Am 25.03.21 um 00:14 schrieb Jason Gunthorpe:
On Wed, Mar 24, 2021 at 09:07:53PM +0100, Thomas Hellström (Intel) wrote:
On 3/24/21 7:31 PM, Christian König wrote:
Am 24.03.21 um 17:38 schrieb Jason Gunthorpe:
On Wed, Mar 24, 2021 at 04:50:14PM +0100, Thomas Hellström (Intel)
wrote:
On 3/24/21 2:48 PM, Jason Gunthorpe wrote:
On Wed, Mar 24, 2021 at 02:35:38PM +0100, Thomas Hellström
(Intel) wrote:
In an ideal world the creation/destruction of page
table levels would
by dynamic at this point, like THP.
Hmm, but I'm not sure what problem we're trying to solve
by changing the
interface in this way?
We are trying to make a sensible driver API to deal with huge pages.
Currently if the core vm requests a huge pud, we give it
one, and if we
can't or don't want to (because of dirty-tracking, for
example, which is
always done on 4K page-level) we just return
VM_FAULT_FALLBACK, and the
fault is retried at a lower level.
Well, my thought would be to move the pte related stuff into
vmf_insert_range instead of recursing back via VM_FAULT_FALLBACK.
I don't know if the locking works out, but it feels cleaner that the
driver tells the vmf how big a page it can stuff in, not the vm
telling the driver to stuff in a certain size page which it might not
want to do.
Some devices want to work on a in-between page size like 64k so they
can't form 2M pages but they can stuff 64k of 4K pages in a batch on
every fault.
Hmm, yes, but we would in that case be limited anyway to insert ranges
smaller than and equal to the fault size to avoid extensive and
possibly
unnecessary checks for contigous memory.
Why? The insert function is walking the page tables, it just updates
things as they are. It learns the arragement for free while doing the
walk.
The device has to always provide consistent data, if it overlaps into
pages that are already populated that is fine so long as it isn't
changing their addresses.
And then if we can't support the full fault size, we'd need to
either presume a size and alignment of the next level or search for
contigous memory in both directions around the fault address,
perhaps unnecessarily as well.
You don't really need to care about levels, the device should be
faulting in the largest memory regions it can within its efficiency.
If it works on 4M pages then it should be faulting 4M pages. The page
size of the underlying CPU doesn't really matter much other than some
tuning to impact how the device's allocator works.
Yes, but then we'd be adding a lot of complexity into this function that is
already provided by the current interface for DAX, for little or no gain, at
least in the drm/ttm setting. Please think of the following situation: You
get a fault, you do an extensive time-consuming scan of your VRAM buffer
object into which the fault goes and determine you can fault 1GB. Now you
hand it to vmf_insert_range() and because the user-space address is
misaligned, or already partly populated because of a previous eviction, you
can only fault single pages, and you end up faulting a full GB of single
pages perhaps for a one-time small update.
Why would "you can only fault single pages" ever be true? If you have
1GB of pages then the vmf_insert_range should allocate enough page
table entries to consume it, regardless of alignment.
Completely agree with Jason. Filling in the CPU page tables is
relatively cheap if you fill in a large continuous range.
In other words filling in 1GiB of a linear range is *much* less overhead
than filling in 1<<18 4KiB faults.
I would say that this is always preferable even if the CPU only wants to
update a single byte.
And why shouldn't DAX switch to this kind of interface anyhow? It is
basically exactly the same problem. The underlying filesystem block
size is *not* necessarily aligned to the CPU page table sizes and DAX
would benefit from better handling of this mismatch.
On top of this, unless we want to do the walk trying increasingly smaller
sizes of vmf_insert_xxx(), we'd have to use apply_to_page_range() and teach
it about transhuge page table entries, because pagewalk.c can't be used (It
can't populate page tables). That also means apply_to_page_range() needs to
be complicated with page table locks since transhuge pages aren't stable and
can be zapped and refaulted under us while we do the walk.
I didn't say it would be simple :) But we also need to stop hacking
around the sides of all this huge page stuff and come up with sensible
APIs that drivers can actually implement correctly. Exposing drivers
to specific kinds of page levels really feels like the wrong level of
abstraction.
Once we start doing this we should do it everywhere, the io_remap_pfn
stuff should be able to create huge special IO pages as well, for
instance.
Oh, yes please!
We easily have 16GiB of VRAM which is linear mapped into the kernel
space for each GPU instance.
Doing that with 1GiB mapping instead of 4KiB would be quite a win.
Regards,
Christian.
On top of this, the user-space address allocator needs to know how large gpu
pages are aligned in buffer objects to have a reasonable chance of aligning
with CPU huge page boundaries which is a requirement to be able to insert a
huge CPU page table entry, so the driver would basically need the drm helper
that can do this alignment anyway.
Don't you have this problem anyhow?
Jason
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