On 3/23/21 4:45 PM, Christian König wrote:
Am 23.03.21 um 16:13 schrieb Michal Hocko:
On Tue 23-03-21 14:56:54, Christian König wrote:
Am 23.03.21 um 14:41 schrieb Michal Hocko:
[...]
Anyway, I am wondering whether the overall approach is sound. Why
don't
you simply use shmem as your backing storage from the beginning and
pin
those pages if they are used by the device?
Yeah, that is exactly what the Intel guys are doing for their
integrated
GPUs :)
Problem is for TTM I need to be able to handle dGPUs and those have all
kinds of funny allocation restrictions. In other words I need to
guarantee
that the allocated memory is coherent accessible to the GPU without
using
SWIOTLB.
The simple case is that the device can only do DMA32, but you also got
device which can only do 40bits or 48bits.
On top of that you also got AGP, CMA and stuff like CPU cache behavior
changes (write back vs. write through, vs. uncached).
OK, so the underlying problem seems to be that gfp mask (thus
mapping_gfp_mask) cannot really reflect your requirements, right? Would
it help if shmem would allow to provide an allocation callback to
override alloc_page_vma which is used currently? I am pretty sure there
will be more to handle but going through shmem for the whole life time
is just so much easier to reason about than some tricks to abuse shmem
just for the swapout path.
Well it's a start, but the pages can have special CPU cache settings.
So direct IO from/to them usually doesn't work as expected.
Additional to that for AGP and CMA I need to make sure that I give
those pages back to the relevant subsystems instead of just dropping
the page reference.
So I would need to block for the swapio to be completed.
Anyway I probably need to revert those patches for now since this
isn't working as we hoped it would.
Thanks for the explanation how stuff works here.
Another alternative here that I've tried before without being successful
would perhaps be to drop shmem completely and, if it's a normal page (no
dma or funny caching attributes) just use add_to_swap_cache()? If it's
something else, try alloc a page with relevant gfp attributes, copy and
add_to_swap_cache()? Or perhaps that doesn't work well from a shrinker
either?
/Thomas
Christian.
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