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Date:   Tue, 17 May 2022 16:32:40 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     Oscar Salvador <osalvador@suse.de>
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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On Tue, May 17, 2022 at 09:47:48AM +0200, Oscar Salvador wrote:
> 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.
>

Yeah, it's a little complicated. All the magic aims to generate
two enumeration from one MAPPER(xxx, config), one is SECTION_xxx_SHIFT,
another is SECTION_xxx = BIT(SECTION_xxx_SHIFT) if the 'config' is
configured. If we want to add a new flag, like the follow patch, just
one line could do that.

  MAPPER(CANNOT_OPTIMIZE_VMEMMAP, CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP)

Without those magic, we have to add 4 lines like follows to do the
similar thing.

  #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
  	SECTION_CANNOT_OPTIMIZE_VMEMMAP_SHIFT,
  #define SECTION_CANNOT_OPTIMIZE_VMEMMAP BIT(SECTION_CANNOT_OPTIMIZE_VMEMMAP_SHIFT)
  #endif

I admit it is more clear but not simplified as above approach.

Both two approaches are fine to me. If we choose the simplified
one, I agree with you I should add more comments to explain
what happens here.

Thanks.