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Date:   Tue, 17 May 2022 09:47:48 +0200
From:   Oscar Salvador <osalvador@suse.de>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     corbet@lwn.net, mike.kravetz@oracle.com, akpm@linux-foundation.org,
        mcgrof@kernel.org, keescook@chromium.org, yzaikin@google.com,
        david@redhat.com, masahiroy@kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, smuchun@gmail.com
Subject: Re: [PATCH v12 3/7] mm: memory_hotplug: enumerate all supported
 section flags
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References: <20220516102211.41557-1-songmuchun@bytedance.com>
 <20220516102211.41557-4-songmuchun@bytedance.com>
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On Mon, May 16, 2022 at 06:22:07PM +0800, Muchun Song wrote:
> We are almost running out of free slots, only one bit is available in the

I would be more precise about what are we running out of. Free slots of
what?

> worst case (powerpc with 256k pages).  However, there are still some free
> slots 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>
> ---
...
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1418,16 +1418,37 @@ 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 {
> +	ENUM_SECTION_FLAG(__SECTION_SHIFT_FLAG_MAPPER)
> +	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

Is this really worth the extra code? And it might be me that I am not
familiar with all this magic, but it looks overcomplicated.
Maybe some comments here and there help clarifying what it is going on
here.

>  static inline struct page *__section_mem_map_addr(struct mem_section *section)
>  {
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 111684878fd9..aef3f041dec7 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -655,12 +655,18 @@ 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
> -- 
> 2.11.0
> 
> 

-- 
Oscar Salvador
SUSE Labs