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

[Alex Goins <agoins@nvidia.com>]

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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? 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.

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.

> 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(-)
> > 
> 
>