect the issue
better and will do so in the next version. This would not affect the
current code (quoted below) that subsequently translates the error to
ENOENT.

> And looking at that particular part:
> 
>         r = &rdt_resources_all[resid];
>         d = rdt_find_domain(r, domid, NULL);
>         if (!d) {
>                 ret = -ENOENT;
>                 goto out;
>         }
> 
> You *could* put in there something like:
> 
> 	seq_printf(m, "Cannot find a domain for resource ID: %d\n", domid);
> 
> and this way it is perfectly clear which error path it is.
> 
> :-)
> 

>From what I understand this scenario should never happen - at the time
knowledge of the resource instance is removed then these mondata files
referring to it will be removed also. Even so, I'd prefer not to make
any user visible changes without a specific requirement or need to do
so. In this case ENOENT is the only error code returned during this user
space interaction so from what I can tell it is currently clear which
error path it is.

Reinette