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

[Ira Weiny <ira.weiny@intel.com>]

On Sat, Jun 08, 2019 at 10:10:36AM +1000, Dave Chinner wrote:
> On Fri, Jun 07, 2019 at 11:25:35AM -0700, Ira Weiny wrote:
> > On Fri, Jun 07, 2019 at 01:04:26PM +0200, Jan Kara wrote:
> > > On Thu 06-06-19 15:03:30, Ira Weiny wrote:
> > > > On Thu, Jun 06, 2019 at 12:42:03PM +0200, Jan Kara wrote:
> > > > > On Wed 05-06-19 18:45:33, ira.weiny@intel.com wrote:
> > > > > > From: Ira Weiny <ira.weiny@intel.com>
> > > > > 
> > > > > So I'd like to actually mandate that you *must* hold the file lease until
> > > > > you unpin all pages in the given range (not just that you have an option to
> > > > > hold a lease). And I believe the kernel should actually enforce this. That
> > > > > way we maintain a sane state that if someone uses a physical location of
> > > > > logical file offset on disk, he has a layout lease. Also once this is done,
> > > > > sysadmin has a reasonably easy way to discover run-away RDMA application
> > > > > and kill it if he wishes so.
> > > > 
> > > > Fair enough.
> > > > 
> > > > I was kind of heading that direction but had not thought this far forward.  I
> > > > was exploring how to have a lease remain on the file even after a "lease
> > > > break".  But that is incompatible with the current semantics of a "layout"
> > > > lease (as currently defined in the kernel).  [In the end I wanted to get an RFC
> > > > out to see what people think of this idea so I did not look at keeping the
> > > > lease.]
> > > > 
> > > > Also hitch is that currently a lease is forcefully broken after
> > > > <sysfs>/lease-break-time.  To do what you suggest I think we would need a new
> > > > lease type with the semantics you describe.
> > > 
> > > I'd do what Dave suggested - add flag to mark lease as unbreakable by
> > > truncate and teach file locking core to handle that. There actually is
> > > support for locks that are not broken after given timeout so there
> > > shouldn't be too many changes need.
> > >  
> > > > Previously I had thought this would be a good idea (for other reasons).  But
> > > > what does everyone think about using a "longterm lease" similar to [1] which
> > > > has the semantics you proppose?  In [1] I was not sure "longterm" was a good
> > > > name but with your proposal I think it makes more sense.
> > > 
> > > As I wrote elsewhere in this thread I think FL_LAYOUT name still makes
> > > sense and I'd add there FL_UNBREAKABLE to mark unusal behavior with
> > > truncate.
> > 
> > Ok I want to make sure I understand what you and Dave are suggesting.
> > 
> > Are you suggesting that we have something like this from user space?
> > 
> > 	fcntl(fd, F_SETLEASE, F_LAYOUT | F_UNBREAKABLE);
> 
> Rather than "unbreakable", perhaps a clearer description of the
> policy it entails is "exclusive"?
> 
> i.e. what we are talking about here is an exclusive lease that
> prevents other processes from changing the layout. i.e. the
> mechanism used to guarantee a lease is exclusive is that the layout
> becomes "unbreakable" at the filesystem level, but the policy we are
> actually presenting to uses is "exclusive access"...

That sounds good.

Ira

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com