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

[Wei Wang]

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