Hi Alex,
adding Daniel as well.
Am 01.10.20 um 20:45 schrieb Alex Goins:
Hi Christian,
On Thu, 1 Oct 2020, Christian König wrote:
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.
We have some mapping requirements that I was hoping I could address by mapping
these pages manually.
Are you sure that it's illegal to access the underlying pages of an exported
DMA-BUF?
yes, I'm 100% sure of that. This was discussed multiple times now on the
mailing list.
There appears to be quite a few usages of this already. See the usage
of drm_prime_sg_to_page_addr_arrays() in vgem, vkms, msm, xen, and etnaviv.
drm_gem_prime_import_dev() uses driver->gem_prime_import_sg_table() when
importing a DMA-BUF from another driver, and the listed drivers then extract the
pages from the given SGT using drm_prime_sg_to_page_addr_arrays(). These pages
can then be mapped and faulted in.
No, exactly that doesn't work correctly.
You are corrupting internal state in struct page while doing so and risk
that userspace is accessing freed up memory.
We really need to find a way to fix the few drivers already doing this.
See commit af33a9190d02 ('drm/vgem: Enable dmabuf import interfaces'). After
importing the pages from the SGT, vgem can fault them in, taking a refcount with
get_page() first. get_page() throws a BUG if the refcount is zero, which it will
hit on each of the 'tail' pages from TTM THP allocations.
All of this currently works fine with TTM DMA-BUFs when the kernel is built with
!CONFIG_TRANSPARENT_HUGEPAGE. However, 'echo never >
/sys/kernel/mm/transparent_hugepage/enabled' doesn't change how TTM allocates
pages.
You need to redirect the mapping to dma_buf_mmap() instead.
Regards,
Christian.
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.
Understood, I figured something like that was the case, so I wanted to get your
input first. Do you know what the problems were, exactly? Practical issues
aside, it seems strange to call something a transparent huge page if it's
non-compound.
Besides making these pages compound, would it be reasonable to split them before
sharing them, in e.g. amdgpu_dma_buf_map (and in other drivers that use TTM)?
That's where it's supposed to make sure that the shared DMA-BUF is accessible by
the target device.
Thanks,
Alex
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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