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

[Dan Williams]

On Wed, Jun 12, 2019 at 3:29 AM Jan Kara <jack@xxxxxxx> wrote:
>
> On Fri 07-06-19 07:52:13, Ira Weiny wrote:
> > On Fri, Jun 07, 2019 at 09:17:29AM -0300, Jason Gunthorpe wrote:
> > > On Fri, Jun 07, 2019 at 12:36:36PM +0200, Jan Kara wrote:
> > >
> > > > Because the pins would be invisible to sysadmin from that point on.
> > >
> > > It is not invisible, it just shows up in a rdma specific kernel
> > > interface. You have to use rdma netlink to see the kernel object
> > > holding this pin.
> > >
> > > If this visibility is the main sticking point I suggest just enhancing
> > > the existing MR reporting to include the file info for current GUP
> > > pins and teaching lsof to collect information from there as well so it
> > > is easy to use.
> > >
> > > If the ownership of the lease transfers to the MR, and we report that
> > > ownership to userspace in a way lsof can find, then I think all the
> > > concerns that have been raised are met, right?
> >
> > I was contemplating some new lsof feature yesterday.  But what I don't
> > think we want is sysadmins to have multiple tools for multiple
> > subsystems.  Or even have to teach lsof something new for every potential
> > new subsystem user of GUP pins.
>
> Agreed.
>
> > I was thinking more along the lines of reporting files which have GUP
> > pins on them directly somewhere (dare I say procfs?) and teaching lsof to
> > report that information.  That would cover any subsystem which does a
> > longterm pin.
>
> So lsof already parses /proc/<pid>/maps to learn about files held open by
> memory mappings. It could parse some other file as well I guess. The good
> thing about that would be that then "longterm pin" structure would just hold
> struct file reference. That would avoid any needs of special behavior on
> file close (the file reference in the "longterm pin" structure would make
> sure struct file and thus the lease stays around, we'd just need to make
> explicit lease unlock block until the "longterm pin" structure is freed).
> The bad thing is that it requires us to come up with a sane new proc
> interface for reporting "longterm pins" and associated struct file. Also we
> need to define what this interface shows if the pinned pages are in DRAM
> (either page cache or anon) and not on NVDIMM.

The anon vs shared detection case is important because a longterm pin
might be blocking a memory-hot-unplug operation if it is pinning
ZONE_MOVABLE memory, but I don't think we want DRAM vs NVDIMM to be an
explicit concern of the interface. For the anon / cached case I expect
it might be useful to put that communication under the memory-blocks
sysfs interface. I.e. a list of pids that are pinning that
memory-block from being hot-unplugged.