Messages in this thread | | | Date | Thu, 16 Nov 2017 21:27:24 +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 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;
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
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