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

From: Ira Weiny <ira.weiny@intel.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: "Jan Kara" <jack@suse.cz>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Theodore Ts'o" <tytso@mit.edu>,
	"Jeff Layton" <jlayton@kernel.org>,
	"Dave Chinner" <david@fromorbit.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	linux-xfs@vger.kernel.org,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-ext4@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal
Date: Wed, 12 Jun 2019 15:13:36 -0700	[thread overview]
Message-ID: <20190612221336.GA27080@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <20190612191421.GM3876@ziepe.ca>

On Wed, Jun 12, 2019 at 04:14:21PM -0300, Jason Gunthorpe wrote:
> On Wed, Jun 12, 2019 at 02:09:07PM +0200, Jan Kara wrote:
> > On Wed 12-06-19 08:47:21, Jason Gunthorpe wrote:
> > > On Wed, Jun 12, 2019 at 12:29:17PM +0200, Jan Kara wrote:
> > > 
> > > > > > 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".
> > > > 
> > > > So out of curiosity: What does RDMA driver do when userspace just closes
> > > > the file pointing to RDMA object? It has to handle that somehow by aborting
> > > > everything that's going on... And I wanted similar behavior here.
> > > 
> > > It aborts *everything* connected to that file descriptor. Destroying
> > > everything avoids creating inconsistencies that destroying a subset
> > > would create.
> > > 
> > > What has been talked about for lease break is not destroying anything
> > > but very selectively saying that one memory region linked to the GUP
> > > is no longer functional.
> > 
> > OK, so what I had in mind was that if RDMA app doesn't play by the rules
> > and closes the file with existing pins (and thus layout lease) we would
> > force it to abort everything. Yes, it is disruptive but then the app didn't
> > obey the rule that it has to maintain file lease while holding pins. Thus
> > such situation should never happen unless the app is malicious / buggy.
> 
> We do have the infrastructure to completely revoke the entire
> *content* of a FD (this is called device disassociate). It is
> basically close without the app doing close. But again it only works
> with some drivers. However, this is more likely something a driver
> could support without a HW change though.
> 
> It is quite destructive as it forcibly kills everything RDMA related
> the process(es) are doing, but it is less violent than SIGKILL, and
> there is perhaps a way for the app to recover from this, if it is
> coded for it.

I don't think many are...  I think most would effectively be "killed" if this
happened to them.

> 
> My preference would be to avoid this scenario, but if it is really
> necessary, we could probably build it with some work.
> 
> The only case we use it today is forced HW hot unplug, so it is rarely
> used and only for an 'emergency' like use case.

I'd really like to avoid this as well.  I think it will be very confusing for
RDMA apps to have their context suddenly be invalid.  I think if we have a way
for admins to ID who is pinning a file the admin can take more appropriate
action on those processes.   Up to and including killing the process.

Ira