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Date:   Thu, 6 Feb 2020 04:09:38 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        virtualization@lists.linux-foundation.org,
        Tyler Sanderson <tysand@google.com>,
        Wei Wang <wei.w.wang@intel.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Nadav Amit <namit@vmware.com>, Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v1 3/3] virtio-balloon: Switch back to OOM handler for
 VIRTIO_BALLOON_F_DEFLATE_ON_OOM
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On Thu, Feb 06, 2020 at 10:05:43AM +0100, David Hildenbrand wrote:
> >> commit bf50e69f63d21091e525185c3ae761412be0ba72
> >> Author: Dave Hansen <dave@linux.vnet.ibm.com>
> >> Date:   Thu Apr 7 10:43:25 2011 -0700
> >>
> >>     virtio balloon: kill tell-host-first logic
> >>
> >>     The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST
> >>     feature bit.  Whenever the bit is set, the guest kernel must
> >>     always tell the host before we free pages back to the allocator.
> >>     Without this feature, we might free a page (and have another
> >>     user touch it) while the hypervisor is unprepared for it.
> >>
> >>     But, if the bit is _not_ set, we are under no obligation to
> >>     reverse the order; we're under no obligation to do _anything_.
> >>     As of now, qemu-kvm defines the bit, but doesn't set it.
> > 
> > Well this is not what the spec says in the end.
> 
> I didn't check the spec, maybe I should do that :)
> 
> > To continue that commit message:
> > 
> >     This patch makes the "tell host first" logic the only case.  This
> >     should make everybody happy, and reduce the amount of untested or
> >     untestable code in the kernel.
> 
> Yeah, but this comment explains that the current deflate is only in
> place, because it makes the code simpler (to support both cases). Of
> course, doing the deflate might result in performance improvements.
> (e.g., MADV_WILLNEED)
> 
> > 
> > you can try proposing the change to the virtio TC, see what do others
> > think.
> 
> We can just drop the comment from this patch for now. The tell_host host
> not be an issue AFAIKS.

I guess it's a good idea.


> -- 
> Thanks,
> 
> David / dhildenb