[PATCH v2 02/29] mm/migrate: Add migrate_device_prepopulated_range
Matthew Brost
matthew.brost at intel.com
Fri Oct 18 07:57:38 UTC 2024
On Fri, Oct 18, 2024 at 06:34:13PM +1100, Alistair Popple wrote:
>
> Matthew Brost <matthew.brost at intel.com> writes:
>
> > On Fri, Oct 18, 2024 at 04:59:05PM +1100, Alistair Popple wrote:
> >>
> >> Matthew Brost <matthew.brost at intel.com> writes:
> >>
> >> > On Fri, Oct 18, 2024 at 08:58:02AM +1100, Alistair Popple wrote:
> >> >>
> >> >> Matthew Brost <matthew.brost at intel.com> writes:
> >> >>
> >> >> > On Thu, Oct 17, 2024 at 04:49:11PM +1100, Alistair Popple wrote:
> >> >> >>
> >> >> >> Matthew Brost <matthew.brost at intel.com> writes:
> >> >> >>
> >> >> >> > On Thu, Oct 17, 2024 at 02:21:13PM +1100, Alistair Popple wrote:
> >> >> >> >>
> >> >> >> >> Matthew Brost <matthew.brost at intel.com> writes:
> >> >> >> >>
> >> >> >> >> > On Thu, Oct 17, 2024 at 12:49:55PM +1100, Alistair Popple wrote:
> >> >> >> >> >>
> >> >> >> >> >> Matthew Brost <matthew.brost at intel.com> writes:
> >> >> >> >> >>
> >> >> >> >> >> > On Wed, Oct 16, 2024 at 04:46:52AM +0000, Matthew Brost wrote:
> >> >> >> >> >> >> On Wed, Oct 16, 2024 at 03:04:06PM +1100, Alistair Popple wrote:
> >> >> >> >> >>
> >> >> >> >> >> [...]
> >> >> >> >> >>
> >> >> >> >> >> >> > > +{
> >> >> >> >> >> >> > > + unsigned long i;
> >> >> >> >> >> >> > > +
> >> >> >> >> >> >> > > + for (i = 0; i < npages; i++) {
> >> >> >> >> >> >> > > + struct page *page = pfn_to_page(src_pfns[i]);
> >> >> >> >> >> >> > > +
> >> >> >> >> >> >> > > + if (!get_page_unless_zero(page)) {
> >> >> >> >> >> >> > > + src_pfns[i] = 0;
> >> >> >> >> >> >> > > + continue;
> >> >> >> >> >> >> > > + }
> >> >> >> >> >> >> > > +
> >> >> >> >> >> >> > > + if (!trylock_page(page)) {
> >> >> >> >> >> >> > > + src_pfns[i] = 0;
> >> >> >> >> >> >> > > + put_page(page);
> >> >> >> >> >> >> > > + continue;
> >> >> >> >> >> >> > > + }
> >> >> >> >> >> >> > > +
> >> >> >> >> >> >> > > + src_pfns[i] = migrate_pfn(src_pfns[i]) | MIGRATE_PFN_MIGRATE;
> >> >> >> >> >> >> >
> >> >> >> >> >> >> > This needs to be converted to use a folio like
> >> >> >> >> >> >> > migrate_device_range(). But more importantly this should be split out as
> >> >> >> >> >> >> > a function that both migrate_device_range() and this function can call
> >> >> >> >> >> >> > given this bit is identical.
> >> >> >> >> >> >> >
> >> >> >> >> >> >>
> >> >> >> >> >> >> Missed the folio conversion and agree a helper shared between this
> >> >> >> >> >> >> function and migrate_device_range would be a good idea. Let add that.
> >> >> >> >> >> >>
> >> >> >> >> >> >
> >> >> >> >> >> > Alistair,
> >> >> >> >> >> >
> >> >> >> >> >> > Ok, I think now I want to go slightly different direction here to give
> >> >> >> >> >> > GPUSVM a bit more control over several eviction scenarios.
> >> >> >> >> >> >
> >> >> >> >> >> > What if I exported the helper discussed above, e.g.,
> >> >> >> >> >> >
> >> >> >> >> >> > 905 unsigned long migrate_device_pfn_lock(unsigned long pfn)
> >> >> >> >> >> > 906 {
> >> >> >> >> >> > 907 struct folio *folio;
> >> >> >> >> >> > 908
> >> >> >> >> >> > 909 folio = folio_get_nontail_page(pfn_to_page(pfn));
> >> >> >> >> >> > 910 if (!folio)
> >> >> >> >> >> > 911 return 0;
> >> >> >> >> >> > 912
> >> >> >> >> >> > 913 if (!folio_trylock(folio)) {
> >> >> >> >> >> > 914 folio_put(folio);
> >> >> >> >> >> > 915 return 0;
> >> >> >> >> >> > 916 }
> >> >> >> >> >> > 917
> >> >> >> >> >> > 918 return migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
> >> >> >> >> >> > 919 }
> >> >> >> >> >> > 920 EXPORT_SYMBOL(migrate_device_pfn_lock);
> >> >> >> >> >> >
> >> >> >> >> >> > And then also export migrate_device_unmap.
> >> >> >> >> >> >
> >> >> >> >> >> > The usage here would be let a driver collect the device pages in virtual
> >> >> >> >> >> > address range via hmm_range_fault, lock device pages under notifier
> >> >> >> >> >> > lock ensuring device pages are valid, drop the notifier lock and call
> >> >> >> >> >> > migrate_device_unmap.
> >> >> >> >> >>
> >> >> >> >> >> I'm still working through this series but that seems a bit dubious, the
> >> >> >> >> >> locking here is pretty subtle and easy to get wrong so seeing some code
> >> >> >> >> >> would help me a lot in understanding what you're suggesting.
> >> >> >> >> >>
> >> >> >> >> >
> >> >> >> >> > For sure locking in tricky, my mistake on not working through this
> >> >> >> >> > before sending out the next rev but it came to mind after sending +
> >> >> >> >> > regarding some late feedback from Thomas about using hmm for eviction
> >> >> >> >> > [2]. His suggestion of using hmm_range_fault to trigger migration
> >> >> >> >> > doesn't work for coherent pages, while something like below does.
> >> >> >> >> >
> >> >> >> >> > [2] https://patchwork.freedesktop.org/patch/610957/?series=137870&rev=1#comment_1125461
> >> >> >> >> >
> >> >> >> >> > Here is a snippet I have locally which seems to work.
> >> >> >> >> >
> >> >> >> >> > 2024 retry:
> >> >> >> >> > 2025 hmm_range.notifier_seq = mmu_interval_read_begin(notifier);
> >> >> >> >> > 2026 hmm_range.hmm_pfns = src;
> >> >> >> >> > 2027
> >> >> >> >> > 2028 while (true) {
> >> >> >> >> > 2029 mmap_read_lock(mm);
> >> >> >> >> > 2030 err = hmm_range_fault(&hmm_range);
> >> >> >> >> > 2031 mmap_read_unlock(mm);
> >> >> >> >> > 2032 if (err == -EBUSY) {
> >> >> >> >> > 2033 if (time_after(jiffies, timeout))
> >> >> >> >> > 2034 break;
> >> >> >> >> > 2035
> >> >> >> >> > 2036 hmm_range.notifier_seq = mmu_interval_read_begin(notifier);
> >> >> >> >> > 2037 continue;
> >> >> >> >> > 2038 }
> >> >> >> >> > 2039 break;
> >> >> >> >> > 2040 }
> >> >> >> >> > 2041 if (err)
> >> >> >> >> > 2042 goto err_put;
> >> >> >> >> > 2043
> >> >> >> >> > 2044 drm_gpusvm_notifier_lock(gpusvm);
> >> >> >> >> > 2045 if (mmu_interval_read_retry(notifier, hmm_range.notifier_seq)) {
> >> >> >> >> > 2046 drm_gpusvm_notifier_unlock(gpusvm);
> >> >> >> >> > 2047 memset(src, 0, sizeof(*src) * npages);
> >> >> >> >> > 2048 goto retry;
> >> >> >> >> > 2049 }
> >> >> >> >> > 2050 for (i = 0; i < npages; ++i) {
> >> >> >> >> > 2051 struct page *page = hmm_pfn_to_page(src[i]);
> >> >> >> >> > 2052
> >> >> >> >> > 2053 if (page && (is_device_private_page(page) ||
> >> >> >> >> > 2054 is_device_coherent_page(page)) && page->zone_device_data)
> >> >> >> >> > 2055 src[i] = src[i] & ~HMM_PFN_FLAGS;
> >> >> >> >> > 2056 else
> >> >> >> >> > 2057 src[i] = 0;
> >> >> >> >> > 2058 if (src[i])
> >> >> >> >> > 2059 src[i] = migrate_device_pfn_lock(src[i]);
> >> >> >> >> > 2060 }
> >> >> >> >> > 2061 drm_gpusvm_notifier_unlock(gpusvm);
> >> >> >> >>
> >> >> >> >> Practically for eviction isn't this much the same as calling
> >> >> >> >> migrate_vma_setup()? And also for eviction as Sima mentioned you
> >> >> >> >> probably shouldn't be looking at mm/vma structs.
> >> >> >> >>
> >> >> >> >
> >> >> >> > hmm_range_fault is just collecting the pages, internally I suppose it
> >> >> >> > does look at a VMA (struct vm_area_struct) but I think the point is
> >> >> >> > drivers should not be looking at VMA here.
> >> >> >>
> >> >> >> migrate_vma_setup() is designed to be called by drivers and needs a vma,
> >> >> >> so in general I don't see a problem with drivers looking up vma's. The
> >> >> >> problem arises specifically for eviction and whether or not that happens
> >> >> >> in the driver or hmm_range_fault() is pretty irrelevant IMHO for the
> >> >> >> issues there (see below).
> >> >> >>
> >> >> >
> >> >> > Ok, if you think it ok for drivers to lookup the VMA then purposed
> >> >> > exporting of migrate_device_pfn_lock & migrate_device_unmap is not
> >> >> > needed, rather just the original function exported in the this patch.
> >> >> >
> >> >> > More below too.
> >> >> >
> >> >> >> >> > 2063 migrate_device_unmap(src, npages, NULL);
> >> >> >> >> > ...
> >> >> >> >> > 2101 migrate_device_pages(src, dst, npages);
> >> >> >> >> > 2102 migrate_device_finalize(src, dst, npages);
> >> >> >> >> >
> >> >> >> >> >
> >> >> >> >> >> > Sima has strongly suggested avoiding a CPUVMA
> >> >> >> >> >> > lookup during eviction cases and this would let me fixup
> >> >> >> >> >> > drm_gpusvm_range_evict in [1] to avoid this.
> >> >> >> >> >>
> >> >> >> >> >> That sounds reasonable but for context do you have a link to the
> >> >> >> >> >> comments/discussion on this? I couldn't readily find it, but I may have
> >> >> >> >> >> just missed it.
> >> >> >> >> >>
> >> >> >> >> >
> >> >> >> >> > See in [4], search for '2. eviction' comment from sima.
> >> >> >> >>
> >> >> >> >> Thanks for pointing that out. For reference here's Sima's comment:
> >> >> >> >>
> >> >> >> >> > 2. eviction
> >> >> >> >> >
> >> >> >> >> > Requirements much like migrate_to_ram, because otherwise we break the
> >> >> >> >> > migration gurantee:
> >> >> >> >> >
> >> >> >> >> > - Only looking at physical memory datastructures and locks, no looking at
> >> >> >> >> > mm/vma structs or relying on those being locked. We rely entirely on
> >> >> >> >> > reverse maps from try_to_migrate to find all the mappings on both cpu
> >> >> >> >> > and gpu side (cpu only zone device swap or migration pte entries ofc).
> >> >> >> >>
> >> >> >> >> I also very much agree with this. That's basically why I added
> >> >> >> >> migrate_device_range(), so that we can forcibly evict pages when the
> >> >> >> >> driver needs them freed (eg. driver unload, low memory, etc.). In
> >> >> >> >> general it is impossible to guarantee eviction og all pages using just
> >> >> >> >> hmm_range_fault().
> >> >> >> >>
> >> >> >> >
> >> >> >> > In this code path we don't have device pages available, hence the
> >> >> >> > purposed collection via hmm_range_fault.
> >> >> >>
> >> >> >> Why don't you have the pfns requiring eviction available? I need to read
> >> >> >> this series in more depth, but generally hmm_range_fault() can't
> >> >> >> gurantee you will find every device page.
> >> >> >>
> >> >> >
> >> >> > There are two cases for eviction in my series:
> >> >> >
> >> >> > 1. TTM decides it needs to move memory. This calls
> >> >> > drm_gpusvm_evict_to_ram. In this case the device pfns are available
> >> >> > directly from drm_gpusvm_devmem so the migrate_device_* calls be used
> >> >> > here albiet with the new function added in this patch as device pfns may
> >> >> > be non-contiguous.
> >> >>
> >> >> That makes sense and is generally what I think of when I'm thinking of
> >> >> eviction. The new function makes sense too - migrate_device_range() was
> >> >> primarily introduced to allow a driver to evict all device-private pages
> >> >> from a GPU so didn't consider non-contiguous cases, etc.
> >> >>
> >> >> > 2. An inconsistent state for VA range occurs (mixed system and device pages,
> >> >> > partial unmap of a range, etc...). Here we want to evict the range ram
> >> >> > to make the state consistent. No device pages are available due to an
> >> >> > intentional disconnect between a virtual range and physical
> >> >> > drm_gpusvm_devmem, thus the device pages have to be looked up. This the
> >> >> > function drm_gpusvm_range_evict. Based on what you tell me, it likely is
> >> >> > fine the way originally coded in v2 (vma lookup + migrate_vma_*) vs
> >> >> > using hmm_range_fault like I have suggested here.
> >> >>
> >> >> Thanks for the explanation. I think vma lookup + migrate_vma_setup() is
> >> >> fine for this usage and is exactly what you want - it was designed to
> >> >> either select all the system memory pages or device-private pages within
> >> >> a VA range and migrate them.
> >> >>
> >> >> FWIW I have toyed with the idea of a combined
> >> >> hmm_range_fault()/migrate_vma_setup() front-end to the rest of the
> >> >> migrate_vma_*() process but haven't come up with something nice as
> >> >> yet. I don't think mixing the two in an open-coded fashion is a good
> >> >> idea though, I'd rather we come up with a new API that addresses the
> >> >> short-comings of migrate_vma_setup().
> >> >>
> >> >
> >> > I think that would good. Here we actually need to lookup multiple VMAs
> >> > and have a sequence of migrate_vma_* calls as it possible for VMAs to
> >> > have changed after the driver range was created. It might be nice to
> >> > hide the VMA lookup from the drivers with an API saying collect and
> >> > migrate all pages of a type in a VA range much like hmm_range_fault. If
> >> > the range spans multiple VMAs that would be hidden from the caller.
> >>
> >> Ok. I wasn't really considering multiple VMAs. UVM and Nouveau don't
> >> really have a requirement to migrate across multiple VMAs but if that's
> >> neccessary I think an API that hides that specifically for working with
> >> migrate_vma_*() might make sense.
> >>
> >
> > We can run into multiple VMA scenarios if a user does something rude
> > like this:
>
> fork and mremap were the other "rude" things we've had fun with. They
> basically mean you can get references to device pages which a driver
> can't track with virtual addresses.
>
Yes, I've tested those two and are fun as well. But both are COW which
hasn't turned out to be too difficult.
> > mmap 0x000000...0x1fffff -> fault migrates 2M to VRAM and creates an internal range to track
> > munmap 0x080000...0x17ffff -> now we have two VMAs instead of one and the range has a hole in it
> >
> > In this scenario, which we believe to rare / unsual, we just evict
> > remaining VRAM pages to SRAM, destroy range, and fixup on next GPU
> > fault.
> >
> >> > Matt
> >> >
> >> >> > Note #2 may be removed or unnecessary at some point if we decide to add
> >> >> > support for ininconsistent state in GPU SVM and Xe. Keeping it simple for
> >> >> > now though. See 'Ranges with mixed system and device pages' in [5].
> >>
> >> As someone not very familiar with some of the DRM layers can I ask why
> >> having virtual address ranges with a mix of system and device pages is
> >> hard to support? It seems to me that in practice it might be quite
> >> difficult to keep a VMA range as exclusively all in system memory or all
> >> in device memory.
> >>
> >
> > A few things that make this difficult are:
> >
> > - Our (Xe) bind code would need to be updated to support this
> > - TTM / DRM buddy allocator doesn't support freeing / reallocation of
> > individual pages rather aligned chunks of initial allocation size
> > (e.g., 2M would be common allocation size).
> > - Spliting ranges would add complications
> >
> > All workable problems but since we are writing a new common
> > implementation trying to keep it as simple as possible for initial merge
> > of the design. Almost certainly at some point we will add support for
> > mixed ranges to the common GPU SVM layer with a driver choosing if it
> > wants mixed or non-mixed ranges via a flag to function calls.
> >
> > wrt to being difficult keeping exclusively in system or vram, in
> > addition to the above case the only other case I have found in which
> > this occurs is CPU and GPU faults to same address range racing. This can
> > cause hmm_range_fault to grab a set mixed pages. In this case again we
> > do an eviction remaining page and restart the GPU fault.
> >
> > I don't have real workloads yet but I do have a very aggressive test
> > case that intentionally does things which could break the design in a
> > highly parallel manner and the design as work. Is it ideal? Maybe not.
> > But getting in a simple design which we can build upon is the current
> > goal.
>
> Taking a simple approach first definitely sounds lie the right approach
> to me. I was just interested in the background because it wasn't
> something I'd run into (though we built on top of something quite
> different to the DRM layer). But I have often thought that the
> interfaces we have between core mm and GPU drivers is still a bit too
> low level at the moment and is calling out for a slightly higher level
> common implementation in the middle so am very interested to see where
> this all goes. Thanks.
The idea is build something common which other DRM drivers can use or
perhaps even pull out of the DRM layer eventually into a common device
layer for SVM.
Matt
>
> - Alistair
>
> > Matt
> >
> >> >> > [5] https://patchwork.freedesktop.org/patch/619819/?series=137870&rev=2
> >> >> >
> >> >> >> >> > [3] https://patchwork.freedesktop.org/patch/610957/?series=137870&rev=1#comment_1110726
> >> >> >> >> > [4] https://lore.kernel.org/all/BYAPR11MB3159A304925168D8B6B4671292692@BYAPR11MB3159.namprd11.prod.outlook.com/T/#m89cd6a37778ba5271d5381ebeb03e1f963856a78
> >> >> >> >> >
> >> >> >> >> >> > It would also make the function exported in this patch unnecessary too
> >> >> >> >> >> > as non-contiguous pfns can be setup on driver side via
> >> >> >> >> >> > migrate_device_pfn_lock and then migrate_device_unmap can be called.
> >> >> >> >> >> > This also another eviction usage in GPUSVM, see drm_gpusvm_evict_to_ram
> >> >> >> >> >> > in [1].
> >> >> >> >> >> >
> >> >> >> >> >> > Do you see an issue exporting migrate_device_pfn_lock,
> >> >> >> >> >> > migrate_device_unmap?
> >> >> >> >> >>
> >> >> >> >> >> If there is a good justification for it I can't see a problem with
> >> >> >> >> >> exporting it. That said I don't really understand why you would
> >> >> >> >> >> want/need to split those steps up but I'll wait to see the code.
> >> >> >> >> >>
> >> >> >> >> >
> >> >> >> >> > It is so the device pages returned from hmm_range_fault, which are only
> >> >> >> >> > guaranteed to be valid under the notifier lock + a seqno check, to be
> >> >> >> >> > locked and ref taken for migration. migrate_device_unmap() can trigger a
> >> >> >> >> > MMU invalidation which takes the notifier lock thus calling the function
> >> >> >> >> > which combines migrate_device_pfn_lock + migrate_device_unmap deadlocks.
> >> >> >> >> >
> >> >> >> >> > I think this flow makes sense and agree in general this likely better
> >> >> >> >> > than looking at a CPUVMA.
> >> >> >> >>
> >> >> >> >> I'm still a bit confused about what is better with this flow if you are
> >> >> >> >> still calling hmm_range_fault(). How is it better than just calling
> >> >> >> >> migrate_vma_setup()? Obviously it will fault the pages in, but it seems
> >> >> >> >
> >> >> >> > The code in rev2 calls migrate_vma_setup but the requires a struct
> >> >> >> > vm_area_struct argument whereas hmm_range_fault does not.
> >> >> >>
> >> >> >> I'm not sure that's a good enough justfication because the problem isn't
> >> >> >> whether you're looking up vma's in driver code or mm code. The problem
> >> >> >> is you are looking up vma's at all and all that goes with that (mainly
> >> >> >> taking mmap lock, etc.)
> >> >> >>
> >> >> >> And for eviction hmm_range_fault() won't even find all the pages because
> >> >> >> their virtual address may have changed - consider what happens in cases
> >> >> >> of mremap(), fork(), etc. So eviction really needs physical pages
> >> >> >> (pfn's), not virtual addresses.
> >> >> >>
> >> >> >
> >> >> > See above, #1 yes we use a physical pages. For #2 it is about making the
> >> >> > state consistent within a virtual address range.
> >> >>
> >> >> Yep, makes sense now. For migration of physical pages you want
> >> >> migrate_device_*, virtual address ranges want migrate_vma_*
> >> >>
> >> >> - Alistair
> >> >>
> >> >> > Matt
> >> >> >
> >> >> >> >> we're talking about eviction here so I don't understand why that would
> >> >> >> >> be relevant. And hmm_range_fault() still requires the VMA, although I
> >> >> >> >> need to look at the patches more closely, probably CPUVMA is a DRM
> >> >> >> >
> >> >> >> > 'hmm_range_fault() still requires the VMA' internal yes, but again not
> >> >> >> > as argument. This is about avoiding a driver side lookup of the VMA.
> >> >> >> >
> >> >> >> > CPUVMA == struct vm_area_struct in this email.
> >> >> >>
> >> >> >> Thanks for the clarification.
> >> >> >>
> >> >> >> - Alistair
> >> >> >>
> >> >> >> > Matt
> >> >> >> >
> >> >> >> >> specific concept?
> >> >> >> >>
> >> >> >> >> Thanks.
> >> >> >> >>
> >> >> >> >> - Alistair
> >> >> >> >>
> >> >> >> >> > Matt
> >> >> >> >> >
> >> >> >> >> >> - Alistair
> >> >> >> >> >>
> >> >> >> >> >> > Matt
> >> >> >> >> >> >
> >> >> >> >> >> > [1] https://patchwork.freedesktop.org/patch/619809/?series=137870&rev=2
> >> >> >> >> >> >
> >> >> >> >> >> >> Matt
> >> >> >> >> >> >>
> >> >> >> >> >> >> > > + }
> >> >> >> >> >> >> > > +
> >> >> >> >> >> >> > > + migrate_device_unmap(src_pfns, npages, NULL);
> >> >> >> >> >> >> > > +
> >> >> >> >> >> >> > > + return 0;
> >> >> >> >> >> >> > > +}
> >> >> >> >> >> >> > > +EXPORT_SYMBOL(migrate_device_prepopulated_range);
> >> >> >> >> >> >> > > +
> >> >> >> >> >> >> > > /*
> >> >> >> >> >> >> > > * Migrate a device coherent folio back to normal memory. The caller should have
> >> >> >> >> >> >> > > * a reference on folio which will be copied to the new folio if migration is
> >> >> >> >> >> >> >
> >> >> >> >> >>
> >> >> >> >>
> >> >> >>
> >> >>
> >>
>
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