Am 09.09.21 um 18:56 schrieb Matthew Auld:
On Thu, 9 Sept 2021 at 17:43, Koenig, Christian
<Christian.Koenig@xxxxxxx> wrote:
Hi Matthew,
this doesn't work, I've already tried something similar.
TTM uses the reverse lookup functionality when migrating BOs between system and device memory. And that doesn't seem to work with pages from a shmem file.
Hmm, what do you mean by reverse lookup functionality? Could you
please point out where that is in the TTM code?
When TTM moves a buffer it must make sure that the buffer is not
accessed by the CPU while moving it.
For this the standard reverse lockup functionality of the Linux kernel
is used to figure out in which page tables a page is mapped and mark
those as invalid. Accessing the buffer object will then cause a page
fault which in turn waits for the buffer move to finish.
But when you back the pages in a TT object with pages from a shmemfile
this reverse lockup functionality doesn't work for some reason. I
couldn't figure out what exactly was going wrong here and didn't looked
deeper, I assumed it's because of not setting up page->mapping and
page->index correctly. Thomas or Daniel might know more.
Apart from that your approach sounds like pretty much what I tried as well.
Regards,
Christian.
Regards,
Christian.
________________________________
Von: Matthew Auld <matthew.william.auld@xxxxxxxxx>
Gesendet: Donnerstag, 9. September 2021 16:56
An: Christian König <ckoenig.leichtzumerken@xxxxxxxxx>; Koenig, Christian <Christian.Koenig@xxxxxxx>
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>; ML dri-devel <dri-devel@xxxxxxxxxxxxxxxxxxxxx>
Betreff: i915 ttm_tt shmem backend
Hi Christian,
We are looking into using shmem as a ttm_tt backend in i915 for cached
system memory objects. We would also like to make such objects visible
to the i915-gem shrinker, so that they may be swapped out or discarded
when under memory pressure.
One idea for handling this is roughly something like:
- Add a new TTM_PAGE_FLAG_SHMEM flag, or similar.
- Skip the ttm_pages_allocated accounting on such objects, similar to
how FLAG_SG is already handled.
- Skip all the page->mapping and page->index related bits, like in
tt_add_mapping, since it looks like these are set and used by shmem.
Not sure what functionally this might break, but looks like it's maybe
only driver specific?
- Skip calling into ttm_bo_swap_out/in and just have
ttm_populate/unpopulate handle this directly for such objects.
- Make such objects visible to the i915-gem shrinker.
Does this approach look acceptable?
