Re: [PATCH v3 2/2] mm: remove odd HAVE_PTE_SPECIAL

[Christophe LEROY <christophe.leroy@c-s.fr>]

Le 11/04/2018 A  10:03, Laurent Dufour a A(C)critA :
> Remove the additional define HAVE_PTE_SPECIAL and rely directly on
> CONFIG_ARCH_HAS_PTE_SPECIAL.
> 
> There is no functional change introduced by this patch
> 
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> ---
>   mm/memory.c | 19 ++++++++-----------
>   1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 96910c625daa..7f7dc7b2a341 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -817,17 +817,12 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
>    * PFNMAP mappings in order to support COWable mappings.
>    *
>    */
> -#ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
> -# define HAVE_PTE_SPECIAL 1
> -#else
> -# define HAVE_PTE_SPECIAL 0
> -#endif
>   struct page *_vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>   			     pte_t pte, bool with_public_device)
>   {
>   	unsigned long pfn = pte_pfn(pte);
>   
> -	if (HAVE_PTE_SPECIAL) {
> +	if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
>   		if (likely(!pte_special(pte)))
>   			goto check_pfn;
>   		if (vma->vm_ops && vma->vm_ops->find_special_page)
> @@ -862,7 +857,7 @@ struct page *_vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>   		return NULL;
>   	}
>   
> -	/* !HAVE_PTE_SPECIAL case follows: */
> +	/* !CONFIG_ARCH_HAS_PTE_SPECIAL case follows: */
>   
>   	if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
>   		if (vma->vm_flags & VM_MIXEDMAP) {
> @@ -881,7 +876,8 @@ struct page *_vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>   
>   	if (is_zero_pfn(pfn))
>   		return NULL;
> -check_pfn:
> +
> +check_pfn: __maybe_unused

See below

>   	if (unlikely(pfn > highest_memmap_pfn)) {
>   		print_bad_pte(vma, addr, pte, NULL);
>   		return NULL;
> @@ -891,7 +887,7 @@ struct page *_vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>   	 * NOTE! We still have PageReserved() pages in the page tables.
>   	 * eg. VDSO mappings can cause them to exist.
>   	 */
> -out:
> +out: __maybe_unused

Why do you need that change ?

There is no reason for the compiler to complain. It would complain if 
the goto was within a #ifdef, but all the purpose of using IS_ENABLED() 
is to allow the compiler to properly handle all possible cases. That's 
all the force of IS_ENABLED() compared to ifdefs, and that the reason 
why they are plebicited, ref Linux Codying style for a detailed explanation.

Christophe


>   	return pfn_to_page(pfn);
>   }
>   
> @@ -904,7 +900,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>   	/*
>   	 * There is no pmd_special() but there may be special pmds, e.g.
>   	 * in a direct-access (dax) mapping, so let's just replicate the
> -	 * !HAVE_PTE_SPECIAL case from vm_normal_page() here.
> +	 * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here.
>   	 */
>   	if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
>   		if (vma->vm_flags & VM_MIXEDMAP) {
> @@ -1933,7 +1929,8 @@ static int __vm_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
>   	 * than insert_pfn).  If a zero_pfn were inserted into a VM_MIXEDMAP
>   	 * without pte special, it would there be refcounted as a normal page.
>   	 */
> -	if (!HAVE_PTE_SPECIAL && !pfn_t_devmap(pfn) && pfn_t_valid(pfn)) {
> +	if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) &&
> +	    !pfn_t_devmap(pfn) && pfn_t_valid(pfn)) {
>   		struct page *page;
>   
>   		/*
>