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

[Matthew Brost]

On Wed, Sep 11, 2024 at 02:53:31PM +1000, Alistair Popple wrote:
> 
> Matthew Brost <matthew.brost@xxxxxxxxx> writes:
> 
> > Avoid multiple CPU page faults to the same device page racing by locking
> > the page in do_swap_page before taking an additional reference to the
> > page. This prevents scenarios where multiple CPU page faults each take
> > an extra reference to a device page, which could abort migration in
> > folio_migrate_mapping. With the device page locked in do_swap_page, the
> > migrate_vma_* functions need to be updated to avoid locking the
> > fault_page argument.
> 
> I added the get_page() and therefore the fault_page argument to deal
> with another lifetime issue. Neither Felix nor I particularly liked the
> solution though (see
> https://lore.kernel.org/all/cover.60659b549d8509ddecafad4f498ee7f03bb23c69.1664366292.git-series.apopple@xxxxxxxxxx/T/#m715589d57716448386ff9af41da27a952d94615a)
> and this seems to make it even uglier, so I have suggested an
> alternative solution below.
> 

Thanks for the ref, will read this through.

> > Prior to this change, a livelock scenario could occur in Xe's (Intel GPU
> > DRM driver) SVM implementation if enough threads faulted the same device
> > page.
> 
> I haven't seen the same in the NVIDIA UVM driver (out-of-tree, I know)

Still a driver.

> 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.

> > 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

> > +				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);
>