linux-kernel - Re: [PATCH] mm: allow to decrease swap.max below actual swap usage

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Message-ID: <1523891242.27555.9.camel@surriel.com>
Date:   Mon, 16 Apr 2018 11:07:22 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Tejun Heo <tj@...nel.org>, Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Shaohua Li <shli@...com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] mm: allow to decrease swap.max below actual swap usage

On Sun, 2018-04-15 at 18:39 -0700, Tejun Heo wrote:
> Hello, Roman.
> 
> The reclaim behavior is a bit worrisome.
> 
> * It disables an entire swap area while reclaim is in progress.  Most
>   systems only have one swap area, so this would disable allocating
>   new swap area for everyone.

That could easily cause OOM kills on systems.

I prefer Tejun's simple approach, of having
the system slowly reduce swap use below the
new maximum limit.

> * The reclaim seems very inefficient.  IIUC, it has to read every
> swap
>   page to see whether the page belongs to the target memcg and for
>   each matching page, which involves walking page mm's and page
>   tables.

One of my Outreachy interns, Kelley Nielsen,
worked on making swap reclaim more efficient
by scanning the virtual address space of
processes.

Unfortunately, we ran into some unforseen
issues with that approach that we never managed
to sort out :(

> Signed-off-by: Tejun Heo <tj@...nel.org>

Acked-by: Rik van Riel <riel@...riel.com>

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