linux-kernel - RE: [virtio-dev] Re: [PATCH v33 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE

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Date:   Wed, 20 Jun 2018 09:11:39 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "liliang.opensource@...il.com" <liliang.opensource@...il.com>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "quan.xu0@...il.com" <quan.xu0@...il.com>,
        "nilal@...hat.com" <nilal@...hat.com>,
        "riel@...hat.com" <riel@...hat.com>,
        "peterx@...hat.com" <peterx@...hat.com>
Subject: RE: [virtio-dev] Re: [PATCH v33 2/4] virtio-balloon:
 VIRTIO_BALLOON_F_FREE_PAGE_HINT

On Tuesday, June 19, 2018 10:43 PM, Michael S. Tsirk wrote:
> On Tue, Jun 19, 2018 at 08:13:37PM +0800, Wei Wang wrote:
> > On 06/19/2018 11:05 AM, Michael S. Tsirkin wrote:
> > > On Tue, Jun 19, 2018 at 01:06:48AM +0000, Wang, Wei W wrote:
> > > > On Monday, June 18, 2018 10:29 AM, Michael S. Tsirkin wrote:
> > > > > On Sat, Jun 16, 2018 at 01:09:44AM +0000, Wang, Wei W wrote:
> > > > > > Not necessarily, I think. We have min(4m_page_blocks / 512,
> > > > > > 1024) above,
> > > > > so the maximum memory that can be reported is 2TB. For larger
> guests, e.g.
> > > > > 4TB, the optimization can still offer 2TB free memory (better
> > > > > than no optimization).
> > > > >
> > > > > Maybe it's better, maybe it isn't. It certainly muddies the waters even
> more.
> > > > > I'd rather we had a better plan. From that POV I like what
> > > > > Matthew Wilcox suggested for this which is to steal the necessary # of
> entries off the list.
> > > > Actually what Matthew suggested doesn't make a difference here.
> > > > That method always steal the first free page blocks, and sure can
> > > > be changed to take more. But all these can be achieved via kmalloc
> > > I'd do get_user_pages really. You don't want pages split, etc.
> 
> Oops sorry. I meant get_free_pages .

Yes, we can use __get_free_pages, and the max allocation is MAX_ORDER - 1, which can report up to 2TB free memory. 

"getting two pages isn't harder", do you mean passing two arrays (two allocations by get_free_pages(,MAX_ORDER -1)) to the mm API?

Please see if the following logic aligns to what you think:

        uint32_t i, max_hints, hints_per_page, hints_per_array, total_arrays;
        unsigned long *arrays;
 
     /*
         * Each array size is MAX_ORDER_NR_PAGES. If one array is not enough to
         * store all the hints, we need to allocate multiple arrays.
         * max_hints: the max number of 4MB free page blocks
         * hints_per_page: the number of hints each page can store
         * hints_per_array: the number of hints an array can store
         * total_arrays: the number of arrays we need
         */
        max_hints = totalram_pages / MAX_ORDER_NR_PAGES;
        hints_per_page = PAGE_SIZE / sizeof(__le64);
        hints_per_array = hints_per_page * MAX_ORDER_NR_PAGES;
        total_arrays = max_hints /  hints_per_array +
                       !!(max_hints % hints_per_array);
        arrays = kmalloc(total_arrays * sizeof(unsigned long), GFP_KERNEL);
        for (i = 0; i < total_arrays; i++) {
                arrays[i] = __get_free_pages(__GFP_ATOMIC | __GFP_NOMEMALLOC, MAX_ORDER - 1);

              if (!arrays[i])
                      goto out;
        }


- the mm API needs to be changed to support storing hints to multiple separated arrays offered by the caller.

Best,
Wei