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Date: Wed, 15 Jun 2022 21:02:37 +0800
From: Muchun Song <songmuchun@...edance.com>
To: David Hildenbrand <david@...hat.com>
Cc: corbet@....net, akpm@...ux-foundation.org, paulmck@...nel.org,
mike.kravetz@...cle.com, osalvador@...e.de,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v2 1/2] mm: memory_hotplug: enumerate all supported
section flags
On Wed, Jun 15, 2022 at 11:35:09AM +0200, David Hildenbrand wrote:
> 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@...edance.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?
>
Thanks for taking a look.
Yeah, it is more readable. This question is also raised by Oscar.
I pasted the reply to 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. I can switch to the following approach
seems you think the following one is better.
Thanks.
> /*
> * 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
>
>
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