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

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Message-ID: <20180627053952-mutt-send-email-mst@kernel.org>
Date:   Wed, 27 Jun 2018 05:41:37 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.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 Wed, Jun 27, 2018 at 09:24:18AM +0800, Wei Wang wrote:
> On 06/26/2018 09:34 PM, Michael S. Tsirkin wrote:
> > On Tue, Jun 26, 2018 at 08:27:44PM +0800, Wei Wang wrote:
> > > 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,
> > A re-factoring - you can share code. The main difference is locking.
> > 
> > > 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
> > How much memory is this allocating anyway?
> > 
> 
> For every 2TB memory that the guest has, we allocate 4MB.

Hmm I guess I'm missing something, I don't see it:


+       max_entries = max_free_page_blocks(ARRAY_ALLOC_ORDER);
+       entries_per_page = PAGE_SIZE / sizeof(__le64);
+       entries_per_array = entries_per_page * (1 << ARRAY_ALLOC_ORDER);
+       max_array_num = max_entries / entries_per_array +
+                       !!(max_entries % entries_per_array);

Looks like you always allocate the max number?


> This is the same
> for both cases.
> For today's guests, usually there will be only one 4MB allocated and passed
> to get_from_free_page_list.
> 
> Best,
> Wei
> 
>