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Re: + mm-sparse-refine-usemap_size-a-little.patch added to -mm tree

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  Re: + mm-sparse-refine-usemap_size-a-little.patch added to -mm tree Michal Hocko <mhocko@kernel.org> - 2017-03-16 10:20 +0100
    Re: + mm-sparse-refine-usemap_size-a-little.patch added to -mm tree Wei Yang <richard.weiyang@gmail.com> - 2017-03-17 02:50 +0100

#1602116 — Re: + mm-sparse-refine-usemap_size-a-little.patch added to -mm tree

[CC Mel]

On Fri 10-03-17 14:41:22, Andrew Morton wrote:
> From: Wei Yang <richard.weiyang@gmail.com>
> Subject: mm/sparse: refine usemap_size() a little
> 
> Current implementation calculates usemap_size in two steps:
>     * calculate number of bytes to cover these bits
>     * calculate number of "unsigned long" to cover these bytes
> 
> It would be more clear by:
>     * calculate number of "unsigned long" to cover these bits
>     * multiple it with sizeof(unsigned long)
> 
> This patch refine usemap_size() a little to make it more easy to
> understand.

I haven't checked deeply yet but reading through 5c0e3066474b ("Fix
corruption of memmap on IA64 SPARSEMEM when mem_section is not a power
of 2") made me ask whether the case described in the commit message
still applies after this change or whether it has been considered at
all.

> Link: http://lkml.kernel.org/r/20170310043713.96871-1-richard.weiyang@gmail.com
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Tejun Heo <tj@kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/sparse.c |    5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff -puN mm/sparse.c~mm-sparse-refine-usemap_size-a-little mm/sparse.c
> --- a/mm/sparse.c~mm-sparse-refine-usemap_size-a-little
> +++ a/mm/sparse.c
> @@ -248,10 +248,7 @@ static int __meminit sparse_init_one_sec
>  
>  unsigned long usemap_size(void)
>  {
> -	unsigned long size_bytes;
> -	size_bytes = roundup(SECTION_BLOCKFLAGS_BITS, 8) / 8;
> -	size_bytes = roundup(size_bytes, sizeof(unsigned long));
> -	return size_bytes;
> +	return BITS_TO_LONGS(SECTION_BLOCKFLAGS_BITS) * sizeof(unsigned long);
>  }
>  
>  #ifdef CONFIG_MEMORY_HOTPLUG
> _
> 
> Patches currently in -mm which might be from richard.weiyang@gmail.com are
> 
> mm-sparse-refine-usemap_size-a-little.patch
> mm-page_alloc-return-0-in-case-this-node-has-no-page-within-the-zone.patch
> 
> --
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-- 
Michal Hocko
SUSE Labs

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On Thu, Mar 16, 2017 at 10:17:18AM +0100, Michal Hocko wrote:
>[CC Mel]
>
>On Fri 10-03-17 14:41:22, Andrew Morton wrote:
>> From: Wei Yang <richard.weiyang@gmail.com>
>> Subject: mm/sparse: refine usemap_size() a little
>> 
>> Current implementation calculates usemap_size in two steps:
>>     * calculate number of bytes to cover these bits
>>     * calculate number of "unsigned long" to cover these bytes
>> 
>> It would be more clear by:
>>     * calculate number of "unsigned long" to cover these bits
>>     * multiple it with sizeof(unsigned long)
>> 
>> This patch refine usemap_size() a little to make it more easy to
>> understand.
>
>I haven't checked deeply yet but reading through 5c0e3066474b ("Fix
>corruption of memmap on IA64 SPARSEMEM when mem_section is not a power
>of 2") made me ask whether the case described in the commit message
>still applies after this change or whether it has been considered at
>all.
>

Hi, Michal

Thanks for your comment.

By looking into the commit 5c0e3066474b, I think it does two things:
1. Recalculate the SECTION_BLOCKFLAGS_BITS
2. Move pageblock_flags out mem_section to make the structure power of 2 in
size.

When we look at the original data structure, pageblock_flags originally is
defined as a bitmap:

    DECLARE_BITMAP(pageblock_flags, SECTION_BLOCKFLAGS_BITS);

Which in turn is:

    #define DECLARE_BITMAP(pageblock_flags,SECTION_BLOCKFLAGS_BITS) \
        unsigned long pageblock_flags[BITS_TO_LONGS(SECTION_BLOCKFLAGS_BITS)]

My patch is using BITS_TO_LONGS() to simplify the code and obviously has the
same effect.

Does this resolve your concern?

-- 
Wei Yang
Help you, Help me

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