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:   Fri, 14 Feb 2020 15:18:55 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
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

>> 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?
> 

Deflation should happen when the guest is out of memory, not when
somebody thinks it's time to reclaim some memory. That's what the
feature promised from the beginning: Only give the guest more memory in
case it *really* needs more memory.

Deflate on oom, not deflate on reclaim/memory pressure. (that's what the
report was all about)

A priority for shrinkers might be a step into the right direction.

-- 
Thanks,

David / dhildenb