Re: [PATCH v2 02/29] mm/migrate: Add migrate_device_prepopulated_range

[Matthew Brost <matthew.brost@intel.com>]

On Thu, Oct 17, 2024 at 12:49:55PM +1100, Alistair Popple wrote:
> 
> Matthew Brost <matthew.brost@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);
2062
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.

[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.

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
> >> > 
>