linux-kernel - Re: [GIT PULL] memblock:fix validation of NUMA coverage

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Date: Fri, 14 Jun 2024 08:01:33 +0200
From: Jan Beulich <jbeulich@...e.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Borislav Petkov <bp@...en8.de>, Narasimhan V <Narasimhan.V@....com>,
 "Paul E. McKenney" <paulmck@...nel.org>, stable@...r.kernel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Mike Rapoport <rppt@...nel.org>
Subject: Re: [GIT PULL] memblock:fix validation of NUMA coverage

On 13.06.2024 19:38, Linus Torvalds wrote:
> On Thu, 13 Jun 2024 at 10:09, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> Is there some broken scripting that people have started using (or have
>> been using for a while and was recently broken)?
> 
> ... and then when I actually pull the code, I note that the problem
> where it checked _one_ bogus value has just been replaced with
> checking _another_ bogus value.
> 
> Christ.
> 
> What if people use a node ID that is simply outside the range
> entirely, instead of one of those special node IDs?
> 
> And now for memblock_set_node() you should apparently use NUMA_NO_NODE
> to not get a warning, but for memblock_set_region_node() apparently
> the right random constant to use is MAX_NUMNODES.
> 
> Does *any* of this make sense? No.
> 
> How about instead of having two random constants - and not having any
> range checking that I see - just have *one* random constant for "I
> have no range", call that NUMA_NO_NODE,

Just to mention it - my understanding is that this is an ongoing process
heading in this very direction. I'm not an mm person at all, so I can't
tell why the conversion wasn't done / can't be done all in one go.

Jan

> and then have a simple helper
> for "do I have a valid range", and make that be
> 
>    static inline bool numa_valid_node(int nid)
>    { return (unsigned int)nid < MAX_NUMNODES; }
> 
> or something like that? Notice that now *all* of
> 
>  - NUMA_NO_NODE (explicitly no node)
> 
>  - MAX_NUMNODES (randomly used no node)
> 
>  - out of range node (who knows wth firmware tables do?)
> 
> will get the same result from that "numa_valid_node()" function.
> 
> And at that point you don't need to care, you don't need to warn, and
> you don't need to have these insane rules where "sometimes you *HAVE*
> to use NUMA_NO_NODE, or we warn, in other cases MAX_NUMNODES is the
> thing".
> 
> Please? IOW, instead of adding a warning for fragile code, then change
> some caller to follow the new rules, JUST FIX THE STUPID FRAGILITY!
> 
> Or hey, just do
> 
>     #define NUMA_NO_NODE MAX_NUMNODES
> 
> and have two names for the *same* constant, instead fo having two
> different constants with strange semantic differences that seem to
> make no sense and where the memblock code itself seems to go
> back-and-forth on it in different contexts.
> 
>               Linus