Re: [RFC PATCH 0/9] Support for Virtio-fs daemon crash reconnection

["Marc-André Lureau" <marcandre.lureau@gmail.com>]

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Hi

On Tue, Dec 15, 2020 at 8:22 PM Jiachen Zhang <
zhangjiachen.jaycee@bytedance.com> wrote:

> Hi, all
>
> We implement virtio-fs crash reconnection in this patchset. The crash
> reconnection of virtiofsd here is completely transparent to guest, no
> remount in guest is needed, even the inflight requests can be handled
> normally after reconnection. We are looking forward to any comments.
>
> Thanks,
> Jiachen
>
>
> OVERVIEW:
>
> To support virtio-fs crash reconnection, we need to support the recovery
> of 1) inflight FUSE request, and 2) virtiofsd internal status information.
>
> Fortunately, QEMU's vhost-user reconnection framework already supports
> inflight I/O tracking by using VHOST_USER_GET_INFLIGHT_FD and
> VHOST_USER_SET_INFLIGHT_FD (see 5ad204bf2 and 5f9ff1eff for details).
> As the FUSE requests are transferred by virtqueue I/O requests, by using
> the vhost-user inflight I/O tracking, we can recover the inflight FUSE
> requests.
>
> To support virtiofsd internal status recovery, we introduce 4 new
> vhost-user message types. As shown in the following diagram, two of them
> are used to persist shared lo_maps and opened fds to QEMU, the other two
> message types are used to restore the status when reconnecting.
>
>                                VHOST_USER_SLAVE_SHM
>                                VHOST_USER_SLAVE_FD
>     +--------------+       Persist       +--------------------+
>     |              <---------------------+                    |
>     |     QEMU     |                     |  Virtio-fs Daemon  |
>     |              +--------------------->                    |
>     +--------------+       Restore       +--------------------+
>             VHOST_USER_SET_SHM
>             VHOST_USER_SET_FD
>
> Although the 4 newly added message types are to support virtiofsd
> reconnection in this patchset, it might be potential in other situation.
> So we keep in mind to make them more general when add them to vhost
> related source files. VHOST_USER_SLAVE_SHM and VHOST_USER_SET_SHM can be
> used for memory sharing between a vhost-user daemon and QEMU,
> VHOST_USER_SLAVE_FD and VHOST_USER_SET_FD would be useful if we want to
> shared opened fds between QEMU process and vhost-user daemon process.
>

Before adding new messages to the already complex vhost-user protocol, can
we evaluate other options?

First thing that came to my mind is that the memory state could be saved to
disk or with a POSIX shared memory object.

Eventually, the protocol could just pass around the fds, and not make a
special treatment for shared memory.

Then I remember systemd has a pretty good API & protocol for this sort of
thing: sd_notify(3) (afaik, it is quite easy to implement a minimal handler)

You can store fds with FDSTORE=1 (with an optional associated FDNAME).
sd_listen_fds() & others to get them back (note: passed by inheritance only
I think). systemd seems to not make shm a special case either, just treat
it like an opened fd to restore.

If we consider backend processes are going to be managed by libvirt or even
a systemd service, is it a better alternative? sd_notify() offers a number
of interesting features as well to monitor services.


>
> USAGE and NOTES:
>
> - The commits are rebased to a recent QEMU master commit b4d939133dca0fa2b.
>
> - ",reconnect=1" should be added to the "-chardev socket" of
> vhost-user-fs-pci
> in the QEMU command line, for example:
>
>     qemu-system-x86_64 ... \
>     -chardev socket,id=char0,path=/tmp/vhostqemu,reconnect=1 \
>     -device vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=myfs \
>     ...
>
> - We add new options for virtiofsd to enable or disable crash reconnection.
> And some options are not supported by crash reconnection. So add following
> options to virtiofsd to enable reconnection:
>
>     virtiofsd ... -o reconnect -o no_mount_ns -o no_flock -o no_posix_lock
>     -o no_xattr ...
>
> - The reasons why virtiofsd-side locking, extended attributes, and mount
> namespace are not supported is explained in the commit message of the 6th
> patch (virtiofsd: Add two new options for crash reconnection).
>
> - The 9th patch is a work-around that will not affect the overall
> correctness.
> We remove the qsort related codes because we found that when resubmit_num
> is
> larger than 64, seccomp will kill the virtiofsd process.
>
> - Support for dax version virtiofsd is very possible and requires almost no
> additional change to this patchset.
>
>
> Jiachen Zhang (9):
>   vhost-user-fs: Add support for reconnection of vhost-user-fs backend
>   vhost: Add vhost-user message types for sending shared memory and file
>     fds
>   vhost-user-fs: Support virtiofsd crash reconnection
>   libvhost-user: Add vhost-user message types for sending shared memory
>     and file fds
>   virtiofsd: Convert the struct lo_map array to a more flatten layout
>   virtiofsd: Add two new options for crash reconnection
>   virtiofsd: Persist/restore lo_map and opened fds to/from QEMU
>   virtiofsd: Ensure crash consistency after reconnection
>   virtiofsd: (work around) Comment qsort in inflight I/O tracking
>
>  contrib/libvhost-user/libvhost-user.c | 106 +++-
>  contrib/libvhost-user/libvhost-user.h |  70 +++
>  docs/interop/vhost-user.rst           |  41 ++
>  hw/virtio/vhost-user-fs.c             | 334 ++++++++++-
>  hw/virtio/vhost-user.c                | 123 ++++
>  hw/virtio/vhost.c                     |  42 ++
>  include/hw/virtio/vhost-backend.h     |   6 +
>  include/hw/virtio/vhost-user-fs.h     |  16 +-
>  include/hw/virtio/vhost.h             |  42 ++
>  tools/virtiofsd/fuse_lowlevel.c       |  24 +-
>  tools/virtiofsd/fuse_virtio.c         |  44 ++
>  tools/virtiofsd/fuse_virtio.h         |   1 +
>  tools/virtiofsd/helper.c              |   9 +
>  tools/virtiofsd/passthrough_helpers.h |   2 +-
>  tools/virtiofsd/passthrough_ll.c      | 830 ++++++++++++++++++--------
>  tools/virtiofsd/passthrough_seccomp.c |   1 +
>  16 files changed, 1413 insertions(+), 278 deletions(-)
>
> --
> 2.20.1
>
>
>

-- 
Marc-André Lureau

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