Re: [Virtio-fs] [PATCH 0/8] virtiofs: Notification queue and blocking posix locks

[Vivek Goyal <vgoyal@redhat.com>]

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
>