Re: [PATCH 1/1] mm/migrate: Trylock device page in do_swap_page

[Felix Kuehling <felix.kuehling@xxxxxxx>]

On 2024-09-18 11:10, Alistair Popple wrote:
> Matthew Brost <matthew.brost@xxxxxxxxx> writes:
>
> > On Wed, Sep 11, 2024 at 02:53:31PM +1000, Alistair Popple wrote:
> > > Matthew Brost <matthew.brost@xxxxxxxxx> writes:
> > >
> > > I haven't seen the same in the NVIDIA UVM driver (out-of-tree, I know)
> > Still a driver.
> Indeed, and I'm happy to answer any questions about our implementation.
>
> > > but theoretically it seems like it should be possible. However we
> > > serialize migrations of the same virtual address range to avoid these
> > > kind of issues as they can happen the other way too (ie. multiple
> > > threads trying to migrate to GPU).
> > >
> > > So I suspect what happens in UVM is that one thread wins and installs
> > > the migration entry while the others fail to get the driver migration
> > > lock and bail out sufficiently early in the fault path to avoid the
> > > live-lock.
> > I had to try hard to show this, doubt an actual user could trigger this.
> >
> > I wrote a test which kicked 8 threads, each thread did a pthread join,
> > and then tried to read the same page. This repeats in loop for like 512
> > pages or something. I needed an exclusive lock in migrate_to_ram vfunc
> > for it to livelock. Without an exclusive lock I think on average I saw
> > about 32k retries (i.e. migrate_to_ram calls on the same page) before a
> > thread won this race.
> >
> >  From reading UVM, pretty sure if you tried hard enough you could trigger
> > a livelock given it appears you take excluvise locks in migrate_to_ram.
> Yes, I suspect you're correct here and that we just haven't tried hard
> enough to trigger it.
>
> > > > Cc: Philip Yang <Philip.Yang@xxxxxxx>
> > > > Cc: Felix Kuehling <felix.kuehling@xxxxxxx>
> > > > Cc: Christian König <christian.koenig@xxxxxxx>
> > > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > > Suggessted-by: Simona Vetter <simona.vetter@xxxxxxxx>
> > > > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > > > ---
> > > >   mm/memory.c         | 13 +++++++---
> > > >   mm/migrate_device.c | 60 +++++++++++++++++++++++++++++++--------------
> > > >   2 files changed, 50 insertions(+), 23 deletions(-)
> > > >
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 3c01d68065be..bbd97d16a96a 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -4046,10 +4046,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > > >   			 * Get a page reference while we know the page can't be
> > > >   			 * freed.
> > > >   			 */
> > > > -			get_page(vmf->page);
> > > > -			pte_unmap_unlock(vmf->pte, vmf->ptl);
> > > > -			ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
> > > > -			put_page(vmf->page);
> > > > +			if (trylock_page(vmf->page)) {
> > > > +				get_page(vmf->page);
> > > > +				pte_unmap_unlock(vmf->pte, vmf->ptl);
> > > This is all beginning to look a lot like migrate_vma_collect_pmd(). So
> > > rather than do this and then have to pass all this context
> > > (ie. fault_page) down to the migrate_vma_* functions could we instead
> > > just do what migrate_vma_collect_pmd() does here? Ie. we already have
> > > the PTL and the page lock so there's no reason we couldn't just setup
> > > the migration entry prior to calling migrate_to_ram().
> > >
> > > Obviously calling migrate_vma_setup() would show the page as not
> > > migrating, but drivers could easily just fill in the src_pfn info after
> > > calling migrate_vma_setup().
> > >
> > > This would eliminate the whole fault_page ugliness.
> > This seems like it would work and agree it likely be cleaner. Let me
> > play around with this and see what I come up with. Multi-tasking a bit
> > so expect a bit of delay here.
> >
> > Thanks for the input,
> > Matt

Thanks! Sorry, I'm late catching up after a vacation. Please keep 
Philip, Christian and myself in the loop with future patches in this area.

Regards,
  Felix


> > > > +				ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
> > > > +				put_page(vmf->page);
> > > > +				unlock_page(vmf->page);
> > > > +			} else {
> > > > +				pte_unmap_unlock(vmf->pte, vmf->ptl);
> > > > +			}
> > > >   		} else if (is_hwpoison_entry(entry)) {
> > > >   			ret = VM_FAULT_HWPOISON;
> > > >   		} else if (is_pte_marker_entry(entry)) {
> > > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> > > > index 6d66dc1c6ffa..049893a5a179 100644
> > > > --- a/mm/migrate_device.c
> > > > +++ b/mm/migrate_device.c
> > > > @@ -60,6 +60,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > > >   				   struct mm_walk *walk)
> > > >   {
> > > >   	struct migrate_vma *migrate = walk->private;
> > > > +	struct folio *fault_folio = migrate->fault_page ?
> > > > +		page_folio(migrate->fault_page) : NULL;
> > > >   	struct vm_area_struct *vma = walk->vma;
> > > >   	struct mm_struct *mm = vma->vm_mm;
> > > >   	unsigned long addr = start, unmapped = 0;
> > > > @@ -88,11 +90,13 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > > >   
> > > >   			folio_get(folio);
> > > >   			spin_unlock(ptl);
> > > > -			if (unlikely(!folio_trylock(folio)))
> > > > +			if (unlikely(fault_folio != folio &&
> > > > +				     !folio_trylock(folio)))
> > > >   				return migrate_vma_collect_skip(start, end,
> > > >   								walk);
> > > >   			ret = split_folio(folio);
> > > > -			folio_unlock(folio);
> > > > +			if (fault_folio != folio)
> > > > +				folio_unlock(folio);
> > > >   			folio_put(folio);
> > > >   			if (ret)
> > > >   				return migrate_vma_collect_skip(start, end,
> > > > @@ -192,7 +196,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > > >   		 * optimisation to avoid walking the rmap later with
> > > >   		 * try_to_migrate().
> > > >   		 */
> > > > -		if (folio_trylock(folio)) {
> > > > +		if (fault_folio == folio || folio_trylock(folio)) {
> > > >   			bool anon_exclusive;
> > > >   			pte_t swp_pte;
> > > >   
> > > > @@ -204,7 +208,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > > >   
> > > >   				if (folio_try_share_anon_rmap_pte(folio, page)) {
> > > >   					set_pte_at(mm, addr, ptep, pte);
> > > > -					folio_unlock(folio);
> > > > +					if (fault_folio != folio)
> > > > +						folio_unlock(folio);
> > > >   					folio_put(folio);
> > > >   					mpfn = 0;
> > > >   					goto next;
> > > > @@ -363,6 +368,8 @@ static unsigned long migrate_device_unmap(unsigned long *src_pfns,
> > > >   					  unsigned long npages,
> > > >   					  struct page *fault_page)
> > > >   {
> > > > +	struct folio *fault_folio = fault_page ?
> > > > +		page_folio(fault_page) : NULL;
> > > >   	unsigned long i, restore = 0;
> > > >   	bool allow_drain = true;
> > > >   	unsigned long unmapped = 0;
> > > > @@ -427,7 +434,8 @@ static unsigned long migrate_device_unmap(unsigned long *src_pfns,
> > > >   		remove_migration_ptes(folio, folio, false);
> > > >   
> > > >   		src_pfns[i] = 0;
> > > > -		folio_unlock(folio);
> > > > +		if (fault_folio != folio)
> > > > +			folio_unlock(folio);
> > > >   		folio_put(folio);
> > > >   		restore--;
> > > >   	}
> > > > @@ -536,6 +544,8 @@ int migrate_vma_setup(struct migrate_vma *args)
> > > >   		return -EINVAL;
> > > >   	if (args->fault_page && !is_device_private_page(args->fault_page))
> > > >   		return -EINVAL;
> > > > +	if (args->fault_page && !PageLocked(args->fault_page))
> > > > +		return -EINVAL;
> > > >   
> > > >   	memset(args->src, 0, sizeof(*args->src) * nr_pages);
> > > >   	args->cpages = 0;
> > > > @@ -799,19 +809,13 @@ void migrate_vma_pages(struct migrate_vma *migrate)
> > > >   }
> > > >   EXPORT_SYMBOL(migrate_vma_pages);
> > > >   
> > > > -/*
> > > > - * migrate_device_finalize() - complete page migration
> > > > - * @src_pfns: src_pfns returned from migrate_device_range()
> > > > - * @dst_pfns: array of pfns allocated by the driver to migrate memory to
> > > > - * @npages: number of pages in the range
> > > > - *
> > > > - * Completes migration of the page by removing special migration entries.
> > > > - * Drivers must ensure copying of page data is complete and visible to the CPU
> > > > - * before calling this.
> > > > - */
> > > > -void migrate_device_finalize(unsigned long *src_pfns,
> > > > -			unsigned long *dst_pfns, unsigned long npages)
> > > > +static void __migrate_device_finalize(unsigned long *src_pfns,
> > > > +				      unsigned long *dst_pfns,
> > > > +				      unsigned long npages,
> > > > +				      struct page *fault_page)
> > > >   {
> > > > +	struct folio *fault_folio = fault_page ?
> > > > +		page_folio(fault_page) : NULL;
> > > >   	unsigned long i;
> > > >   
> > > >   	for (i = 0; i < npages; i++) {
> > > > @@ -838,7 +842,8 @@ void migrate_device_finalize(unsigned long *src_pfns,
> > > >   		src = page_folio(page);
> > > >   		dst = page_folio(newpage);
> > > >   		remove_migration_ptes(src, dst, false);
> > > > -		folio_unlock(src);
> > > > +		if (fault_folio != src)
> > > > +			folio_unlock(src);
> > > >   
> > > >   		if (is_zone_device_page(page))
> > > >   			put_page(page);
> > > > @@ -854,6 +859,22 @@ void migrate_device_finalize(unsigned long *src_pfns,
> > > >   		}
> > > >   	}
> > > >   }
> > > > +
> > > > +/*
> > > > + * migrate_device_finalize() - complete page migration
> > > > + * @src_pfns: src_pfns returned from migrate_device_range()
> > > > + * @dst_pfns: array of pfns allocated by the driver to migrate memory to
> > > > + * @npages: number of pages in the range
> > > > + *
> > > > + * Completes migration of the page by removing special migration entries.
> > > > + * Drivers must ensure copying of page data is complete and visible to the CPU
> > > > + * before calling this.
> > > > + */
> > > > +void migrate_device_finalize(unsigned long *src_pfns,
> > > > +			unsigned long *dst_pfns, unsigned long npages)
> > > > +{
> > > > +	return __migrate_device_finalize(src_pfns, dst_pfns, npages, NULL);
> > > > +}
> > > >   EXPORT_SYMBOL(migrate_device_finalize);
> > > >   
> > > >   /**
> > > > @@ -869,7 +890,8 @@ EXPORT_SYMBOL(migrate_device_finalize);
> > > >    */
> > > >   void migrate_vma_finalize(struct migrate_vma *migrate)
> > > >   {
> > > > -	migrate_device_finalize(migrate->src, migrate->dst, migrate->npages);
> > > > +	__migrate_device_finalize(migrate->src, migrate->dst, migrate->npages,
> > > > +				  migrate->fault_page);
> > > >   }
> > > >   EXPORT_SYMBOL(migrate_vma_finalize);