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

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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
>
>> > +				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);
>> 






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