linux-kernel - Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK

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Date:   Sun, 12 Mar 2017 02:07:14 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        virtio-dev@...ts.oasis-open.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        Liang Li <liang.z.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        Amit Shah <amit.shah@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Liang Li <liliang324@...il.com>
Subject: Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of
 VIRTIO_BALLOON_F_CHUNK_TRANSFER

On Sat, Mar 11, 2017 at 07:59:31PM +0800, Wei Wang wrote:
> On 03/11/2017 01:11 AM, Matthew Wilcox wrote:
> > On Fri, Mar 10, 2017 at 05:58:28PM +0200, Michael S. Tsirkin wrote:
> > > One of the issues of current balloon is the 4k page size
> > > assumption. For example if you free a huge page you
> > > have to split it up and pass 4k chunks to host.
> > > Quite often host can't free these 4k chunks at all (e.g.
> > > when it's using huge tlb fs).
> > > It's even sillier for architectures with base page size >4k.
> > I completely agree with you that we should be able to pass a hugepage
> > as a single chunk.  Also we shouldn't assume that host and guest have
> > the same page size.  I think we can come up with a scheme that actually
> > lets us encode that into a 64-bit word, something like this:
> > 
> > bit 0 clear => bits 1-11 encode a page count, bits 12-63 encode a PFN, page size 4k.
> > bit 0 set, bit 1 clear => bits 2-12 encode a page count, bits 13-63 encode a PFN, page size 8k
> > bits 0+1 set, bit 2 clear => bits 3-13 for page count, bits 14-63 for PFN, page size 16k.
> > bits 0-2 set, bit 3 clear => bits 4-14 for page count, bits 15-63 for PFN, page size 32k
> > bits 0-3 set, bit 4 clear => bits 5-15 for page count, bits 16-63 for PFN, page size 64k
> > 
> > That means we can always pass 2048 pages (of whatever page size) in a single chunk.  And
> > we support arbitrary power of two page sizes.  I suggest something like this:
> > 
> > u64 page_to_chunk(struct page *page)
> > {
> > 	u64 chunk = page_to_pfn(page) << PAGE_SHIFT;
> > 	chunk |= (1UL << compound_order(page)) - 1;
> > }
> > 
> > (note this is a single page of order N, so we leave the page count bits
> > set to 0, meaning one page).
> > 
> 
> I'm thinking what if the guest needs to transfer these much physically
> continuous
> memory to host: 1GB+2MB+64KB+32KB+16KB+4KB.
> Is it going to use Six 64-bit chunks? Would it be simpler if we just
> use the 128-bit chunk format (we can drop the previous normal 64-bit
> format)?
> 
> Best,
> Wei

I think I prefer that as a more straightforward approach, but
I can live with either approach.


-- 
MST