linux-kernel - Re: [PATCH] mm: use managed_zone() for more exact check in zone iteration

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Date:   Fri, 16 Nov 2018 04:41:19 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Wei Yang <richard.weiyang@...il.com>, mhocko@...e.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use managed_zone() for more exact check in zone
 iteration

On Thu, Nov 15, 2018 at 01:37:35PM -0800, Andrew Morton wrote:
>On Thu, 15 Nov 2018 07:50:40 +0800 Wei Yang <richard.weiyang@...il.com> wrote:
>
>> For one zone, there are three digits to describe its space range:
>> 
>>     spanned_pages
>>     present_pages
>>     managed_pages
>> 
>> The detailed meaning is written in include/linux/mmzone.h. This patch
>> concerns about the last two.
>> 
>>     present_pages is physical pages existing within the zone
>>     managed_pages is present pages managed by the buddy system
>> 
>> >From the definition, managed_pages is a more strict condition than
>> present_pages.
>> 
>> There are two functions using zone's present_pages as a boundary:
>> 
>>     populated_zone()
>>     for_each_populated_zone()
>> 
>> By going through the kernel tree, most of their users are willing to
>> access pages managed by the buddy system, which means it is more exact
>> to check zone's managed_pages for a validation.
>> 
>> This patch replaces those checks on present_pages to managed_pages by:
>> 
>>     * change for_each_populated_zone() to for_each_managed_zone()
>>     * convert for_each_populated_zone() to for_each_zone() and check
>>       populated_zone() where is necessary
>>     * change populated_zone() to managed_zone() at proper places
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
>> 
>> ---
>> 
>> Michal, after last mail, I did one more thing to replace
>> populated_zone() with managed_zone() at proper places.
>> 
>> One thing I am not sure is those places in mm/compaction.c. I have
>> chaged them. If not, please let me know.
>> 
>> BTW, I did a boot up test with the patched kernel and looks smooth.
>
>Seems sensible, but a bit scary.  A basic boot test is unlikely to
>expose subtle gremlins.
>

Agree.

>Worse, the situations in which managed_zone() != populated_zone() are
>rare(?), so it will take a long time for problems to be discovered, I
>expect.

Hmm... I created a virtual machine with 4 nodes, which has total 6
populated zones. All of them are different.

This is a little bit out of my expactation.

>
>I'll toss it in there for now, let's see who breaks :(

Thanks.

-- 
Wei Yang
Help you, Help me