[PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[Pingfan Liu]

Currently, NR_PAGEBLOCK_BITS and MIGRATE_TYPES are not associated by code.
If someone adds extra migrate type, then he may forget to enlarge the
NR_PAGEBLOCK_BITS. Hence it requires some way to fix.
NR_PAGEBLOCK_BITS depends on MIGRATE_TYPES, while these macro
spread on two different .h file with reverse dependency, it is a little
hard to refer to MIGRATE_TYPES in pageblock-flag.h. This patch tries to
remind such relation in compiling-time.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
---
 include/linux/pageblock-flags.h | 5 +++--
 mm/page_alloc.c                 | 3 ++-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
index 9132c5c..fe0aec4 100644
--- a/include/linux/pageblock-flags.h
+++ b/include/linux/pageblock-flags.h
@@ -25,11 +25,12 @@
 
 #include <linux/types.h>
 
+#define PB_migratetype_bits 3
 /* Bit indices that affect a whole block of pages */
 enum pageblock_bits {
 	PB_migrate,
-	PB_migrate_end = PB_migrate + 3 - 1,
-			/* 3 bits required for migrate types */
+	PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
+			/* n bits required for migrate types */
 	PB_migrate_skip,/* If set the block is skipped by compaction */
 
 	/*
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ec9cc4..1a22d8d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -425,7 +425,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 	unsigned long bitidx, word_bitidx;
 	unsigned long old_word, word;
 
-	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
+	BUILD_BUG_ON(order_base_2(MIGRATE_TYPES)
+		!= (PB_migratetype_bits - 1));
 
 	bitmap = get_pageblock_bitmap(page, pfn);
 	bitidx = pfn_to_bitidx(page, pfn);
-- 
2.7.4

Re: [PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[Vlastimil Babka]

On 12/7/18 5:53 AM, Pingfan Liu wrote:
> Currently, NR_PAGEBLOCK_BITS and MIGRATE_TYPES are not associated by code.
> If someone adds extra migrate type, then he may forget to enlarge the
> NR_PAGEBLOCK_BITS. Hence it requires some way to fix.
> NR_PAGEBLOCK_BITS depends on MIGRATE_TYPES, while these macro
> spread on two different .h file with reverse dependency, it is a little
> hard to refer to MIGRATE_TYPES in pageblock-flag.h. This patch tries to
> remind such relation in compiling-time.
> 
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  include/linux/pageblock-flags.h | 5 +++--
>  mm/page_alloc.c                 | 3 ++-
>  2 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
> index 9132c5c..fe0aec4 100644
> --- a/include/linux/pageblock-flags.h
> +++ b/include/linux/pageblock-flags.h
> @@ -25,11 +25,12 @@
>  
>  #include <linux/types.h>
>  
> +#define PB_migratetype_bits 3
>  /* Bit indices that affect a whole block of pages */
>  enum pageblock_bits {
>  	PB_migrate,
> -	PB_migrate_end = PB_migrate + 3 - 1,
> -			/* 3 bits required for migrate types */
> +	PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
> +			/* n bits required for migrate types */
>  	PB_migrate_skip,/* If set the block is skipped by compaction */
>  
>  	/*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2ec9cc4..1a22d8d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -425,7 +425,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>  	unsigned long bitidx, word_bitidx;
>  	unsigned long old_word, word;
>  
> -	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);

Why delete this one? It's for something a bit different and also still
valid.

> +	BUILD_BUG_ON(order_base_2(MIGRATE_TYPES)
> +		!= (PB_migratetype_bits - 1));

I think this should use the '>' operator. It's fine if there are less
types than what can fit into 3 bits. AFAICS for !CONFIG_DMA and
!CONFIG_MEMORY_ISOLATION there are just 4 types that fit into 2 bits...

>  
>  	bitmap = get_pageblock_bitmap(page, pfn);
>  	bitidx = pfn_to_bitidx(page, pfn);
> 

Re: [PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 3918 bytes --]

Hi Pingfan,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[also build test ERROR on v4.20-rc5 next-20181207]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Pingfan-Liu/mm-pageblock-throw-compiling-time-error-if-pageblock_bits-can-not-hold-MIGRATE_TYPES/20181207-161514
config: x86_64-randconfig-x017-201848 (attached as .config)
compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

   In file included from include/asm-generic/bug.h:5:0,
                    from arch/x86/include/asm/bug.h:47,
                    from include/linux/bug.h:5,
                    from include/linux/mmdebug.h:5,
                    from include/linux/mm.h:9,
                    from mm/page_alloc.c:18:
   mm/page_alloc.c: In function 'set_pfnblock_flags_mask':
>> include/linux/compiler.h:372:38: error: call to '__compiletime_assert_429' declared with attribute error: BUILD_BUG_ON failed: order_base_2(MIGRATE_TYPES) != (PB_migratetype_bits - 1)
     _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
                                         ^
   include/linux/compiler.h:353:4: note: in definition of macro '__compiletime_assert'
       prefix ## suffix();    \
       ^~~~~~
   include/linux/compiler.h:372:2: note: in expansion of macro '_compiletime_assert'
     _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
     ^~~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:45:37: note: in expansion of macro 'compiletime_assert'
    #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                        ^~~~~~~~~~~~~~~~~~
   include/linux/build_bug.h:69:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
     BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
     ^~~~~~~~~~~~~~~~
   mm/page_alloc.c:428:2: note: in expansion of macro 'BUILD_BUG_ON'
     BUILD_BUG_ON(order_base_2(MIGRATE_TYPES)
     ^~~~~~~~~~~~

vim +/__compiletime_assert_429 +372 include/linux/compiler.h

9a8ab1c3 Daniel Santos 2013-02-21  358  
9a8ab1c3 Daniel Santos 2013-02-21  359  #define _compiletime_assert(condition, msg, prefix, suffix) \
9a8ab1c3 Daniel Santos 2013-02-21  360  	__compiletime_assert(condition, msg, prefix, suffix)
9a8ab1c3 Daniel Santos 2013-02-21  361  
9a8ab1c3 Daniel Santos 2013-02-21  362  /**
9a8ab1c3 Daniel Santos 2013-02-21  363   * compiletime_assert - break build and emit msg if condition is false
9a8ab1c3 Daniel Santos 2013-02-21  364   * @condition: a compile-time constant condition to check
9a8ab1c3 Daniel Santos 2013-02-21  365   * @msg:       a message to emit if condition is false
9a8ab1c3 Daniel Santos 2013-02-21  366   *
9a8ab1c3 Daniel Santos 2013-02-21  367   * In tradition of POSIX assert, this macro will break the build if the
9a8ab1c3 Daniel Santos 2013-02-21  368   * supplied condition is *false*, emitting the supplied error message if the
9a8ab1c3 Daniel Santos 2013-02-21  369   * compiler has support to do so.
9a8ab1c3 Daniel Santos 2013-02-21  370   */
9a8ab1c3 Daniel Santos 2013-02-21  371  #define compiletime_assert(condition, msg) \
9a8ab1c3 Daniel Santos 2013-02-21 @372  	_compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
9a8ab1c3 Daniel Santos 2013-02-21  373  

:::::: The code at line 372 was first introduced by commit
:::::: 9a8ab1c39970a4938a72d94e6fd13be88a797590 bug.h, compiler.h: introduce compiletime_assert & BUILD_BUG_ON_MSG

:::::: TO: Daniel Santos <daniel.santos@pobox.com>
:::::: CC: Linus Torvalds <torvalds@linux-foundation.org>

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
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Re: [PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[Pingfan Liu]

On Fri, Dec 7, 2018 at 3:36 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 12/7/18 5:53 AM, Pingfan Liu wrote:
> > Currently, NR_PAGEBLOCK_BITS and MIGRATE_TYPES are not associated by code.
> > If someone adds extra migrate type, then he may forget to enlarge the
> > NR_PAGEBLOCK_BITS. Hence it requires some way to fix.
> > NR_PAGEBLOCK_BITS depends on MIGRATE_TYPES, while these macro
> > spread on two different .h file with reverse dependency, it is a little
> > hard to refer to MIGRATE_TYPES in pageblock-flag.h. This patch tries to
> > remind such relation in compiling-time.
> >
> > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > ---
> >  include/linux/pageblock-flags.h | 5 +++--
> >  mm/page_alloc.c                 | 3 ++-
> >  2 files changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
> > index 9132c5c..fe0aec4 100644
> > --- a/include/linux/pageblock-flags.h
> > +++ b/include/linux/pageblock-flags.h
> > @@ -25,11 +25,12 @@
> >
> >  #include <linux/types.h>
> >
> > +#define PB_migratetype_bits 3
> >  /* Bit indices that affect a whole block of pages */
> >  enum pageblock_bits {
> >       PB_migrate,
> > -     PB_migrate_end = PB_migrate + 3 - 1,
> > -                     /* 3 bits required for migrate types */
> > +     PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
> > +                     /* n bits required for migrate types */
> >       PB_migrate_skip,/* If set the block is skipped by compaction */
> >
> >       /*
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 2ec9cc4..1a22d8d 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -425,7 +425,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
> >       unsigned long bitidx, word_bitidx;
> >       unsigned long old_word, word;
> >
> > -     BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
>
> Why delete this one? It's for something a bit different and also still
> valid.
>

I think it is dependent on PB_migratetype_bits (plus 1 dedicated bit
for skip), hence the later one implies it.
> > +     BUILD_BUG_ON(order_base_2(MIGRATE_TYPES)
> > +             != (PB_migratetype_bits - 1));
>
> I think this should use the '>' operator. It's fine if there are less

If ">" is chosen, then it allows wasted bits. I had thought it is not
the purpose of the design, hence disard the ">". Otherwise, what about
using warning on ">"?
> types than what can fit into 3 bits. AFAICS for !CONFIG_DMA and
> !CONFIG_MEMORY_ISOLATION there are just 4 types that fit into 2 bits...
>
Oh, yes, you are right. How about this:
#if defined(CONFIG_DMA) || defined(CONFIG_MEMORY_ISOLATION)
#define PB_migratetype_bits 3
#else
#define PB_migratetype_bits 2
#endif

Thanks for your kindly review.

Regards,
Pingfan
> >
> >       bitmap = get_pageblock_bitmap(page, pfn);
> >       bitidx = pfn_to_bitidx(page, pfn);
> >
>

Re: [PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[Vlastimil Babka]

On 12/10/18 4:15 AM, Pingfan Liu wrote:
> On Fri, Dec 7, 2018 at 3:36 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> On 12/7/18 5:53 AM, Pingfan Liu wrote:
>>> Currently, NR_PAGEBLOCK_BITS and MIGRATE_TYPES are not associated by code.
>>> If someone adds extra migrate type, then he may forget to enlarge the
>>> NR_PAGEBLOCK_BITS. Hence it requires some way to fix.
>>> NR_PAGEBLOCK_BITS depends on MIGRATE_TYPES, while these macro
>>> spread on two different .h file with reverse dependency, it is a little
>>> hard to refer to MIGRATE_TYPES in pageblock-flag.h. This patch tries to
>>> remind such relation in compiling-time.
>>>
>>> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>> Cc: Oscar Salvador <osalvador@suse.de>
>>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>>> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>> ---
>>>  include/linux/pageblock-flags.h | 5 +++--
>>>  mm/page_alloc.c                 | 3 ++-
>>>  2 files changed, 5 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
>>> index 9132c5c..fe0aec4 100644
>>> --- a/include/linux/pageblock-flags.h
>>> +++ b/include/linux/pageblock-flags.h
>>> @@ -25,11 +25,12 @@
>>>
>>>  #include <linux/types.h>
>>>
>>> +#define PB_migratetype_bits 3
>>>  /* Bit indices that affect a whole block of pages */
>>>  enum pageblock_bits {
>>>       PB_migrate,
>>> -     PB_migrate_end = PB_migrate + 3 - 1,
>>> -                     /* 3 bits required for migrate types */
>>> +     PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
>>> +                     /* n bits required for migrate types */
>>>       PB_migrate_skip,/* If set the block is skipped by compaction */
>>>
>>>       /*
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 2ec9cc4..1a22d8d 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -425,7 +425,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>>>       unsigned long bitidx, word_bitidx;
>>>       unsigned long old_word, word;
>>>
>>> -     BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
>>
>> Why delete this one? It's for something a bit different and also still
>> valid.
>>
> 
> I think it is dependent on PB_migratetype_bits (plus 1 dedicated bit
> for skip), hence the later one implies it.

No, NR_PAGEBLOCK_BITS simply to be 4 (or other number that can divide
the number of bits in a word with no remainder), or
set_pfnblock_flags_mask() will not function correctly.

>>> +     BUILD_BUG_ON(order_base_2(MIGRATE_TYPES)
>>> +             != (PB_migratetype_bits - 1));
>>
>> I think this should use the '>' operator. It's fine if there are less
> 
> If ">" is chosen, then it allows wasted bits. I had thought it is not
> the purpose of the design, hence disard the ">".

We do allow wasted bits, because we need NR_PAGEBLOCK_BITS to be 4 in
any case, as explained above.

> Otherwise, what about
> using warning on ">"?

I think it would be needed.

>> types than what can fit into 3 bits. AFAICS for !CONFIG_DMA and
>> !CONFIG_MEMORY_ISOLATION there are just 4 types that fit into 2 bits...
>>
> Oh, yes, you are right. How about this:
> #if defined(CONFIG_DMA) || defined(CONFIG_MEMORY_ISOLATION)
> #define PB_migratetype_bits 3
> #else
> #define PB_migratetype_bits 2
> #endif

I think it's not necessary. Really, we need to have 4 NR_PAGEBLOCK_BITS
with the current code, and this leaves us with 3 bits for migratetype.
The only thing to check is whether migratetypes fit into these 3 bits, IMHO.

> Thanks for your kindly review.
> 
> Regards,
> Pingfan
>>>
>>>       bitmap = get_pageblock_bitmap(page, pfn);
>>>       bitidx = pfn_to_bitidx(page, pfn);
>>>
>>

Re: [PATCHv2] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[Pingfan Liu]

On Mon, Dec 10, 2018 at 3:46 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 12/10/18 4:15 AM, Pingfan Liu wrote:
> > On Fri, Dec 7, 2018 at 3:36 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> >>
> >> On 12/7/18 5:53 AM, Pingfan Liu wrote:
> >>> Currently, NR_PAGEBLOCK_BITS and MIGRATE_TYPES are not associated by code.
> >>> If someone adds extra migrate type, then he may forget to enlarge the
> >>> NR_PAGEBLOCK_BITS. Hence it requires some way to fix.
> >>> NR_PAGEBLOCK_BITS depends on MIGRATE_TYPES, while these macro
> >>> spread on two different .h file with reverse dependency, it is a little
> >>> hard to refer to MIGRATE_TYPES in pageblock-flag.h. This patch tries to
> >>> remind such relation in compiling-time.
> >>>
> >>> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> >>> Cc: Andrew Morton <akpm@linux-foundation.org>
> >>> Cc: Michal Hocko <mhocko@suse.com>
> >>> Cc: Pavel Tatashin <pavel.tatashin@microsoft.com>
> >>> Cc: Vlastimil Babka <vbabka@suse.cz>
> >>> Cc: Oscar Salvador <osalvador@suse.de>
> >>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> >>> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >>> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >>> ---
> >>>  include/linux/pageblock-flags.h | 5 +++--
> >>>  mm/page_alloc.c                 | 3 ++-
> >>>  2 files changed, 5 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
> >>> index 9132c5c..fe0aec4 100644
> >>> --- a/include/linux/pageblock-flags.h
> >>> +++ b/include/linux/pageblock-flags.h
> >>> @@ -25,11 +25,12 @@
> >>>
> >>>  #include <linux/types.h>
> >>>
> >>> +#define PB_migratetype_bits 3
> >>>  /* Bit indices that affect a whole block of pages */
> >>>  enum pageblock_bits {
> >>>       PB_migrate,
> >>> -     PB_migrate_end = PB_migrate + 3 - 1,
> >>> -                     /* 3 bits required for migrate types */
> >>> +     PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
> >>> +                     /* n bits required for migrate types */
> >>>       PB_migrate_skip,/* If set the block is skipped by compaction */
> >>>
> >>>       /*
> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >>> index 2ec9cc4..1a22d8d 100644
> >>> --- a/mm/page_alloc.c
> >>> +++ b/mm/page_alloc.c
> >>> @@ -425,7 +425,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
> >>>       unsigned long bitidx, word_bitidx;
> >>>       unsigned long old_word, word;
> >>>
> >>> -     BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
> >>
> >> Why delete this one? It's for something a bit different and also still
> >> valid.
> >>
> >
> > I think it is dependent on PB_migratetype_bits (plus 1 dedicated bit
> > for skip), hence the later one implies it.
>
> No, NR_PAGEBLOCK_BITS simply to be 4 (or other number that can divide
> the number of bits in a word with no remainder), or
> set_pfnblock_flags_mask() will not function correctly.
>

Yes, you are right. It will break the atomic.
> >>> +     BUILD_BUG_ON(order_base_2(MIGRATE_TYPES)
> >>> +             != (PB_migratetype_bits - 1));
> >>
> >> I think this should use the '>' operator. It's fine if there are less
> >
> > If ">" is chosen, then it allows wasted bits. I had thought it is not
> > the purpose of the design, hence disard the ">".
>
> We do allow wasted bits, because we need NR_PAGEBLOCK_BITS to be 4 in
> any case, as explained above.
>

Right!
> > Otherwise, what about
> > using warning on ">"?
>
> I think it would be needed.
>
OK.
> >> types than what can fit into 3 bits. AFAICS for !CONFIG_DMA and
> >> !CONFIG_MEMORY_ISOLATION there are just 4 types that fit into 2 bits...
> >>
> > Oh, yes, you are right. How about this:
> > #if defined(CONFIG_DMA) || defined(CONFIG_MEMORY_ISOLATION)
> > #define PB_migratetype_bits 3
> > #else
> > #define PB_migratetype_bits 2
> > #endif
>
> I think it's not necessary. Really, we need to have 4 NR_PAGEBLOCK_BITS
> with the current code, and this leaves us with 3 bits for migratetype.
> The only thing to check is whether migratetypes fit into these 3 bits, IMHO.
>
OK. I will update v3.

Thanks and regards,
Pingfan