linux-kernel - Re: [PATCH v2 2/2] mm,nodemask: Use nr_node

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Date:   Fri, 4 Aug 2023 10:14:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mateusz Guzik <mjguzik@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        tglx@...utronix.de, rppt@...nel.org
Subject: Re: [PATCH v2 2/2] mm,nodemask: Use nr_node_ids

On Thu, Aug 03, 2023 at 01:41:42PM -0700, Yury Norov wrote:
> Consider MAX_NUMNODES == 64 and nr_node_ids == 4. Then
> small_nodemask_bits == 64.
> 
> The nodes_full() will set all 64 bits:
> 
>   #define nodes_full(nodemask) __nodes_full(&(nodemask), small_nodemask_bits)
>   static inline bool __nodes_full(const nodemask_t *srcp, unsigned int nbits)
>   {
>           return bitmap_full(srcp->bits, nbits);
>   }
> 
> And the following nodes_weight() will return 64:
> 
>   #define nodes_weight(nodemask) __nodes_weight(&(nodemask), small_nodemask_bits)
>   static inline int __nodes_weight(const nodemask_t *srcp, unsigned int nbits)
>   {
>           return bitmap_weight(srcp->bits, nbits);
>   }

That would be a straight up bug. You're asking it: tell me how many of
these 4 bits are set, and they you say: 64. At which point I'll suggest
they go back to primary school and do the counting lessons again.

Hmm?