Re: [PATCH v4 4/8] xen/arm: introduce put_page_nr and get_page_nr

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 17 May 2022, Penny Zheng wrote:
> Later, we need to add the right amount of references, which should be
> the number of borrower domains, to the owner domain. Since we only have
> get_page() to increment the page reference by 1, a loop is needed per
> page, which is inefficient and time-consuming.
> 
> To save the loop time, this commit introduces a set of new helpers
> put_page_nr() and get_page_nr() to increment/drop the page reference by nr.
> 
> Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> v4 changes:
> - make sure that the right equation return is at least equal to n
> - simplify the underflow
> ---
> v3 changes:
> - check overflow with "n"
> - remove spurious change
> - bring back the check that we enter the loop only when count_info is
> greater than 0
> ---
> v2 change:
> - new commit
> ---
>  xen/arch/arm/include/asm/mm.h |  4 ++++
>  xen/arch/arm/mm.c             | 42 +++++++++++++++++++++++++++--------
>  2 files changed, 37 insertions(+), 9 deletions(-)
> 
> diff --git a/xen/arch/arm/include/asm/mm.h b/xen/arch/arm/include/asm/mm.h
> index 424aaf2823..c737d51e4d 100644
> --- a/xen/arch/arm/include/asm/mm.h
> +++ b/xen/arch/arm/include/asm/mm.h
> @@ -347,6 +347,10 @@ void free_init_memory(void);
>  int guest_physmap_mark_populate_on_demand(struct domain *d, unsigned long 
> gfn,
>                                            unsigned int order);
>  
> +extern bool get_page_nr(struct page_info *page, const struct domain *domain,
> +                        unsigned long nr);
> +extern void put_page_nr(struct page_info *page, unsigned long nr);
> +
>  extern void put_page_type(struct page_info *page);
>  static inline void put_page_and_type(struct page_info *page)
>  {
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 7b1f2f4906..8c8a8f6378 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -1537,21 +1537,29 @@ long arch_memory_op(int op, 
> XEN_GUEST_HANDLE_PARAM(void) arg)
>      return 0;
>  }
>  
> -struct domain *page_get_owner_and_reference(struct page_info *page)
> +static struct domain *page_get_owner_and_nr_reference(struct page_info *page,
> +                                                      unsigned long nr)
>  {
>      unsigned long x, y = page->count_info;
>      struct domain *owner;
>  
> +    /* Restrict nr to avoid "double" overflow */
> +    if ( nr >= PGC_count_mask )
> +    {
> +        ASSERT_UNREACHABLE();
> +        return NULL;
> +    }
> +
>      do {
>          x = y;
>          /*
>           * Count ==  0: Page is not allocated, so we cannot take a reference.
>           * Count == -1: Reference count would wrap, which is invalid.
>           */
> -        if ( unlikely(((x + 1) & PGC_count_mask) <= 1) )
> +        if ( unlikely(((x + nr) & PGC_count_mask) <= nr) )
>              return NULL;
>      }
> -    while ( (y = cmpxchg(&page->count_info, x, x + 1)) != x );
> +    while ( (y = cmpxchg(&page->count_info, x, x + nr)) != x );
>  
>      owner = page_get_owner(page);
>      ASSERT(owner);
> @@ -1559,14 +1567,19 @@ struct domain *page_get_owner_and_reference(struct 
> page_info *page)
>      return owner;
>  }
>  
> -void put_page(struct page_info *page)
> +struct domain *page_get_owner_and_reference(struct page_info *page)
> +{
> +    return page_get_owner_and_nr_reference(page, 1);
> +}
> +
> +void put_page_nr(struct page_info *page, unsigned long nr)
>  {
>      unsigned long nx, x, y = page->count_info;
>  
>      do {
> -        ASSERT((y & PGC_count_mask) != 0);
> +        ASSERT((y & PGC_count_mask) >= nr);
>          x  = y;
> -        nx = x - 1;
> +        nx = x - nr;
>      }
>      while ( unlikely((y = cmpxchg(&page->count_info, x, nx)) != x) );
>  
> @@ -1576,19 +1589,30 @@ void put_page(struct page_info *page)
>      }
>  }
>  
> -bool get_page(struct page_info *page, const struct domain *domain)
> +void put_page(struct page_info *page)
> +{
> +    put_page_nr(page, 1);
> +}
> +
> +bool get_page_nr(struct page_info *page, const struct domain *domain,
> +                 unsigned long nr)
>  {
> -    const struct domain *owner = page_get_owner_and_reference(page);
> +    const struct domain *owner = page_get_owner_and_nr_reference(page, nr);
>  
>      if ( likely(owner == domain) )
>          return true;
>  
>      if ( owner != NULL )
> -        put_page(page);
> +        put_page_nr(page, nr);
>  
>      return false;
>  }
>  
> +bool get_page(struct page_info *page, const struct domain *domain)
> +{
> +    return get_page_nr(page, domain, 1);
> +}
> +
>  /* Common code requires get_page_type and put_page_type.
>   * We don't care about typecounts so we just do the minimum to make it
>   * happy. */
> -- 
> 2.25.1
>