Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

[Jason Gunthorpe]

On Wed, Feb 06, 2019 at 03:30:27PM -0800, Dan Williams wrote:
> On Wed, Feb 6, 2019 at 3:21 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Wed, Feb 06, 2019 at 02:44:45PM -0800, Dan Williams wrote:
> >
> > > > Do they need to stick with xfs?
> > >
> > > Can you clarify the motivation for that question? This problem exists
> > > for any filesystem that implements an mmap that where the physical
> > > page backing the mapping is identical to the physical storage location
> > > for the file data.
> >
> > .. and needs to dynamicaly change that mapping. Which is not really
> > something inherent to the general idea of a filesystem. A file system
> > that had *strictly static* block assignments would work fine.
> >
> > Not all filesystem even implement hole punch.
> >
> > Not all filesystem implement reflink.
> >
> > ftruncate doesn't *have* to instantly return the free blocks to
> > allocation pool.
> >
> > ie this is not a DAX & RDMA issue but a XFS & RDMA issue.
> >
> > Replacing XFS is probably not be reasonable, but I wonder if a XFS--
> > operating mode could exist that had enough features removed to be
> > safe?
> 
> You're describing the current situation, i.e. Linux already implements
> this, it's called Device-DAX and some users of RDMA find it
> insufficient. The choices are to continue to tell them "no", or say
> "yes, but you need to submit to lease coordination".

Device-DAX is not what I'm imagining when I say XFS--.

I mean more like XFS with all features that require rellocation of
blocks disabled.

Forbidding hold punch, reflink, cow, etc, doesn't devolve back to
device-dax.

Jason