Re: [dpdk-dev] [PATCH] vhost: fix segfault as handle set_mem_table message

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

Hi Maxime and Victor,

> -----Original Message-----
> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> Sent: Tuesday, December 5, 2017 10:28 PM
> To: Yuanhan Liu; Tan, Jianfeng; Victor Kaplansky
> Cc: dev@dpdk.org; stable@dpdk.org; Yang, Yi Y
> Subject: Re: [PATCH] vhost: fix segfault as handle set_mem_table message
> 
> 
> 
> On 12/05/2017 03:19 PM, Yuanhan Liu wrote:
> > On Tue, Nov 28, 2017 at 01:09:29PM +0100, Maxime Coquelin wrote:
> >>
> >>
> >> On 11/15/2017 12:41 PM, Jianfeng Tan wrote:
> >>> In a running VM, operations (like device attach/detach) will
> >>> trigger the QEMU to resend set_mem_table to vhost-user backend.
> >>>
> >>> DPDK vhost-user handles this message rudely by unmap all existing
> >>> regions and map new ones. This might lead to segfault if there
> >>> is pmd thread just trying to touch those unmapped memory regions.
> >>>
> >>> But for most cases, except VM memory hotplug,
> 
> FYI, Victor is working on implementing a lock-less protection mechanism
> to prevent crashes in such cases. It is intended first to protect
> log_base in case of multiqueue + live-migration, but would solve thi
> issue too.

Bring this issue back for discussion.

Reported by SPDK guys, even with per-queue lock, they could still run into crash as of memory hot plug or unplug.
In detail, you add the lock in an internal struct, vhost_queue which is, unfortunately, not visible to the external datapath, like vhost-scsi in SPDK.

The memory hot plug and unplug would be the main issue from SPDK side so far. For this specific issue, I think we can continue this patch to filter out the changed regions, and keep unchanged regions not remapped.

But I know that the per-vq lock is not only to resolve the memory table issue, some other vhost-user messages could also lead to problems? If yes, shall we take a step back, to think about how to solve this kind of issues for backends, like vhost-scsi.

Thoughts?

Thanks,
Jianfeng

> 
> >>>> QEMU still sends the
> >>> set_mem_table message even the memory regions are not changed as
> >>> QEMU vhost-user filters out those not backed by file (fd > 0).
> >>>
> >>> To fix this case, we add a check in the handler to see if the
> >>> memory regions are really changed; if not, we just keep old memory
> >>> regions.
> >>>
> >>> Fixes: 8f972312b8f4 ("vhost: support vhost-user")
> >>>
> >>> CC: stable@dpdk.org
> >>>
> >>> CC: Yuanhan Liu <yliu@fridaylinux.org>
> >>> CC: Maxime Coquelin <maxime.coquelin@redhat.com>
> >>>
> >>> Reported-by: Yang Zhang <zy107165@alibaba-inc.com>
> >>> Reported-by: Xin Long <longxin.xl@alibaba-inc.com>
> >>> Signed-off-by: Yi Yang <yi.y.yang@intel.com>
> >>> Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
> >>> ---
> >>>   lib/librte_vhost/vhost_user.c | 33
> +++++++++++++++++++++++++++++++++
> >>>   1 file changed, 33 insertions(+)
> >>
> >> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> >
> > Applied to dpdk-next-virtio.
> >
> > Thanks.
> >
> > 	--yliu
> >
> 
> Maxime