[PATCH 2/2] gpu/i915: use HMM mirror for userptr buffer object.

[Joonas Lahtinen]

Hi Jerome,

Bit late reply, but here goes :)

We're working quite hard to avoid pinning any pages unless they're in
the GPU page tables. And when they are in the GPU page tables, they must
be pinned for whole of that duration, for the reason that our GPUs can
not take a fault. And to avoid thrashing GPU page tables, we do leave
objects in page tables with the expectation that smart userspace
recycles buffers.

So what I understand of your proposal, it wouldn't really make a
difference for us in the amount of pinned pages (which I agree,
we'd love to see going down). When we're unable to take a fault,
the first use effectively forces us to pin any pages and keep them
pinned to avoid thrashing GPU page tables.

So from i915 perspective, it just seems to be mostly an exchange of
an API to an another for getting the pages. You already mentioned
the fast path is being worked on, which is an obvious difference.
But is there some other improvement one would be expecting, beyond
the page pinning?

Also, is the requirement for a single non-file-backed VMA in the
plans of being eliminated or is that inherent restriction of the
HMM_MIRROR feature? We're currently not imposing such a limitation.

Regards, Joonas

Quoting jglisse at redhat.com (2018-09-10 03:57:36)
> From: Jérôme Glisse <jglisse at redhat.com>
> 
> This replace existing code that rely on get_user_page() aka GUP with
> code that now use HMM mirror to mirror a range of virtual address as
> a buffer object accessible by the GPU. There is no functional changes
> from userspace point of view.
> 
> From kernel point of view we no longer pin pages for userptr buffer
> object which is a welcome change (i am assuming that everyone dislike
> page pin as i do).
> 
> Another change, from kernel point of view, is that it does no longer
> have a fast path with get_user_pages_fast() this can eventually added
> back through HMM.
> 
> Signed-off-by: Jérôme Glisse <jglisse at redhat.com>
> Cc: dri-devel at lists.freedesktop.org
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> Cc: intel-gfx at lists.freedesktop.org