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

[Jan Kara <jack@suse.cz>]

On Mon 11-02-19 09:22:58, Dan Williams wrote:
> On Mon, Feb 11, 2019 at 2:24 AM Jan Kara <jack@suse.cz> wrote:
> >
> > On Fri 08-02-19 12:50:37, Dan Williams wrote:
> > > On Fri, Feb 8, 2019 at 3:11 AM Jan Kara <jack@suse.cz> wrote:
> > > >
> > > > On Fri 08-02-19 15:43:02, Dave Chinner wrote:
> > > > > On Thu, Feb 07, 2019 at 04:55:37PM +0000, Christopher Lameter wrote:
> > > > > > One approach that may be a clean way to solve this:
> > > > > > 3. Filesystems that allow bypass of the page cache (like XFS / DAX) will
> > > > > >    provide the virtual mapping when the PIN is done and DO NO OPERATIONS
> > > > > >    on the longterm pinned range until the long term pin is removed.
> > > > >
> > > > > So, ummm, how do we do block allocation then, which is done on
> > > > > demand during writes?
> > > > >
> > > > > IOWs, this requires the application to set up the file in the
> > > > > correct state for the filesystem to lock it down so somebody else
> > > > > can write to it.  That means the file can't be sparse, it can't be
> > > > > preallocated (i.e. can't contain unwritten extents), it must have zeroes
> > > > > written to it's full size before being shared because otherwise it
> > > > > exposes stale data to the remote client (secure sites are going to
> > > > > love that!), they can't be extended, etc.
> > > > >
> > > > > IOWs, once the file is prepped and leased out for RDMA, it becomes
> > > > > an immutable for the purposes of local access.
> > > > >
> > > > > Which, essentially we can already do. Prep the file, map it
> > > > > read/write, mark it immutable, then pin it via the longterm gup
> > > > > interface which can do the necessary checks.
> > > >
> > > > Hum, and what will you do if the immutable file that is target for RDMA
> > > > will be a source of reflink? That seems to be currently allowed for
> > > > immutable files but RDMA store would be effectively corrupting the data of
> > > > the target inode. But we could treat it similarly as swapfiles - those also
> > > > have to deal with writes to blocks beyond filesystem control. In fact the
> > > > similarity seems to be quite large there. What do you think?
> > >
> > > This sounds so familiar...
> > >
> > >     https://lwn.net/Articles/726481/
> > >
> > > I'm not opposed to trying again, but leases was what crawled out
> > > smoking crater when this last proposal was nuked.
> >
> > Umm, don't think this is that similar to daxctl() discussion. We are not
> > speaking about providing any new userspace API for this.
> 
> I thought explicit userspace API was one of the outcomes, i.e. that we
> can't depend on this behavior being an implicit side effect of a page
> pin?

I was thinking an implicit sideeffect of gup_longterm() call. Similarly as
swapon(2) does not require the file to be marked in any special way. But
OTOH I agree that RDMA is a less controlled usage than swapon so it is
questionable. I'd still require something like CAP_LINUX_IMMUTABLE at least
for gup_longterm() calls that end up pinning the file.

Inspired by Christoph's idea you reference in [2], maybe gup_longterm()
will succeed only if there is FL_LAYOUT lease for the range being pinned
and we don't allow the lease to be released until there's a pinned page in
the range. And we make the file protected (i.e. treat it like swapfile) if
there's any such lease in it. But this is just a rough sketch and needs more
thinking.

> > Also I think the
> > situation about leases has somewhat cleared up with this discussion - ODP
> > hardware does not need leases since it can use MMU notifiers, for non-ODP
> > hardware it is difficult to handle leases as such hardware has only one big
> > kill-everything call and using that would effectively mean lot of work on
> > the userspace side to resetup everything to make things useful if workable
> > at all.
> >
> > So my proposal would be:
> >
> > 1) ODP hardward uses gup_fast() like direct IO and uses MMU notifiers to do
> > its teardown when fs needs it.
> >
> > 2) Hardware not capable of tearing down pins from MMU notifiers will have
> > to use gup_longterm() (we may actually rename it to a more suitable name).
> > FS may just refuse such calls (for normal page cache backed file, it will
> > just return success but for DAX file it will do sanity checks whether the
> > file is fully allocated etc. like we currently do for swapfiles) but if
> > gup_longterm() returns success, it will provide the same guarantees as for
> > swapfiles. So the only thing that we need is some call from gup_longterm()
> > to a filesystem callback to tell it - this file is going to be used by a
> > third party as an IO buffer, don't touch it. And we can (and should)
> > probably refactor the handling to be shared between swapfiles and
> > gup_longterm().
> 
> Yes, lets pursue this. At the risk of "arguing past 'yes'" this is a
> solution I thought we dax folks walked away from in the original
> MAP_DIRECT discussion [1]. Here is where leases were the response to
> MAP_DIRECT [2]. ...and here is where we had tame discussions about
> implications of notifying memory-registrations of lease break events
> [3].

Yeah, thanks for the references.

> I honestly don't like the idea that random subsystems can pin down
> file blocks as a side effect of gup on the result of mmap. Recall that
> it's not just RDMA that wants this guarantee. It seems safer to have
> the file be in an explicit block-allocation-immutable-mode so that the
> fallocate man page can describe this error case. Otherwise how would
> you describe the scenarios under which FALLOC_FL_PUNCH_HOLE fails?

So with requiring lease for gup_longterm() to succeed (and the
FALLOC_FL_PUNCH_HOLE failure being keyed from the existence of such lease),
does it look more reasonable to you?

> [1]: https://lwn.net/Articles/736333/
> [2]: https://www.mail-archive.com/linux-nvdimm@lists.01.org/msg06437.html
> [3]: https://www.mail-archive.com/linux-nvdimm@lists.01.org/msg06499.html

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR