linux-kernel - RE: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for VIRTIO_BALLOON_F_DEFLATE_ON

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Date:   Thu, 6 Feb 2020 08:57:05 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Tyler Sanderson <tysand@...gle.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Nadav Amit <namit@...are.com>, Michal Hocko <mhocko@...nel.org>
Subject: RE: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for
 VIRTIO_BALLOON_F_DEFLATE_ON_OOM

On Thursday, February 6, 2020 12:34 AM, 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.
> 
> 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."

Not sure if we need to go back to OOM, which has many drawbacks as we discussed.
Just posted out another approach, which is simple.

Best,
Wei