linux-kernel - Re: [PATCH v34 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE

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Date:   Tue, 26 Jun 2018 20:27:44 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        torvalds@...ux-foundation.org, pbonzini@...hat.com,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu0@...il.com, nilal@...hat.com, riel@...hat.com,
        peterx@...hat.com
Subject: Re: [PATCH v34 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

On 06/26/2018 11:56 AM, Michael S. Tsirkin wrote:
> On Tue, Jun 26, 2018 at 11:46:35AM +0800, Wei Wang wrote:
>

>>
>>>
>>>> +	if (!arrays)
>>>> +		return NULL;
>>>> +
>>>> +	for (i = 0; i < max_array_num; i++) {
>>> So we are getting a ton of memory here just to free it up a bit later.
>>> Why doesn't get_from_free_page_list get the pages from free list for us?
>>> We could also avoid the 1st allocation then - just build a list
>>> of these.
>> That wouldn't be a good choice for us. If we check how the regular
>> allocation works, there are many many things we need to consider when pages
>> are allocated to users.
>> For example, we need to take care of the nr_free
>> counter, we need to check the watermark and perform the related actions.
>> Also the folks working on arch_alloc_page to monitor page allocation
>> activities would get a surprise..if page allocation is allowed to work in
>> this way.
>>
> mm/ code is well positioned to handle all this correctly.

I'm afraid that would be a re-implementation of the alloc functions, and 
that would be much more complex than what we have. I think your idea of 
passing a list of pages is better.

Best,
Wei