Re: [RFC PATCH 14/15] mm: introduce MIN_MAX_ORDER to replace MAX_ORDER as compile time constant.

[Mike Rapoport]

On Thu, Aug 05, 2021 at 03:02:52PM -0400, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
> 
> For other MAX_ORDER uses (described below), there is no need or too much
> hassle to convert certain static array to dynamic ones. Add
> MIN_MAX_ORDER to serve as compile time constant in place of MAX_ORDER.
> 
> ARM64 hypervisor maintains its own free page list and does not import
> any core kernel symbols, so soon-to-be runtime variable MAX_ORDER is not
> accessible in ARM64 hypervisor code. Also there is no need to allocating
> very large pages.
> 
> In SLAB/SLOB/SLUB, 2-D array kmalloc_caches uses MAX_ORDER in its second
> dimension. It is too much hassle to allocate memory for kmalloc_caches
> before any proper memory allocator is set up.
> 
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Quentin Perret <qperret@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  arch/arm64/kvm/hyp/include/nvhe/gfp.h | 2 +-
>  arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 3 ++-
>  include/linux/mmzone.h                | 3 +++
>  include/linux/slab.h                  | 8 ++++----
>  mm/slab.c                             | 2 +-
>  mm/slub.c                             | 7 ++++---
>  6 files changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> index fb0f523d1492..c774b4a98336 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> @@ -16,7 +16,7 @@ struct hyp_pool {
>  	 * API at EL2.
>  	 */
>  	hyp_spinlock_t lock;
> -	struct list_head free_area[MAX_ORDER];
> +	struct list_head free_area[MIN_MAX_ORDER];
>  	phys_addr_t range_start;
>  	phys_addr_t range_end;
>  	unsigned short max_order;
> diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> index 41fc25bdfb34..a1cc1b648de0 100644
> --- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> +++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> @@ -226,7 +226,8 @@ int hyp_pool_init(struct hyp_pool *pool, u64 pfn, unsigned int nr_pages,
>  	int i;
>  
>  	hyp_spin_lock_init(&pool->lock);
> -	pool->max_order = min(MAX_ORDER, get_order(nr_pages << PAGE_SHIFT));
> +
> +	pool->max_order = min(MIN_MAX_ORDER, get_order(nr_pages << PAGE_SHIFT));
>  	for (i = 0; i < pool->max_order; i++)
>  		INIT_LIST_HEAD(&pool->free_area[i]);
>  	pool->range_start = phys;
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 09aafc05aef4..379dada82d4b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -27,11 +27,14 @@
>  #ifndef CONFIG_ARCH_FORCE_MAX_ORDER
>  #ifdef CONFIG_SET_MAX_ORDER
>  #define MAX_ORDER CONFIG_SET_MAX_ORDER
> +#define MIN_MAX_ORDER CONFIG_SET_MAX_ORDER
>  #else
>  #define MAX_ORDER 11
> +#define MIN_MAX_ORDER MAX_ORDER
>  #endif /* CONFIG_SET_MAX_ORDER */
>  #else
>  #define MAX_ORDER CONFIG_ARCH_FORCE_MAX_ORDER
> +#define MIN_MAX_ORDER CONFIG_ARCH_FORCE_MAX_ORDER
>  #endif /* CONFIG_ARCH_FORCE_MAX_ORDER */
>  #define MAX_ORDER_NR_PAGES (1 << (MAX_ORDER - 1))

The end result of this #ifdef explosion looks entirely unreadable:

/* Free memory management - zoned buddy allocator.  */
#ifndef CONFIG_ARCH_FORCE_MAX_ORDER
#ifdef CONFIG_SET_MAX_ORDER
/* Defined in mm/page_alloc.c */
extern int buddy_alloc_max_order;

#define MAX_ORDER buddy_alloc_max_order
#define MIN_MAX_ORDER CONFIG_SET_MAX_ORDER
#else
#define MAX_ORDER 11
#define MIN_MAX_ORDER MAX_ORDER
#endif /* CONFIG_SET_MAX_ORDER */
#else

#ifdef CONFIG_SPARSEMEM_VMEMMAP
/* Defined in mm/page_alloc.c */
extern int buddy_alloc_max_order;

#define MAX_ORDER buddy_alloc_max_order
#else
#define MAX_ORDER CONFIG_ARCH_FORCE_MAX_ORDER
#endif /* CONFIG_SPARSEMEM_VMEMMAP */
#define MIN_MAX_ORDER CONFIG_ARCH_FORCE_MAX_ORDER
#endif /* CONFIG_ARCH_FORCE_MAX_ORDER */

> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 2c0d80cca6b8..d8747c158db6 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -244,8 +244,8 @@ static inline void __check_heap_object(const void *ptr, unsigned long n,
>   * to do various tricks to work around compiler limitations in order to
>   * ensure proper constant folding.
>   */
> -#define KMALLOC_SHIFT_HIGH	((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
> -				(MAX_ORDER + PAGE_SHIFT - 1) : 25)
> +#define KMALLOC_SHIFT_HIGH	((MIN_MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
> +				(MIN_MAX_ORDER + PAGE_SHIFT - 1) : 25)
>  #define KMALLOC_SHIFT_MAX	KMALLOC_SHIFT_HIGH
>  #ifndef KMALLOC_SHIFT_LOW
>  #define KMALLOC_SHIFT_LOW	5
> @@ -258,7 +258,7 @@ static inline void __check_heap_object(const void *ptr, unsigned long n,
>   * (PAGE_SIZE*2).  Larger requests are passed to the page allocator.
>   */
>  #define KMALLOC_SHIFT_HIGH	(PAGE_SHIFT + 1)
> -#define KMALLOC_SHIFT_MAX	(MAX_ORDER + PAGE_SHIFT - 1)
> +#define KMALLOC_SHIFT_MAX	(MIN_MAX_ORDER + PAGE_SHIFT - 1)
>  #ifndef KMALLOC_SHIFT_LOW
>  #define KMALLOC_SHIFT_LOW	3
>  #endif
> @@ -271,7 +271,7 @@ static inline void __check_heap_object(const void *ptr, unsigned long n,
>   * be allocated from the same page.
>   */
>  #define KMALLOC_SHIFT_HIGH	PAGE_SHIFT
> -#define KMALLOC_SHIFT_MAX	(MAX_ORDER + PAGE_SHIFT - 1)
> +#define KMALLOC_SHIFT_MAX	(MIN_MAX_ORDER + PAGE_SHIFT - 1)
>  #ifndef KMALLOC_SHIFT_LOW
>  #define KMALLOC_SHIFT_LOW	3
>  #endif
> diff --git a/mm/slab.c b/mm/slab.c
> index d0f725637663..0041de8ec0e9 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -466,7 +466,7 @@ static int __init slab_max_order_setup(char *str)
>  {
>  	get_option(&str, &slab_max_order);
>  	slab_max_order = slab_max_order < 0 ? 0 :
> -				min(slab_max_order, MAX_ORDER - 1);
> +				min(slab_max_order, MIN_MAX_ORDER - 1);
>  	slab_max_order_set = true;
>  
>  	return 1;
> diff --git a/mm/slub.c b/mm/slub.c
> index b6c5205252eb..228e4a77c678 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3564,8 +3564,9 @@ static inline int calculate_order(unsigned int size)
>  	/*
>  	 * Doh this slab cannot be placed using slub_max_order.
>  	 */
> -	order = slab_order(size, 1, MAX_ORDER, 1);
> -	if (order < MAX_ORDER)
> +
> +	order = slab_order(size, 1, MIN_MAX_ORDER, 1);
> +	if (order < MIN_MAX_ORDER)
>  		return order;
>  	return -ENOSYS;
>  }
> @@ -4079,7 +4080,7 @@ __setup("slub_min_order=", setup_slub_min_order);
>  static int __init setup_slub_max_order(char *str)
>  {
>  	get_option(&str, (int *)&slub_max_order);
> -	slub_max_order = min(slub_max_order, (unsigned int)MAX_ORDER - 1);
> +	slub_max_order = min(slub_max_order, (unsigned int)MIN_MAX_ORDER - 1);
>  
>  	return 1;
>  }
> -- 
> 2.30.2
> 

-- 
Sincerely yours,
Mike.