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

[Alistair Popple <apopple@nvidia.com>]

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.

> +			} 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.

> +				     !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.

>  				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.

>  				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.

 - 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);