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    SubjectRe: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA
    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@ziepe.ca> 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

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