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Date:   Fri, 14 Feb 2020 15:06:41 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org,
        Tyler Sanderson <tysand@...gle.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Wei Wang <wei.w.wang@...el.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for
 VIRTIO_BALLOON_F_DEFLATE_ON_OOM

On Wed 05-02-20 17:34:02, David Hildenbrand wrote:
> Commit 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> changed the behavior when deflation happens automatically. Instead of
> deflating when called by the OOM handler, the shrinker is used.
> 
> However, the balloon is not simply some slab cache that should be
> shrunk when under memory pressure. The shrinker does not have a concept of
> priorities, so this behavior cannot be configured.

Adding a priority to the shrinker doesn't sound like a big problem to
me. Shrinkers already get shrink_control data structure already and
priority could be added there.

> There was a report that this results in undesired side effects when
> inflating the balloon to shrink the page cache. [1]
> 	"When inflating the balloon against page cache (i.e. no free memory
> 	 remains) vmscan.c will both shrink page cache, but also invoke the
> 	 shrinkers -- including the balloon's shrinker. So the balloon
> 	 driver allocates memory which requires reclaim, vmscan gets this
> 	 memory by shrinking the balloon, and then the driver adds the
> 	 memory back to the balloon. Basically a busy no-op."
> 
> The name "deflate on OOM" makes it pretty clear when deflation should
> happen - after other approaches to reclaim memory failed, not while
> reclaiming. This allows to minimize the footprint of a guest - memory
> will only be taken out of the balloon when really needed.
> 
> Especially, a drop_slab() will result in the whole balloon getting
> deflated - undesired.

Could you explain why some more? drop_caches shouldn't be really used in
any production workloads and if somebody really wants all the cache to
be dropped then why is balloon any different?

-- 
Michal Hocko
SUSE Labs

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