Messages in this thread | | | Date | Fri, 17 Nov 2017 19:35:03 +0800 | From | Wei Wang <> | Subject | Re: [virtio-dev] Re: [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ |
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On 11/16/2017 09:27 PM, Wei Wang wrote: > 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 <wei.w.wang@intel.com> >>> Signed-off-by: Liang Li <liang.z.li@intel.com> >>> Cc: Michael S. Tsirkin <mst@redhat.com> >>> Cc: Michal Hocko <mhocko@kernel.org> >>> --- >>> >>> + >>> +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; >
Thought about it more, I think it would be better to put this logic to virtio_balloon:
send_free_page_cmd_id(vb, &vb->start_cmd_id); if (page_poisoning_enabled() && !IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY)) walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages); send_free_page_cmd_id(vb, &vb->stop_cmd_id);
walk_free_mem_block() should be a more generic API, and this potential page poisoning issue is specific to live migration which is only one use case of this function, so I think it is better to handle it in the special use case itself.
Best, Wei
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