Re: [PATCH v2 03/29] mm/migrate: Trylock device page in do_swap_page

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

On Fri, Nov 29, 2024 at 10:31:32AM +1100, Alistair Popple wrote:
> 
> Matthew Brost <matthew.brost@intel.com> writes:
> 
> > Avoid multiple CPU page faults to the same device page racing by trying
> > to lock the page in do_swap_page before taking an extra 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 being locked in
> > do_swap_page, the migrate_vma_* functions need to be updated to avoid
> > locking the fault_page argument.
> >
> > 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.
> >
> > Cc: Philip Yang <Philip.Yang@amd.com>
> > Cc: Felix Kuehling <felix.kuehling@amd.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Suggessted-by: Simona Vetter <simona.vetter@ffwll.ch>
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >  mm/memory.c         | 13 ++++++---
> >  mm/migrate_device.c | 69 ++++++++++++++++++++++++++++++---------------
> >  2 files changed, 56 insertions(+), 26 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 2366578015ad..b72bde782611 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4252,10 +4252,15 @@ 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);
> > +				ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
> > +				put_page(vmf->page);
> > +				unlock_page(vmf->page);
> 
> Isn't the order wrong here? In the common case put_page() will have just
> dropped the last reference on the page and freed it so the unlock_page()
> needs to happen first.
> 

Yes, this appears wrong. I haven't seen this show up as a problem but
certainly a put should be done after dropping the lock. 

> > +			} 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 f163c2131022..2477d39f57be 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 &&
> 
> We don't currently support large ZONE_DEVICE pages so we should never
> get here. I think a WARN_ON_ONCE(fault_folio == folio) and bail would be
> better.
> 

Sure will fix.

> > +				     !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, 0);
> >  
> >  		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++) {
> > @@ -824,7 +828,8 @@ void migrate_device_finalize(unsigned long *src_pfns,
> >  
> >  		if (!page) {
> >  			if (dst) {
> > -				folio_unlock(dst);
> > +				if (fault_folio != dst)
> > +					folio_unlock(dst);
> 
> How could the destination page be the faulting page? I think we can drop
> this check.
> 

Yea, will drop.

> >  				folio_put(dst);
> >  			}
> >  			continue;
> > @@ -834,14 +839,16 @@ void migrate_device_finalize(unsigned long *src_pfns,
> >  
> >  		if (!(src_pfns[i] & MIGRATE_PFN_MIGRATE) || !dst) {
> >  			if (dst) {
> > -				folio_unlock(dst);
> > +				if (fault_folio != dst)
> > +					folio_unlock(dst);
> 
> Likewise.
> 

Same here.

> >  				folio_put(dst);
> >  			}
> >  			dst = src;
> >  		}
> >  
> >  		remove_migration_ptes(src, dst, 0);
> > -		folio_unlock(src);
> > +		if (fault_folio != src)
> > +			folio_unlock(src);
> >  
> >  		if (folio_is_zone_device(src))
> >  			folio_put(src);
> > @@ -849,7 +856,8 @@ void migrate_device_finalize(unsigned long *src_pfns,
> >  			folio_putback_lru(src);
> >  
> >  		if (dst != src) {
> > -			folio_unlock(dst);
> > +			if (fault_folio != dst)
> > +				folio_unlock(dst);
> 
> This one also seems unnecessary.
>

Same here.

Thanks - Matt
 
>  - Alistair
> 
> >  			if (folio_is_zone_device(dst))
> >  				folio_put(dst);
> >  			else
> > @@ -857,6 +865,22 @@ 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);
> >  
> >  /**
> > @@ -872,7 +896,8 @@ 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);
>