Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

[Daniel Vetter]

On Thu, Mar 25, 2021 at 8:50 AM Christian König
<christian.koenig@xxxxxxx> wrote:
>
> 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.

io_remap_pfn is for userspace mmaps. Kernel mappings should be as big
as possible already I think for everything.
-Daniel


> 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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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch