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).