[PATCH RFC 0/1] drm/ttm: Allocate transparent huge pages without clearing __GFP_COMP

[Christian König]

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(-)
>>>