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

[Jiachen Zhang]

On Wed, Dec 16, 2020 at 11:36 PM Marc-André Lureau <
marcandre.lureau at gmail.com> wrote:

> Hi
>
> On Tue, Dec 15, 2020 at 8:22 PM Jiachen Zhang <
> zhangjiachen.jaycee at 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.
>
>

Thanks for the suggestions. Actually, we choose to save all state
information to QEMU because a virtiofsd has the same lifecycle as its
QEMU master. However, saving things to a file do avoid communication with
QEMU, and we no longer need to increase the complexity of vhost-user
protocol. The suggestion to save fds to the systemd is also very reasonable
if we don't consider the lifecycle issues, we will try it.

All the best,
Jiachen



>>
>> 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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