[Xen-changelog] [xen master] x86/memshr: fix preemption in relinquish_shared_pages()

[patchbot@xxxxxxx]

commit a0070f7a5ad8652c74c685a0ee5f10215402279d
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Dec 17 16:39:39 2013 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Dec 17 16:39:39 2013 +0100

    x86/memshr: fix preemption in relinquish_shared_pages()
    
    For one, should hypercall_preempt_check() return false the first time
    it gets called, it would never have got called again (because count,
    being checked for equality, didn't get reset to zero).
    
    And then, if there were a huge range of unshared pages, with count not
    getting incremented at all in that case there would also not be any
    preemption.
    
    Fix this by using a biased increment (ratio 1:16 for unshared vs shared
    pages), and flushing the count to zero in case of a "false" return from
    hypercall_preempt_check().
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
---
 xen/arch/x86/mm/mem_sharing.c |   18 ++++++++++++------
 1 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index c94856b..4a5d9e8 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -1290,15 +1290,21 @@ int relinquish_shared_pages(struct domain *d)
             set_rc = p2m->set_entry(p2m, gfn, _mfn(0), PAGE_ORDER_4K,
                                     p2m_invalid, p2m_access_rwx);
             ASSERT(set_rc != 0);
-            count++;
+            count += 0x10;
         }
+        else
+            ++count;
 
-        /* Preempt every 2MiB. Arbitrary */
-        if ( (count == 512) && hypercall_preempt_check() )
+        /* Preempt every 2MiB (shared) or 32MiB (unshared) - arbitrary. */
+        if ( count >= 0x2000 )
         {
-            p2m->next_shared_gfn_to_relinquish = gfn + 1;
-            rc = -EAGAIN;
-            break;
+            if ( hypercall_preempt_check() )
+            {
+                p2m->next_shared_gfn_to_relinquish = gfn + 1;
+                rc = -EAGAIN;
+                break;
+            }
+            count = 0;
         }
     }
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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