linux-kernel - Re: [PATCH] mm, memcg: Do not high throttle allocators based on wraparound

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Message-ID: <20200331170410.GB972283@chrisdown.name>
Date:   Tue, 31 Mar 2020 18:04:10 +0100
From:   Chris Down <chris@...isdown.name>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Jakub Kicinski <kuba@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] mm, memcg: Do not high throttle allocators based on
 wraparound

Michal Hocko writes:
>I find this paragraph rather confusing. This is essentially an unsigned
>underflow when any of the memcg up the hierarchy is below the high
>limit, right?  There doesn't really seem anything complex in such a
>hierarchy.

The conditions to trigger the bug itself are easy, but having it obviously 
visible in tests requires a moderately complex hierarchy, since in the basic 
case ancestor_usage is "similar enough" to the test leaf cgroup's usage.