Hi Stefano
> -----Original Message-----
> From: Stefano Stabellini <sstabell...@kernel.org>
> Sent: Saturday, May 7, 2022 9:09 AM
> To: Penny Zheng <penny.zh...@arm.com>
> Cc: xen-devel@lists.xenproject.org; Wei Chen <wei.c...@arm.com>; Stefano
> Stabellini <sstabell...@kernel.org>; Julien Grall <jul...@xen.org>; Bertrand
> Marquis <bertrand.marq...@arm.com>; Volodymyr Babchuk
> <volodymyr_babc...@epam.com>
> Subject: Re: [PATCH v2 4/9] xen/arm: introduce put_page_nr and get_page_nr
>
> 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.zh...@arm.com>
> > ---
> > 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)); ?
>
> > x = y;
> > - nx = x - 1;
> > + nx = x - nr;
> > }
> > while ( unlikely((y = cmpxchg(&page->count_info, x, nx)) != x) );
> >
> > if ( unlikely((nx & PGC_count_mask) == 0) )
> > - {
> > free_domheap_page(page);
> > - }
> > }
> >
> > -bool get_page(struct page_info *page, const struct domain *domain)
> > +void put_page(struct page_info *page)
> > {
> > - const struct domain *owner = page_get_owner_and_reference(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_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
> >
