[PATCH v2 02/29] mm/migrate: Add migrate_device_prepopulated_range
Alistair Popple
apopple at nvidia.com
Fri Oct 18 05:59:05 UTC 2024
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.
> 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.
>> > [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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