[Virtio-fs] [PATCH 0/8] virtiofs: Notification queue and blocking posix locks
Vivek Goyal
vgoyal at redhat.com
Thu Sep 30 15:43:29 UTC 2021
On Thu, Sep 30, 2021 at 10:38:42AM -0400, Vivek Goyal wrote:
> Hi,
>
> As of now we do not support blocking remote posix locks with virtiofs.
> Well fuse client does not care but server returns -EOPNOTSUPP.
Posted corresponding qemu/virtiofsd changes here.
https://listman.redhat.com/archives/virtio-fs/2021-September/msg00153.html
Thanks
Vivek
>
> There are couple of reasons to not support it yet.
>
> - If virtiofsd is single threaded or does not have a thread pool just
> to handle requests which can block for a long time, virtiofsd will
> stop processing new requests and virtiofs will come to a halt.
> To the extent that further unlock request will not make progress
> and deadlock will result. This can be taken care of by creating
> a custom thread pool in virtiofsd just to hanlde lock requests.
>
> - If client sends a blocking lock request and blocks, then it will
> consume descriptors in vring. If enough processes block, it is
> possible that vring does not have capacity to send more requests
> till some response comes back and descriptors are free. This can
> also lead to deadlock where an unlock request can't be sent to
> virtiofsd now. Also this will stop virtiofs operation as well as
> new filesystem requests can't be sent.
>
> To avoid this issue, idea was suggested thatn when a blocking
> lock request is sent by client, do not block it. Immediately
> send a reply saying client process should wait for a notification
> which will let it know once lock is available. This will make
> sure descriptors in virtqueue are not kept busy while we are
> waiting for lock and future unlock and other file system requests
> can continue to make progress.
>
> This first requires a notion of notification queue and virtiosfd
> being able to send notifications to client. This patch series
> implements that as well.
>
> As of now only one notification type has been implemented but now
> infrastructure is in place and other use cases should be easily
> add more type of notifications as need be.
>
> We don't yet have the capability to interrupt the process which
> is waiting for the posix lock. And reason for that is that virtiofs
> does not support capability to interrupt yet. That's a TODO item
> for later.
>
> Please have a look.
>
> Thanks
> Vivek
>
> Vivek Goyal (8):
> virtiofs: Disable interrupt requests properly
> virtiofs: Fix a comment about fuse_dev allocation
> virtiofs: Add an index to keep track of first request queue
> virtiofs: Decouple queue index and queue type
> virtiofs: Add a virtqueue for notifications
> virtiofs: Add a helper to end request and decrement inflight number
> virtiofs: Add new notification type FUSE_NOTIFY_LOCK
> virtiofs: Handle reordering of reply and notification event
>
> fs/fuse/virtio_fs.c | 438 ++++++++++++++++++++++++++++++---
> include/uapi/linux/fuse.h | 11 +-
> include/uapi/linux/virtio_fs.h | 5 +
> 3 files changed, 412 insertions(+), 42 deletions(-)
>
> --
> 2.31.1
>
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