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

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Message-ID: <20190206234132.GB15234@ziepe.ca>
Date:   Wed, 6 Feb 2019 16:41:32 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Doug Ledford <dledford@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Ira Weiny <ira.weiny@...el.com>,
        lsf-pc@...ts.linux-foundation.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [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@...pe.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