linux-ext4 - Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

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Date:   Wed, 12 Jun 2019 16:14:21 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jan Kara <jack@...e.cz>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        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 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.

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.

Jason