Hi Alex,
first of all accessing the underlying page of an exported DMA-buf is
illegal! So I'm not 100% sure what you're intentions are here, please
explain further.
Then the reason for TTM not using compound pages is that we can't
guarantee that they are mapped as a whole to userspace.
The result is that the kernel sometimes tried to de-compound them which
created a bunch of problems.
So yes this is completely intentional.
Regards,
Christian.
Am 01.10.20 um 00:18 schrieb Alex Goins:
Hi Christian,
I've been looking into the DMA-BUFs exported from AMDGPU / TTM. Would
you mind giving some input on this?
I noticed that your changes implementing transparent huge page support
in TTM are allocating them as non-compound. I understand that using
multiorder non-compound pages is common in device drivers, but I think
this can cause a problem when these pages are exported to other drivers.
It's possible for other drivers to access the DMA-BUF's pages via
gem_prime_import_sg_table(), but without context from TTM, it's
impossible for the importing driver to make sense of them; they simply
appear as individual pages, with only the first page having a non-zero
refcount. Making TTM's THP allocations compound puts them more in line
with the standard definition of a THP, and allows DMA-BUF-importing
drivers to make sense of the pages within.
I would like to propose making these allocations compound, but based on
patch history, it looks like the decision to make them non-compound was
intentional, as there were difficulties figuring out how to map them
into CPU page tables. I did some cursory testing with compound THPs, and
nothing seems obviously broken. I was also able to map compound THP
DMA-BUFs into userspace without issue, and access their contents. Are
you aware of any other potential consequences?
Commit 5c42c64f7d54 ("drm/ttm: fix the fix for huge compound pages") should
probably also be reverted if this is applied.
Thanks,
Alex
Alex Goins (1):
drm-ttm: Allocate compound transparent huge pages
drivers/gpu/drm/ttm/ttm_page_alloc.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
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