From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Wang Subject: Re: [virtio-dev] Re: [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ Date: Thu, 16 Nov 2017 21:27:24 +0800 Message-ID: <5A0D923C.4020807@intel.com> References: <1509696786-1597-1-git-send-email-wei.w.wang@intel.com> <1509696786-1597-7-git-send-email-wei.w.wang@intel.com> <20171115220743-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, mhocko@kernel.org, akpm@linux-foundation.org, mawilcox@microsoft.com, david@redhat.com, penguin-kernel@I-love.SAKURA.ne.jp, cornelia.huck@de.ibm.com, mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, willy@infradead.org, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu@aliyun.com To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20171115220743-mutt-send-email-mst@kernel.org> Sender: owner-linux-mm@kvack.org List-Id: kvm.vger.kernel.org On 11/16/2017 04:32 AM, Michael S. Tsirkin wrote: > On Fri, Nov 03, 2017 at 04:13:06PM +0800, Wei Wang wrote: >> Negotiation of the VIRTIO_BALLOON_F_FREE_PAGE_VQ feature indicates the >> support of reporting hints of guest free pages to the host via >> virtio-balloon. The host requests the guest to report the free pages by >> sending commands via the virtio-balloon configuration registers. >> >> When the guest starts to report, the first element added to the free page >> vq is a sequence id of the start reporting command. The id is given by >> the host, and it indicates whether the following free pages correspond >> to the command. For example, the host may stop the report and start again >> with a new command id. The obsolete pages for the previous start command >> can be detected by the id dismatching on the host. The id is added to the >> vq using an output buffer, and the free pages are added to the vq using >> input buffer. >> >> Here are some explainations about the added configuration registers: >> - host2guest_cmd: a register used by the host to send commands to the >> guest. >> - guest2host_cmd: written by the guest to ACK to the host about the >> commands that have been received. The host will clear the corresponding >> bits on the host2guest_cmd register. The guest also uses this register >> to send commands to the host (e.g. when finish free page reporting). >> - free_page_cmd_id: the sequence id of the free page report command >> given by the host. >> >> Signed-off-by: Wei Wang >> Signed-off-by: Liang Li >> Cc: Michael S. Tsirkin >> Cc: Michal Hocko >> --- >> >> + >> +static void report_free_page(struct work_struct *work) >> +{ >> + struct virtio_balloon *vb; >> + >> + vb = container_of(work, struct virtio_balloon, report_free_page_work); >> + report_free_page_cmd_id(vb); >> + walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages); >> + /* >> + * The last few free page blocks that were added may not reach the >> + * batch size, but need a kick to notify the device to handle them. >> + */ >> + virtqueue_kick(vb->free_page_vq); >> + report_free_page_end(vb); >> +} >> + > I think there's an issue here: if pages are poisoned and hypervisor > subsequently drops them, testing them after allocation will > trigger a false positive. > > The specific configuration: > > PAGE_POISONING on > PAGE_POISONING_NO_SANITY off > PAGE_POISONING_ZERO off > > > Solutions: > 1. disable the feature in that configuration > suggested as an initial step Thanks for the finding. Similar to this option: I'm thinking could we make walk_free_mem_block() simply return if that option is on? That is, at the beginning of the function: if (!page_poisoning_enabled()) return; I think in most usages, people would not choose to use the poisoning option due to the added overhead. Probably we could make it a separate fix patch of this report following patch 5 to explain the above reasons in the commit. > 2. pass poison value to host so it can validate page content > before it drops it > 3. pass poison value to host so it can init allocated pages with that value > > In fact one nice side effect would be that unmap > becomes safe even though free list is not locked anymore. I haven't got this point yet, how would it bring performance benefit? > It would be interesting to see whether this last has > any value performance-wise. > Best, Wei -- 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