On 12/4/19 3:40 PM, Christian König wrote:
Am 04.12.19 um 13:32 schrieb Thomas Hellström (VMware):
On 12/4/19 1:08 PM, Christian König wrote:
Am 04.12.19 um 12:36 schrieb Thomas Hellström (VMware):
On 12/4/19 12:11 PM, Christian König wrote:
Am 03.12.19 um 14:22 schrieb Thomas Hellström (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
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