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

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Date:   Fri, 7 Jun 2019 07:52:13 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Jan Kara <jack@...e.cz>, Dan Williams <dan.j.williams@...el.com>,
        Theodore Ts'o <tytso@....edu>,
        Jeff Layton <jlayton@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-ext4@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

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