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

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Date:   Sat, 11 Mar 2017 19:59:31 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     "Michael S. Tsirkin" <mst@...hat.com>,
        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 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