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Date:   Thu, 6 Feb 2020 04:31:01 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     "Wang, Wei W" <wei.w.wang@intel.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "virtualization@lists.linux-foundation.org" 
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        "alexander.h.duyck@linux.intel.com" 
        <alexander.h.duyck@linux.intel.com>,
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        "mhocko@kernel.org" <mhocko@kernel.org>,
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        "penguin-kernel@i-love.sakura.ne.jp" 
        <penguin-kernel@i-love.sakura.ne.jp>
Subject: Re: [PATCH RFC] virtio_balloon: conservative balloon page shrinking
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On Thu, Feb 06, 2020 at 09:27:04AM +0000, Wang, Wei W wrote:
> On Thursday, February 6, 2020 5:04 PM, Michael S. Tsirkin wrote:
> > virtio_balloon_shrinker_count(struct shrinker *shrinker,
> > >  					struct virtio_balloon, shrinker);
> > >  	unsigned long count;
> > >
> > > -	count = vb->num_pages / VIRTIO_BALLOON_PAGES_PER_PAGE;
> > > +	if (conservative_shrinker && global_node_page_state(NR_FILE_PAGES))
> > 
> > I'd rather have an API for that in mm/. In particular, do we want other
> > shrinkers to run, not just pagecache? To pick an example I'm familiar
> > with, kvm mmu cache for nested virt?
> 
> We could make it extendable:
> 
> #define BALLOON_SHRINKER_AFTER_PAGE_CACHE	(1 << 0)
> #define BALLOON_SHRINKER_AFTER_KVM_MMU_CACHE	(1 << 1)
> ...
> 
> uint64_t conservative_shrinker;
> if ((conservative_shrinker | BALLOON_SHRINKER_AFTER_PAGE_CACHE) && global_node_page_state(NR_FILE_PAGES))
> 	return 0;
> 
> For now, we probably only need BALLOON_SHRINKER_AFTER_PAGE_CACHE.
> 
> Best,
> Wei

How about just making this a last resort thing to be compatible with
existing hypervisors? if someone wants to change behaviour
that really should use a feature bit ...

-- 
MST