From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f200.google.com (mail-qk0-f200.google.com [209.85.220.200]) by kanga.kvack.org (Postfix) with ESMTP id 003AE28092C for ; Fri, 10 Mar 2017 14:35:26 -0500 (EST) Received: by mail-qk0-f200.google.com with SMTP id c85so197119407qkg.0 for ; Fri, 10 Mar 2017 11:35:25 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id t56si8810135qta.24.2017.03.10.11.35.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 10 Mar 2017 11:35:25 -0800 (PST) Date: Fri, 10 Mar 2017 21:35:21 +0200 From: "Michael S. Tsirkin" Subject: Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK_TRANSFER Message-ID: <20170310211102-mutt-send-email-mst@kernel.org> References: <1488519630-89058-1-git-send-email-wei.w.wang@intel.com> <1488519630-89058-4-git-send-email-wei.w.wang@intel.com> <20170309141411.GZ16328@bombadil.infradead.org> <58C28FF8.5040403@intel.com> <20170310175349-mutt-send-email-mst@kernel.org> <20170310171143.GA16328@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170310171143.GA16328@bombadil.infradead.org> Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: Wei Wang , virtio-dev@lists.oasis-open.org, kvm@vger.kernel.org, qemu-devel@nongnu.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-mm@kvack.org, Liang Li , Paolo Bonzini , Cornelia Huck , Amit Shah , Dave Hansen , Andrea Arcangeli , David Hildenbrand , Liang Li On Fri, Mar 10, 2017 at 09:11:44AM -0800, 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; > } You need to fill in the size, do you not? > > (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. I think we will still need something like this down the road. The point is that not all hosts are able to use 4k pages in a balloon. So it's pointless for guest to pass 4k pages to such a host, and we need host to tell guest the page size it needs. However that's a separate feature that can wait until another day. > > - 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? Not entirely sure but I think would have been prudent to leave some free if possible. Your encoding seems to use them all up, so be it. -- MST -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org