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

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Message-ID: <20181116095720.GE14706@dhcp22.suse.cz>
Date:   Fri, 16 Nov 2018 10:57:20 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Wei Yang <richard.weiyang@...il.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 15-11-18 13:37:35, Andrew Morton wrote:
[...]
> 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.

We would basically have to deplete the whole zone by the bootmem
allocator or pull out all pages from the page allocator. E.g. memory
hotplug decreases both managed and present counters. I am actually not
sure that is 100% correct (put on my TODO list to check). There is no
consistency in that regards.

That being said, I will review the patch (today hopefully) but
fundamentally most users should indeed care about managed pages when
iterating zones with memory. There should be a good reason why they
might want to look at reserved pages.

-- 
Michal Hocko
SUSE Labs