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

From: Matthew Wilcox
Date: Fri Mar 10 2017 - 12:12:04 EST

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).

> Two things to consider:
> - host should pass its base page size to guest
> this can be a separate patch and for now we can fall back on 12 bit if not there

With this encoding scheme, I don't think we need to do this? As long as
it's *at least* 12 bit, then we're fine.

> - guest should pass full huge pages to host
> this should be done correctly to avoid breaking up huge pages
> I would say yes let's use a single format but drop the "normal chunk"
> and always use the extended one.
> Also, size is in units of 4k, right? Please document that low 12 bit
> are reserved, they will be handy as e.g. flags.

What per-chunk flags are you thinking would be useful?