linux-kernel - Re: [PATCH] mm: don't zero ballooned pages

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Date:   Mon, 31 Jul 2017 16:34:58 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        mawilcox@...rosoft.com, dave.hansen@...el.com,
        akpm@...ux-foundation.org, zhenwei.pi@...runcloud.com
Subject: Re: [PATCH] mm: don't zero ballooned pages

On 07/31/2017 03:43 PM, Michal Hocko wrote:
> On Mon 31-07-17 15:41:49, Wei Wang wrote:
>> On 07/31/2017 02:55 PM, Michal Hocko wrote:
>>> On Mon 31-07-17 12:13:33, Wei Wang wrote:
>>>> Ballooned pages will be marked as MADV_DONTNEED by the hypervisor and
>>>> shouldn't be given to the host ksmd to scan.
>>> Could you point me where this MADV_DONTNEED is done, please?
>> Sure. It's done in the hypervisor when the balloon pages are received.
>>
>> Please see line 40 at
>> https://github.com/qemu/qemu/blob/master/hw/virtio/virtio-balloon.c
> Thanks. Are all hypervisors which are using this API doing this?


The implementation may be different across different hypervisors.
But the underlying concept is the same - they unmap the balloon
pages from the guest and those pages will be given to other guests
or host processes to use.

Regardless of the implementation, I think it is an improper operation
to make the memory KSM mergeable when the memory does not
belong to the guest anymore.


Best,
Wei