Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

[Ira Weiny]

On Fri, Jun 07, 2019 at 09:17:29AM -0300, Jason Gunthorpe wrote:
> On Fri, Jun 07, 2019 at 12:36:36PM +0200, Jan Kara wrote:
> 
> > Because the pins would be invisible to sysadmin from that point on. 
> 
> It is not invisible, it just shows up in a rdma specific kernel
> interface. You have to use rdma netlink to see the kernel object
> holding this pin.
> 
> If this visibility is the main sticking point I suggest just enhancing
> the existing MR reporting to include the file info for current GUP
> pins and teaching lsof to collect information from there as well so it
> is easy to use.
> 
> If the ownership of the lease transfers to the MR, and we report that
> ownership to userspace in a way lsof can find, then I think all the
> concerns that have been raised are met, right?

I was contemplating some new lsof feature yesterday.  But what I don't think we
want is sysadmins to have multiple tools for multiple subsystems.  Or even have
to teach lsof something new for every potential new subsystem user of GUP pins.

I was thinking more along the lines of reporting files which have GUP pins on
them directly somewhere (dare I say procfs?) and teaching lsof to report that
information.  That would cover any subsystem which does a longterm pin.

> 
> > ugly to live so we have to come up with something better. The best I can
> > currently come up with is to have a method associated with the lease that
> > would invalidate the RDMA context that holds the pins in the same way that
> > a file close would do it.
> 
> This is back to requiring all RDMA HW to have some new behavior they
> currently don't have..
> 
> The main objection to the current ODP & DAX solution is that very
> little HW can actually implement it, having the alternative still
> require HW support doesn't seem like progress.
> 
> I think we will eventually start seein some HW be able to do this
> invalidation, but it won't be universal, and I'd rather leave it
> optional, for recovery from truely catastrophic errors (ie my DAX is
> on fire, I need to unplug it).

Agreed.  I think software wise there is not much some of the devices can do
with such an "invalidate".

Ira