[PATCH v16 0/4] RDMA: Add dma-buf support

[Daniel Vetter]

On Fri, Feb 05, 2021 at 11:43:19AM -0400, Jason Gunthorpe wrote:
> On Fri, Feb 05, 2021 at 04:39:47PM +0100, Daniel Vetter wrote:
> 
> > > And again, for slightly older hardware, without pinning to VRAM there is
> > > no way to use this solution here for peer-to-peer. So I'm glad to see that
> > > so far you're not ruling out the pinning option.
> > 
> > Since HMM and ZONE_DEVICE came up, I'm kinda tempted to make ZONE_DEVICE
> > ZONE_MOVEABLE (at least if you don't have a pinned vram contigent in your
> > cgroups) or something like that, so we could benefit from the work to make
> > sure pin_user_pages and all these never end up in there?
> 
> ZONE_DEVICE should already not be returned from GUP.
> 
> I've understood in the hmm casse the idea was a CPU touch of some
> ZONE_DEVICE pages would trigger a migration to CPU memory, GUP would
> want to follow the same logic, presumably it comes for free with the
> fault handler somehow

Oh I didn't know this, I thought the proposed p2p direct i/o patches would
just use the fact that underneath ZONE_DEVICE there's "normal" struct
pages. And so I got worried that maybe also pin_user_pages can creep in.
But I didn't read the patches in full detail:

https://lore.kernel.org/linux-block/20201106170036.18713-12-logang@deltatee.com/

But if you're saying that this all needs specific code and all the gup/pup
code we have is excluded, I think we can make sure that we're not ever
building features that requiring time-unlimited pinning of ZONE_DEVICE.
Which I think we want.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch