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

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Date:   Wed, 12 Jun 2019 15:54:19 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        "Theodore Ts'o" <tytso@....edu>, Jeff Layton <jlayton@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

On Wed, Jun 12, 2019 at 3:12 PM Ira Weiny <ira.weiny@...el.com> wrote:
>
> 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.

Can RDMA context invalidation, "device disassociate", be inflicted on
a process from the outside? Identifying the pid of a pin holder only
leaves SIGKILL of the entire process as the remediation for revoking a
pin, and I assume admins would use the finer grained invalidation
where it was available.