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Date:   Wed, 15 Jun 2022 11:35:09 +0200
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To:     Muchun Song <songmuchun@bytedance.com>, corbet@lwn.net,
        akpm@linux-foundation.org, paulmck@kernel.org,
        mike.kravetz@oracle.com, osalvador@suse.de
Cc:     linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, duanxiongchun@bytedance.com, smuchun@gmail.com
References: <20220520025538.21144-1-songmuchun@bytedance.com>
 <20220520025538.21144-2-songmuchun@bytedance.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v2 1/2] mm: memory_hotplug: enumerate all supported
 section flags
In-Reply-To: <20220520025538.21144-2-songmuchun@bytedance.com>
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On 20.05.22 04:55, Muchun Song wrote:
> We are almost running out of section flags, only one bit is available in
> the worst case (powerpc with 256k pages).  However, there are still some
> free bits (in ->section_mem_map) on other architectures (e.g. x86_64 has
> 10 bits available, arm64 has 8 bits available with worst case of 64K
> pages).  We have hard coded those numbers in code, it is inconvenient to
> use those bits on other architectures except powerpc.  So transfer those
> section flags to enumeration to make it easy to add new section flags in
> the future.  Also, move SECTION_TAINT_ZONE_DEVICE into the scope of
> CONFIG_ZONE_DEVICE to save a bit on non-zone-device case.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Sorry for the late reply. This looks overly complicated to me.

IOW, staring at that patch I don't quite like what I am seeing.


Something like the following is *a lot* easier to read than some
MAPPER macro magic. What speaks against it?

/*
 * Section bits use the lower unused bits in the ->section_mem_map
 */
enum {
	SECTION_MARKED_PRESENT_BIT = 0,
	SECTION_HAS_MEM_MAP_BIT,
	...
#ifdef ZONE_DEVICE
	SECTION_TAINT_ZONE_DEVICE_BIT
#endif
}

#define SECTION_MARKED_PRESENT	   (1ULL << SECTION_MARKED_PRESENT_BIT)
...
#ifdef ZONE_DEVICE
#define SECTION_TAINT_ZONE_DEVICE  (1ULL << SECTION_TAINT_ZONE_DEVICE_BIT)
#endif /* ZONE_DEVICE */


> ---
>  include/linux/kconfig.h |  1 +
>  include/linux/mmzone.h  | 54 +++++++++++++++++++++++++++++++++++++++++--------
>  mm/memory_hotplug.c     |  6 ++++++
>  3 files changed, 53 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
> index 20d1079e92b4..7044032b9f42 100644
> --- a/include/linux/kconfig.h
> +++ b/include/linux/kconfig.h
> @@ -10,6 +10,7 @@
>  #define __LITTLE_ENDIAN 1234
>  #endif
>  
> +#define __ARG_PLACEHOLDER_ 0,
>  #define __ARG_PLACEHOLDER_1 0,
>  #define __take_second_arg(__ignored, val, ...) val
>  
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 299259cfe462..2cf2a76535ab 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1422,16 +1422,47 @@ extern size_t mem_section_usage_size(void);
>   *      (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the
>   *      worst combination is powerpc with 256k pages,
>   *      which results in PFN_SECTION_SHIFT equal 6.
> - * To sum it up, at least 6 bits are available.
> + * To sum it up, at least 6 bits are available on all architectures.
> + * However, we can exceed 6 bits on some other architectures except
> + * powerpc (e.g. 15 bits are available on x86_64, 13 bits are available
> + * with the worst case of 64K pages on arm64) if we make sure the
> + * exceeded bit is not applicable to powerpc.
>   */
> -#define SECTION_MARKED_PRESENT		(1UL<<0)
> -#define SECTION_HAS_MEM_MAP		(1UL<<1)
> -#define SECTION_IS_ONLINE		(1UL<<2)
> -#define SECTION_IS_EARLY		(1UL<<3)
> -#define SECTION_TAINT_ZONE_DEVICE	(1UL<<4)
> -#define SECTION_MAP_LAST_BIT		(1UL<<5)
> +#define ENUM_SECTION_FLAG(MAPPER)						\
> +	MAPPER(MARKED_PRESENT)							\
> +	MAPPER(HAS_MEM_MAP)							\
> +	MAPPER(IS_ONLINE)							\
> +	MAPPER(IS_EARLY)							\
> +	MAPPER(TAINT_ZONE_DEVICE, CONFIG_ZONE_DEVICE)				\
> +	MAPPER(MAP_LAST_BIT)
> +
> +#define __SECTION_SHIFT_FLAG_MAPPER_0(x)
> +#define __SECTION_SHIFT_FLAG_MAPPER_1(x)	SECTION_##x##_SHIFT,
> +#define __SECTION_SHIFT_FLAG_MAPPER(x, ...)	\
> +	__PASTE(__SECTION_SHIFT_FLAG_MAPPER_, IS_ENABLED(__VA_ARGS__))(x)
> +
> +#define __SECTION_FLAG_MAPPER_0(x)
> +#define __SECTION_FLAG_MAPPER_1(x)		SECTION_##x = BIT(SECTION_##x##_SHIFT),
> +#define __SECTION_FLAG_MAPPER(x, ...)		\
> +	__PASTE(__SECTION_FLAG_MAPPER_, IS_ENABLED(__VA_ARGS__))(x)
> +
> +enum {
> +	/*
> +	 * Generate a series of enumeration flags like SECTION_$name_SHIFT.
> +	 * Each entry in ENUM_SECTION_FLAG() macro will be generated to one
> +	 * enumeration iff the 2nd parameter of MAPPER() is defined or absent.
> +	 * The $name comes from the 1st parameter of MAPPER() macro.
> +	 */
> +	ENUM_SECTION_FLAG(__SECTION_SHIFT_FLAG_MAPPER)
> +	/*
> +	 * Generate a series of enumeration flags like:
> +	 *   SECTION_$name = BIT(SECTION_$name_SHIFT)
> +	 */
> +	ENUM_SECTION_FLAG(__SECTION_FLAG_MAPPER)
> +};
> +
>  #define SECTION_MAP_MASK		(~(SECTION_MAP_LAST_BIT-1))
> -#define SECTION_NID_SHIFT		6
> +#define SECTION_NID_SHIFT		SECTION_MAP_LAST_BIT_SHIFT
>  
>  static inline struct page *__section_mem_map_addr(struct mem_section *section)
>  {
> @@ -1470,12 +1501,19 @@ static inline int online_section(struct mem_section *section)
>  	return (section && (section->section_mem_map & SECTION_IS_ONLINE));
>  }
>  
> +#ifdef CONFIG_ZONE_DEVICE
>  static inline int online_device_section(struct mem_section *section)
>  {
>  	unsigned long flags = SECTION_IS_ONLINE | SECTION_TAINT_ZONE_DEVICE;
>  
>  	return section && ((section->section_mem_map & flags) == flags);
>  }
> +#else
> +static inline int online_device_section(struct mem_section *section)
> +{
> +	return 0;
> +}
> +#endif
>  
>  static inline int online_section_nr(unsigned long nr)
>  {
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 1213d0c67a53..3b360eda933f 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -672,12 +672,18 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
>  
>  }
>  
> +#ifdef CONFIG_ZONE_DEVICE
>  static void section_taint_zone_device(unsigned long pfn)
>  {
>  	struct mem_section *ms = __pfn_to_section(pfn);
>  
>  	ms->section_mem_map |= SECTION_TAINT_ZONE_DEVICE;
>  }
> +#else
> +static inline void section_taint_zone_device(unsigned long pfn)
> +{
> +}
> +#endif
>  
>  /*
>   * Associate the pfn range with the given zone, initializing the memmaps


-- 
Thanks,

David / dhildenb