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

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Message-ID: <20250618061143.6470-1-lizhe.67@bytedance.com>
Date: Wed, 18 Jun 2025 14:11:43 +0800
From: lizhe.67@...edance.com
To: david@...hat.com
Cc: akpm@...ux-foundation.org,
	alex.williamson@...hat.com,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	lizhe.67@...edance.com,
	peterx@...hat.com
Subject: Re: [PATCH v4 3/3] vfio/type1: optimize vfio_unpin_pages_remote() for large folio

On Tue, 17 Jun 2025 15:47:09 +0200, david@...hat.com wrote:
 
> > 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.

Yes, the page_isolation case does not require the 'min' logic. Since
there are already places in the current kernel code where
folio_remaining_pages() is used without needing min, we could simply
create a custom wrapper function based on folio_remaining_pages() only
in those specific scenarios where min is necessary.

Following this line of thinking, the wrapper function in vfio would
look something like this.

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -801,16 +801,40 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
        return pinned;
 }
 
+static inline unsigned long vfio_folio_remaining_pages(
+               struct folio *folio, struct page *start_page,
+               unsigned long max_pages)
+{
+       if (!folio_test_large(folio))
+               return 1;
+       return min(max_pages,
+                  folio_remaining_pages(folio, start_page));
+}
+

---

Does this approach seem acceptable to you?

Thanks,
Zhe