Re: [PATCH] mm/hugepages: fix "orig_pud" set but not used

[Andrew Morton]

On Thu, 28 Feb 2019 19:49:03 -0500 Qian Cai <cai@xxxxxx> wrote:

> The commit a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent
> hugepages") introduced pudp_huge_get_and_clear_full() but no one uses
> its return code, so just make it void.
> 
> mm/huge_memory.c: In function 'zap_huge_pud':
> mm/huge_memory.c:1982:8: warning: variable 'orig_pud' set but not used
> [-Wunused-but-set-variable]
>   pud_t orig_pud;
>         ^~~~~~~~
> 
> ...
>
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -167,11 +167,11 @@ static inline pmd_t pmdp_huge_get_and_clear_full(struct mm_struct *mm,
>  #endif
>  
>  #ifndef __HAVE_ARCH_PUDP_HUGE_GET_AND_CLEAR_FULL
> -static inline pud_t pudp_huge_get_and_clear_full(struct mm_struct *mm,
> -					    unsigned long address, pud_t *pudp,
> -					    int full)
> +static inline void pudp_huge_get_and_clear_full(struct mm_struct *mm,
> +						unsigned long address,
> +						pud_t *pudp, int full)
>  {
> -	return pudp_huge_get_and_clear(mm, address, pudp);
> +	pudp_huge_get_and_clear(mm, address, pudp);
>  }

Not sure this is a good change.  Future callers might want that return
value.

> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1979,7 +1979,6 @@ spinlock_t *__pud_trans_huge_lock(pud_t *pud, struct vm_area_struct *vma)
>  int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		 pud_t *pud, unsigned long addr)
>  {
> -	pud_t orig_pud;
>  	spinlock_t *ptl;
>  
>  	ptl = __pud_trans_huge_lock(pud, vma);
> @@ -1991,8 +1990,7 @@ int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  	 * pgtable_trans_huge_withdraw after finishing pudp related
>  	 * operations.
>  	 */
> -	orig_pud = pudp_huge_get_and_clear_full(tlb->mm, addr, pud,
> -			tlb->fullmm);
> +	pudp_huge_get_and_clear_full(tlb->mm, addr, pud, tlb->fullmm);

In fact this code perhaps should be passing orig_pud into
pudp_huge_get_and_clear_full().  That could depend on what future
per-arch implementations of pudp_huge_get_and_clear_full() choose to
do.

Anyway, I'll await Matthew's feedback.