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

[Jason Gunthorpe]

On Thu, Jun 13, 2019 at 02:13:21PM -0700, Ira Weiny wrote:
> On Thu, Jun 13, 2019 at 08:27:55AM -0700, Matthew Wilcox wrote:
> > On Thu, Jun 13, 2019 at 10:25:55AM +1000, Dave Chinner wrote:
> > > e.g. Process A has an exclusive layout lease on file F. It does an
> > > IO to file F. The filesystem IO path checks that Process A owns the
> > > lease on the file and so skips straight through layout breaking
> > > because it owns the lease and is allowed to modify the layout. It
> > > then takes the inode metadata locks to allocate new space and write
> > > new data.
> > > 
> > > Process B now tries to write to file F. The FS checks whether
> > > Process B owns a layout lease on file F. It doesn't, so then it
> > > tries to break the layout lease so the IO can proceed. The layout
> > > breaking code sees that process A has an exclusive layout lease
> > > granted, and so returns -ETXTBSY to process B - it is not allowed to
> > > break the lease and so the IO fails with -ETXTBSY.
> > 
> > This description doesn't match the behaviour that RDMA wants either.
> > Even if Process A has a lease on the file, an IO from Process A which
> > results in blocks being freed from the file is going to result in the
> > RDMA device being able to write to blocks which are now freed (and
> > potentially reallocated to another file).
> 
> I don't understand why this would not work for RDMA?  As long as the layout
> does not change the page pins can remain in place.

Because process A had a layout lease (and presumably a MR) and the
layout was still modified in way that invalidates the RDMA MR.

Jason