Re: [PATCH 6/8] drm: Add a drm_get_unmapped_area() helper

[Thomas HellstrÃm (VMware)]

On 12/4/19 3:40 PM, Christian KÃnig wrote:
> Am 04.12.19 um 13:32 schrieb Thomas HellstrÃm (VMware):
> > On 12/4/19 1:08 PM, Christian KÃnig wrote:
> > > Am 04.12.19 um 12:36 schrieb Thomas HellstrÃm (VMware):
> > > > On 12/4/19 12:11 PM, Christian KÃnig wrote:
> > > > > Am 03.12.19 um 14:22 schrieb Thomas HellstrÃm (VMware):
> > > > > > From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> > > > > >
> > > > > > This helper is used to align user-space buffer object addresses to
> > > > > > huge page boundaries, minimizing the chance of alignment mismatch
> > > > > > between user-space addresses and physical addresses.
> > > > >
> > > > > Mhm, I'm wondering if that is really such a good idea.
> > > >
> > > > Could you elaborate? What drawbacks do you see?
> > >
> > > Main problem for me seems to be that I don't fully understand what 
> > > the get_unmapped_area callback is doing.
> >
> > It makes sure that, if there is a chance that we could use huge 
> > page-table entries, virtual address huge page boundaries are 
> > perfectly aligned to physical address huge page boundaries, which is 
> > if not a CPU hardware requirement, at least a kernel requirement 
> > currently.
> >
> >
> > > For example why do we need to use drm_vma_offset_lookup_locked() to 
> > > adjust the pgoff?
> > >
> > > The mapped offset should be completely irrelevant for finding some 
> > > piece of userspace address space or am I totally off here?
> >
> >
> > Because the unmodified pgoff assumes that physical address boundaries 
> > are perfectly aligned with file offset boundaries, which is typical 
> > for all other subsystems.
> >
> > That's not true for TTM, however, where a buffer object start 
> > physical address may be huge page aligned, but the file offset is 
> > always page aligned. We could of course change that to align also 
> > file offsets to huge page size boundaries, but with the above 
> > adjustment, that's not needed. I opted for the adjustment.
>
> I would opt for aligning the file offsets instead.

Yes but that adds additional complexity and considerations which made me 
think that lookup was the by far simplest choice:

- We need to modify the vma manager to care about alignments.
- Fragmentation issues. Do we want to align > 1G BOs
- For which drivers do we want to do this, how do we handle drivers that 
want to opt out in TTM mmap()?
- Non TTM drivers. Could they still reuse the same get_unmapped_area.

> Now that you explained it that the rest of the kernel enforces this 
> actually makes sense.

So is that an ack?


Thanks,

Thomas



> Regards,
> Christian.
>
> > Thanks,
> >
> > Thomas