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

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Date:   Fri, 17 Mar 2017 09:47:43 +0800
From:   Wei Yang <richard.weiyang@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, richard.weiyang@...il.com,
        tj@...nel.org, mm-commits@...r.kernel.org,
        Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org
Subject: Re: + mm-sparse-refine-usemap_size-a-little.patch added to -mm tree

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@...il.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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