RE: [PATCH v2 4/9] xen/arm: introduce put_page_nr and get_page_nr

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sat, 7 May 2022, Penny Zheng wrote:
> > On Fri, 6 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>
> > > ---
> > > v2 change:
> > > - new commit
> > > ---
> > >  xen/arch/arm/include/asm/mm.h |  4 ++++
> > >  xen/arch/arm/mm.c             | 36 +++++++++++++++++++++++++----------
> > >  2 files changed, 30 insertions(+), 10 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..e565979f3c
> > > 100644
> > > --- a/xen/arch/arm/mm.c
> > > +++ b/xen/arch/arm/mm.c
> > > @@ -1537,7 +1537,8 @@ 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;
> > > @@ -1545,13 +1546,14 @@ struct domain
> > *page_get_owner_and_reference(struct page_info *page)
> > >      do {
> > >          x = y;
> > >          /*
> > > +         * Consider the minimum case(nr = 1):
> > >           * 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) )
> > >              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);
> > > @@ -1559,36 +1561,50 @@ 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 - nr) & PGC_count_mask) >= 0);
> > 
> > Why this change? The original ASSERT is to check that we enter the loop only
> > when count_info is greater than 0. It should still apply even for 
> > put_page_nr
> > without modifications?
> > 
> > 
> 
> Oh, I understand wrongly about the ASSERT. I thought it was to
> check the result count_info after the loop will be not smaller than 0.
> 
> Do you think we still need to ensure that? Maybe ASSERT( ((y & 
> PGC_count_mask) != 0) && (((y - nr) & PGC_count_mask) >= 0)); ?

I think it should be:

ASSERT((y & PGC_count_mask) >= nr);