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Re: [PATCH v3 4/8] xen/arm: introduce put_page_nr and get_page_nr





On 12/05/2022 10:11, 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>
---
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             | 36 ++++++++++++++++++++++++++---------
  2 files changed, 31 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..a9461e07aa 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;
@@ -1548,10 +1549,10 @@ struct domain *page_get_owner_and_reference(struct 
page_info *page)
           * 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) <= 1) )

This check looks wrong to me. You want to make sure that the right equation return is at least equal to n otherwise.

Furthermore, I think we need to restrict 'nr' to PGC_count_mask to fully catch any overflow.

Before the loop, the code would look like:

/* Restrict nr to avoid "double" overflow */
if ( nr >= PGC_count_mask )
{
    ASSERT_UNREACHABLE();
    return NULL;
}

The check in the loop would look like:

if ( unlikely((x + nr) & PGC_count_mask) <= n )

That said, it might be easier to read the overflow check if we do:

count = x & PGC_count_mask;
if ( !count || ((PGC_count_mask - count) <= n) )

I haven't measured and check which of the two options would result to better code and performance (get_page() is often called).

              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 +1560,20 @@ 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) != 0) &&
+               (((y - nr) & PGC_count_mask) >= 0));

I think there are a potential underflow here if 'y' is smaller than 'nr'. But on v2, Stefano suggest to use ASSERT((y & PGC_count_mask) >= nr); which I think is sufficient here.

          x  = y;
-        nx = x - 1;
+        nx = x - nr;
      }
      while ( unlikely((y = cmpxchg(&page->count_info, x, nx)) != x) );
@@ -1576,19 +1583,30 @@ 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)
  {
-    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. */

Cheers,

--
Julien Grall



 


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