Re: [PATCH] mm: Fix clang W=1 compiler warnings

[Jakub Kicinski <kuba@kernel.org>]

On Fri, 31 Jan 2025 11:12:23 -0800 Bart Van Assche wrote:
> Commit 30c2de0a267c ("mm/vmstat: fix a W=1 clang compiler warning")
> suppresses some but not all compiler warnings that are reported by clang
> when building mm/ with W=1. Hence revert commit 30c2de0a267c and instead
> make NR_LRU_BASE and NR_ZONE_LRU_BASE integer constants instead of
> enumeration constants. Additionally, convert the item##_NORMAL constants
> from type enum vm_event_item into int before subtracting ZONE_NORMAL. The
> latter constant has type enum zone_type.

Could we possibly please still consider taking this in for 6.14? :(
Since the warning comes from vmstat.h pretty much every object file
generates this warning. clang 19 is getting more widely used now, 
its making it hard to see new warnings.

> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 9540b41894da..bc139c704538 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -135,11 +135,20 @@ enum numa_stat_item {
>  #define NR_VM_NUMA_EVENT_ITEMS 0
>  #endif
>  
> +/*
> + * NR_ZONE_LRU_BASE and NR_VM_ZONE_STAT_ITEMS are often added to enumeration
> + * constants of another type than enum_zone_stat_item. Define these constants
> + * as an integer instead of enum node_stat_item to prevent that the compiler
> + * warns about enumeration type mismatches when these constants are used.
> + */
> +#define NR_ZONE_LRU_BASE	(__NR_ZONE_LRU_BASE + 0)
> +#define NR_VM_ZONE_STAT_ITEMS	(__NR_VM_ZONE_STAT_ITEMS + 0)
> +
>  enum zone_stat_item {
>  	/* First 128 byte cacheline (assuming 64 bit words) */
>  	NR_FREE_PAGES,
> -	NR_ZONE_LRU_BASE, /* Used only for compaction and reclaim retry */
> -	NR_ZONE_INACTIVE_ANON = NR_ZONE_LRU_BASE,
> +	__NR_ZONE_LRU_BASE, /* Used only for compaction and reclaim retry */
> +	NR_ZONE_INACTIVE_ANON = __NR_ZONE_LRU_BASE,
>  	NR_ZONE_ACTIVE_ANON,
>  	NR_ZONE_INACTIVE_FILE,
>  	NR_ZONE_ACTIVE_FILE,
> @@ -155,11 +164,18 @@ enum zone_stat_item {
>  #ifdef CONFIG_UNACCEPTED_MEMORY
>  	NR_UNACCEPTED,
>  #endif
> -	NR_VM_ZONE_STAT_ITEMS };
> +	__NR_VM_ZONE_STAT_ITEMS };
> +
> +/*
> + * enum lru_list constants are often added to NR_LRU_BASE. Define NR_LRU_BASE
> + * as an integer instead of enum node_stat_item to prevent that the compiler
> + * warns about enumeration type mismatches.
> + */
> +#define NR_LRU_BASE (__NR_LRU_BASE + 0)
>  
>  enum node_stat_item {
> -	NR_LRU_BASE,
> -	NR_INACTIVE_ANON = NR_LRU_BASE, /* must match order of LRU_[IN]ACTIVE */
> +	__NR_LRU_BASE,
> +	NR_INACTIVE_ANON = __NR_LRU_BASE, /*must match order of LRU_[IN]ACTIVE*/
>  	NR_ACTIVE_ANON,		/*  "     "     "   "       "         */
>  	NR_INACTIVE_FILE,	/*  "     "     "   "       "         */
>  	NR_ACTIVE_FILE,		/*  "     "     "   "       "         */
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 9f3a04345b86..72d1974ee81f 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -135,8 +135,13 @@ static inline void vm_events_fold_cpu(int cpu)
>  #define count_vm_vma_lock_event(x) do {} while (0)
>  #endif
>  
> +/*
> + * item##_NORMAL has type enum vm_event_item while ZONE_NORMAL and zid have
> + * type enum zone_type. Suppress compiler warnings about mixing different
> + * enumeration types by converting item##_NORMAL into an int with '+ 0'.
> + */
>  #define __count_zid_vm_events(item, zid, delta) \
> -	__count_vm_events(item##_NORMAL - ZONE_NORMAL + zid, delta)
> +	__count_vm_events((item##_NORMAL + 0) - ZONE_NORMAL + zid, delta)
>  
>  /*
>   * Zone and node-based page accounting with per cpu differentials.
> @@ -515,7 +520,7 @@ static inline const char *node_stat_name(enum node_stat_item item)
>  
>  static inline const char *lru_list_name(enum lru_list lru)
>  {
> -	return node_stat_name(NR_LRU_BASE + (enum node_stat_item)lru) + 3; // skip "nr_"
> +	return node_stat_name(NR_LRU_BASE + lru) + 3; // skip "nr_"
>  }
>  
>  #if defined(CONFIG_VM_EVENT_COUNTERS) || defined(CONFIG_MEMCG)
> 
>