linux-kernel - Re: [PATCH] include/linux/nodemask.h: Use nr_node_ids instead of MAX_NUMNODES in nodemask_pr

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Date:   Tue, 18 Dec 2018 16:41:18 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Oscar Salvador <osalvador@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] include/linux/nodemask.h: Use nr_node_ids instead of
 MAX_NUMNODES in nodemask_pr_args

On 12/18/2018 03:41 PM, Andrew Morton wrote:
> On Tue, 18 Dec 2018 12:30:31 -0500 Waiman Long <longman@...hat.com> wrote:
>
>> When viewing the /proc/<pid>/status file, one can see output lines like
>>
>>   Cpus_allowed:	ffffffff,ffffffff,ffffffff
>>   Cpus_allowed_list:	0-95
>>   Mems_allowed:	00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0000000f
>>   Mems_allowed_list:	0-3
> Oh dear.
>
>> It looks really strange that so many bits are displayed in the
>> "Mems_allowed:" line.  Whereas the "Cpus_allowed:" line is perfectly
>> fine.
>>
>> It is because the nodemask_pr_args() macro uses MAX_NUMNODES as the
>> number of nodes to iterate. The cpumask_pr_args() macro uses nr_cpu_ids
>> instead of MAX_CPUS. For nodes, there is a corresponding nr_node_ids.
>> So it makes sense to use nr_node_ids instead to be consistent with
>> "Cpus_allowed:".
>>
>> With that change, the "Mems_allowed:" line becomes
>>
>>   Mems_allowed:	f
> OK, I guess.  There is a risk of breaking existing userspace, but any
> half-decent parser should treat the above as equivalent.
>
> However there are quite a lot of nodemask_pr_args() callers in the tree
> and this patch potentially changes behaviour at all those sites. 
> Please work through all those callers and show how the output is
> altered so we can consider the patch's complete effect.
>
>
The /proc/<pid>/status file is the only one that will be affected by
this patch. I believe the "Mems_allowed_list" is what most users are
using, not the "Mems_allowed".

Cheers,
Longman