Re: [PATCH v3 2/2] vfio/type1: optimize vfio_unpin_pages_remote() for large folio

[David Hildenbrand <david@redhat.com>]

On 16.06.25 13:13, lizhe.67@bytedance.com wrote:
> On Mon, 16 Jun 2025 10:14:23 +0200, david@redhat.com wrote:
> 
>>>    drivers/vfio/vfio_iommu_type1.c | 55 +++++++++++++++++++++++++++------
>>>    1 file changed, 46 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>> index e952bf8bdfab..09ecc546ece8 100644
>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>> @@ -469,17 +469,28 @@ static bool is_invalid_reserved_pfn(unsigned long pfn)
>>>    	return true;
>>>    }
>>>    
>>> -static int put_pfn(unsigned long pfn, int prot)
>>> +static inline void _put_pfns(struct page *page, int npages, int prot)
>>>    {
>>> -	if (!is_invalid_reserved_pfn(pfn)) {
>>> -		struct page *page = pfn_to_page(pfn);
>>> +	unpin_user_page_range_dirty_lock(page, npages, prot & IOMMU_WRITE);
>>> +}
>>>    
>>> -		unpin_user_pages_dirty_lock(&page, 1, prot & IOMMU_WRITE);
>>> -		return 1;
>>> +/*
>>> + * The caller must ensure that these npages PFNs belong to the same folio.
>>> + */
>>> +static inline int put_pfns(unsigned long pfn, int npages, int prot)
>>> +{
>>> +	if (!is_invalid_reserved_pfn(pfn)) {
>>> +		_put_pfns(pfn_to_page(pfn), npages, prot);
>>> +		return npages;
>>>    	}
>>>    	return 0;
>>>    }
>>>    
>>> +static inline int put_pfn(unsigned long pfn, int prot)
>>> +{
>>> +	return put_pfns(pfn, 1, prot);
>>> +}
>>> +
>>>    #define VFIO_BATCH_MAX_CAPACITY (PAGE_SIZE / sizeof(struct page *))
>>>    
>>>    static void __vfio_batch_init(struct vfio_batch *batch, bool single)
>>> @@ -806,11 +817,37 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
>>>    				    bool do_accounting)
>>>    {
>>>    	long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
>>> -	long i;
>>>    
>>> -	for (i = 0; i < npage; i++)
>>> -		if (put_pfn(pfn++, dma->prot))
>>> -			unlocked++;
>>> +	while (npage) {
>>> +		long nr_pages = 1;
>>> +
>>> +		if (!is_invalid_reserved_pfn(pfn)) {
>>> +			struct page *page = pfn_to_page(pfn);
>>> +			struct folio *folio = page_folio(page);
>>> +			long folio_pages_num = folio_nr_pages(folio);
>>> +
>>> +			/*
>>> +			 * For a folio, it represents a physically
>>> +			 * contiguous set of bytes, and all of its pages
>>> +			 * share the same invalid/reserved state.
>>> +			 *
>>> +			 * Here, our PFNs are contiguous. Therefore, if we
>>> +			 * detect that the current PFN belongs to a large
>>> +			 * folio, we can batch the operations for the next
>>> +			 * nr_pages PFNs.
>>> +			 */
>>> +			if (folio_pages_num > 1)
>>> +				nr_pages = min_t(long, npage,
>>> +					folio_pages_num -
>>> +					folio_page_idx(folio, page));
>>> +
>>> +			_put_pfns(page, nr_pages, dma->prot);
>>
>>
>> This is sneaky. You interpret the page pointer a an actual page array,
>> assuming that it would give you the right values when advancing nr_pages
>> in that array.
>>
>> This is mostly true, but with !CONFIG_SPARSEMEM_VMEMMAP it is not
>> universally true for very large folios (e.g., in a 1 GiB hugetlb folio
>> when we cross the 128 MiB mark on x86).
>>
>> Not sure if that could already trigger here, but it is subtle.
> 
> As previously mentioned in the email, the code here functions
> correctly.
> 
>>> +			unlocked += nr_pages;
>>
>> We could do slightly better here, as we already have the folio. We would
>> add a unpin_user_folio_dirty_locked() similar to unpin_user_folio().
>>
>> Instead of _put_pfns, we would be calling
>>
>> unpin_user_folio_dirty_locked(folio, nr_pages, dma->prot & IOMMU_WRITE);
> 
> Thank you so much for your suggestion. Does this implementation of
> unpin_user_folio_dirty_locked() look viable to you?
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index fdda6b16263b..567c9dae9088 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1689,6 +1689,8 @@ void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
>   				      bool make_dirty);
>   void unpin_user_pages(struct page **pages, unsigned long npages);
>   void unpin_user_folio(struct folio *folio, unsigned long npages);
> +void unpin_user_folio_dirty_locked(struct folio *folio,
> +		unsigned long npages, bool make_dirty);
>   void unpin_folios(struct folio **folios, unsigned long nfolios);
>   
>   static inline bool is_cow_mapping(vm_flags_t flags)
> diff --git a/mm/gup.c b/mm/gup.c
> index 84461d384ae2..2f1e14a79463 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -360,11 +360,8 @@ void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
>   
>   	for (i = 0; i < npages; i += nr) {
>   		folio = gup_folio_range_next(page, npages, i, &nr);
> -		if (make_dirty && !folio_test_dirty(folio)) {
> -			folio_lock(folio);
> -			folio_mark_dirty(folio);
> -			folio_unlock(folio);
> -		}
> +		if (make_dirty && !folio_test_dirty(folio))
> +			folio_mark_dirty_lock(folio);
>   		gup_put_folio(folio, nr, FOLL_PIN);

We can call unpin_user_folio_dirty_locked(). :)

>   	}
>   }
> @@ -435,6 +432,26 @@ void unpin_user_folio(struct folio *folio, unsigned long npages)
>   }
>   EXPORT_SYMBOL(unpin_user_folio);
>   
> +/**
> + * unpin_user_folio_dirty_locked() - release pages of a folio and
> + * optionally dirty

"conditionally mark a folio dirty and unpin it"

Because that's the sequence in which it is done.

> + *
> + * @folio:  pointer to folio to be released
> + * @npages: number of pages of same folio

Can we change that to "nrefs" or rather "npins"?

> + * @make_dirty: whether to mark the folio dirty
> + *
> + * Mark the folio as being modified if @make_dirty is true. Then
> + * release npages of the folio.

Similarly, adjust the doc here.

> + */
> +void unpin_user_folio_dirty_locked(struct folio *folio,
> +		unsigned long npages, bool make_dirty)
> +{
> +	if (make_dirty && !folio_test_dirty(folio))
> +		folio_mark_dirty_lock(folio);
> +	gup_put_folio(folio, npages, FOLL_PIN);
> +}
> +EXPORT_SYMBOL(unpin_user_folio_dirty_locked);

Yes, should probably go into a separate cleanup patch.

-- 
Cheers,

David / dhildenb