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Subject: Re: [PATCH v21 2/5 RESEND] virtio-balloon: VIRTIO_BALLOON_F_SG

On 01/11/2018 07:06 PM, Tetsuo Handa wrote:
Wei Wang wrote:
Michael, could we merge patch 3-5 first?
No! I'm repeatedly asking you to propose only VIRTIO_BALLOON_F_SG changes.
Please don't ignore me.

Patch 4 depends on patch 2. Thus, back to patch 2.

There is not strict dependence per se. I plan to split the two features into 2 series, and post out 3-5 first, and the corresponding hypervisor code.
After that's done, I'll get back to the discussion of patch 2.

Now, proceeding to patch 4.

Your patch is trying to call add_one_sg() for multiple times based on

+	/*
+	 * This is expected to never fail: there is always at least 1 entry
+	 * available on the vq, because when the vq is full the worker thread
+	 * that adds the sg will be put into sleep until at least 1 entry is
+	 * available to use.
+	 */

This will be more clear in the new version which is not together with patch 2.

Now, I suspect we need to add VIRTIO_BALLOON_F_FREE_PAGE_VQ flag. I want to see
the patch for the hypervisor side which makes use of VIRTIO_BALLOON_F_FREE_PAGE_VQ
flag because its usage becomes tricky. Between the guest kernel obtains snapshot of
free memory blocks and the hypervisor is told that some pages are currently free,
these pages can become in use. That is, I don't think

   The second feature enables the optimization of the 1st round memory
   transfer - the hypervisor can skip the transfer of guest free pages in the
   1st round.

is accurate. The hypervisor is allowed to mark pages which are told as "currently
unused" by the guest kernel as "write-protected" before starting the 1st round.
Then, the hypervisor performs copying all pages except write-protected pages as
the 1st round. Then, the 2nd and later rounds will be the same. That is,
VIRTIO_BALLOON_F_FREE_PAGE_VQ requires the hypervisor to do 0th round as
preparation. Thus, I want to see the patch for the hypervisor side.

Now, what if all free pages in the guest kernel were reserved as ballooned pages?
There will be no free pages which VIRTIO_BALLOON_F_FREE_PAGE_VQ flag would help.
The hypervisor will have to copy all pages because all pages are either currently
in-use or currently in balloons. After ballooning to appropriate size, there will
be little free memory in the guest kernel, and the hypervisor already knows which
pages are in the balloon. Thus, the hypervisor can skip copying the content of
pages in the balloon, without using VIRTIO_BALLOON_F_FREE_PAGE_VQ flag.

Then, why can't we do "inflate the balloon up to reasonable level (e.g. no need to
wait for reclaim and no need to deflate)" instead of "find all the free pages as of
specific moment" ? That is, code for VIRTIO_BALLOON_F_DEFLATE_ON_OOM could be reused

I think you misunderstood the work, which seems not easy to explain everything from the beginning here. I wish to review patch 4 (I'll send out a new independent version) with Michael if possible.
I'll discuss with you about patch 2 later. Thanks.


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