linux-kernel - Re: [PATCH v4 3/3] vfio/type1: optimize vfio_unpin_pages

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Message-ID: <ce2af146-499b-4fce-8095-6c5471fdf288@redhat.com>
Date: Tue, 17 Jun 2025 15:47:09 +0200
From: David Hildenbrand <david@...hat.com>
To: lizhe.67@...edance.com
Cc: akpm@...ux-foundation.org, alex.williamson@...hat.com,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 peterx@...hat.com
Subject: Re: [PATCH v4 3/3] vfio/type1: optimize vfio_unpin_pages_remote() for
 large folio

On 17.06.25 14:42, lizhe.67@...edance.com wrote:
> On Tue, 17 Jun 2025 11:49:43 +0200, david@...hat.com wrote:
>   
>> On 17.06.25 11:47, lizhe.67@...edance.com wrote:
>>> On Tue, 17 Jun 2025 09:43:56 +0200, david@...hat.com wrote:
>>>    
>>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>>> index e952bf8bdfab..d7653f4c10d5 100644
>>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>>> @@ -801,16 +801,43 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>>>>>            return pinned;
>>>>>     }
>>>>>     
>>>>> +/* Returned number includes the provided current page. */
>>>>> +static inline unsigned long folio_remaining_pages(struct folio *folio,
>>>>> +               struct page *page, unsigned long max_pages)
>>>>> +{
>>>>> +       if (!folio_test_large(folio))
>>>>> +               return 1;
>>>>> +       return min_t(unsigned long, max_pages,
>>>>> +                    folio_nr_pages(folio) - folio_page_idx(folio, page));
>>>>> +}
>>>>
>>>> Note that I think that should go somewhere into mm.h, and also get used
>>>> by GUP. So factoring it out from GUP and then using it here.
>>>
>>> I think I need to separate this out into a distinct patch within the
>>> patchset. Is that correct?
>>
>> Yes, that's what I would do.
> 
> How do you think of this implementation?
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 242b05671502..eb91f99ea973 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2165,6 +2165,23 @@ static inline long folio_nr_pages(const struct folio *folio)
>          return folio_large_nr_pages(folio);
>   }
>   
> +/*
> + * folio_remaining_pages - Counts the number of pages from a given
> + * start page to the end of the folio.
> + *
> + * @folio: Pointer to folio
> + * @start_page: The starting page from which to begin counting.
> + *
> + * Returned number includes the provided start page.
> + *
> + * The caller must ensure that @start_page belongs to @folio.
> + */
> +static inline unsigned long folio_remaining_pages(struct folio *folio,
> +               struct page *start_page)
> +{
> +       return folio_nr_pages(folio) - folio_page_idx(folio, start_page);
> +}
> +
>   /* Only hugetlbfs can allocate folios larger than MAX_ORDER */
>   #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
>   #define MAX_FOLIO_NR_PAGES     (1UL << PUD_ORDER)
> diff --git a/mm/gup.c b/mm/gup.c
> index 15debead5f5b..14ae2e3088b4 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -242,7 +242,7 @@ static inline struct folio *gup_folio_range_next(struct page *start,
>   
>          if (folio_test_large(folio))
>                  nr = min_t(unsigned int, npages - i,
> -                          folio_nr_pages(folio) - folio_page_idx(folio, next));
> +                          folio_remaining_pages(folio, next));
>   
>          *ntails = nr;
>          return folio;
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index b2fc5266e3d2..34e85258060c 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -96,7 +96,7 @@ static struct page *has_unmovable_pages(unsigned long start_pfn, unsigned long e
>                                  return page;
>                          }
>   
> -                       skip_pages = folio_nr_pages(folio) - folio_page_idx(folio, page);
> +                       skip_pages = folio_remaining_pages(folio, page);
>                          pfn += skip_pages - 1;
>                          continue;
>                  }
> ---

Guess I would have pulled the "min" in there, but passing something like 
ULONG_MAX for the page_isolation case also looks rather ugly.

-- 
Cheers,

David / dhildenb