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(-)