linux-kernel - Re: [PATCH v4 1/2] mm: memory_hotplug: enumerate all supported section flags

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Message-ID: <YrAs1LGbExHI/mcw@FVFYT0MHHV2J.usts.net>
Date:   Mon, 20 Jun 2022 16:16:20 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     akpm@...ux-foundation.org, corbet@....net, mike.kravetz@...cle.com,
        osalvador@...e.de, paulmck@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v4 1/2] mm: memory_hotplug: enumerate all supported
 section flags

On Mon, Jun 20, 2022 at 09:51:42AM +0200, David Hildenbrand wrote:
> On 19.06.22 15:38, 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>
> > ---
> >  include/linux/mmzone.h | 44 +++++++++++++++++++++++++++++++++++---------
> >  mm/memory_hotplug.c    |  6 ++++++
> >  mm/sparse.c            |  2 +-
> >  3 files changed, 42 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index aab70355d64f..932843c6459b 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -1418,16 +1418,35 @@ 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 SECTION_MAP_MASK		(~(SECTION_MAP_LAST_BIT-1))
> > -#define SECTION_NID_SHIFT		6
> > +enum {
> > +	SECTION_MARKED_PRESENT_BIT,
> > +	SECTION_HAS_MEM_MAP_BIT,
> > +	SECTION_IS_ONLINE_BIT,
> > +	SECTION_IS_EARLY_BIT,
> > +#ifdef CONFIG_ZONE_DEVICE
> > +	SECTION_TAINT_ZONE_DEVICE_BIT,
> > +#endif
> > +	SECTION_MAP_LAST_BIT,
> > +};
> > +
> > +enum {
> > +	SECTION_MARKED_PRESENT		= BIT(SECTION_MARKED_PRESENT_BIT),
> > +	SECTION_HAS_MEM_MAP		= BIT(SECTION_HAS_MEM_MAP_BIT),
> > +	SECTION_IS_ONLINE		= BIT(SECTION_IS_ONLINE_BIT),
> > +	SECTION_IS_EARLY		= BIT(SECTION_IS_EARLY_BIT),
> > +#ifdef CONFIG_ZONE_DEVICE
> > +	SECTION_TAINT_ZONE_DEVICE	= BIT(SECTION_TAINT_ZONE_DEVICE_BIT),
> > +#endif
> > +};
> 
> I can understand the reason for the other enum, to auto-assing numbers.
> What's the underlying reason for the enum here? Personally, I'd just
> stay with defines, so I'm curious :)
>

Oh, just my personal preference. I can replace those with defines in
next version if you prefer it. :-)

Thanks.

> LGTM
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 
>