[Virtio-fs] multiple virtiofsd's + overlapping shared subtree -> enforced compatible xattr maps?

[Harry G. Coin]

On 7/1/21 3:33 AM, Dr. David Alan Gilbert wrote:
> * Harry G. Coin (hgcoin at gmail.com) wrote:
>> If two or more instances of virtiofsd have a common full or partially
>> shared directory tree path -- how will those instances 'enforce' a
>> 'compatible' xattr map lest the host+guest(s) have quite the selinux
>> attribute salad?
> Virtiofsd instances are independent; it's upto whatever runs the daemons
> to pick the options in a sane way.

While that's the current case, because the intention was to rely on the
underlying fs to manage contention among virtiofsds: the moment
xattrmaps happened virtiofsd entered the layer all network file system
daemons face.  Roughly speaking: a shift from one process per client to
some IPC or threaded approach with a 'manager' process/thread that
coordinates 'compatible' xattrmaps, acl maps, 'root squashing' etc. etc..

>>   How much does virtiofsd need to feel like nfsd...?
> What does nfsd do for this?

A couple attempts over the last decade, the most landed in 5.9: 
https://www.phoronix.com/scan.php?page=news_item&px=Linux-5.9-NFS-Server-User-Xattr