[Xen-devel] [PATCH 6 of 6] x86/mm: Don't claim a slot on the paging ring if we might not need it

[Tim Deegan <tim@xxxxxxx>]

# HG changeset patch
# User Tim Deegan <tim@xxxxxxx>
# Date 1330013929 0
# Node ID 516f96172e67ff463c371357e0ab93724c8b9048
# Parent  510d80343793227bd39b9a5d4d3b61b9a3f776e4
x86/mm: Don't claim a slot on the paging ring if we might not need it.

This avoids the possibility of hitting a wait queue for a slot and then
finding out later that it wasn't necessary.

Signed-off-by: Tim Deegan <tim@xxxxxxx>

diff -r 510d80343793 -r 516f96172e67 xen/arch/x86/mm/p2m.c
--- a/xen/arch/x86/mm/p2m.c     Thu Feb 23 16:18:09 2012 +0000
+++ b/xen/arch/x86/mm/p2m.c     Thu Feb 23 16:18:49 2012 +0000
@@ -1001,19 +1001,23 @@ void p2m_mem_paging_populate(struct doma
     p2m_access_t a;
     mfn_t mfn;
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
-    int send_request = 0;
+    int rc, send_request = 0;
 
-    /* We're paging. There should be a ring */
-    int rc = mem_event_claim_slot(d, &d->mem_event->paging);
-    if ( rc == -ENOSYS )
+    if ( !d->mem_event->paging.ring_page )
     {
         gdprintk(XENLOG_ERR, "Domain %hu paging gfn %lx yet no ring "
                              "in place\n", d->domain_id, gfn);
         domain_crash(d);
         return;
     }
-    else if ( rc < 0 )
-        return;
+
+    /* Foreign users must grab a ring entry before doing any work since
+     * they might not be able to get one later.  Local users shouldn't
+     * grab a slot until they know they need it, to avoid going on a
+     * wait-queue unnecessarily. */
+    if ( d != current->domain )
+        if ( mem_event_claim_slot(d, &d->mem_event->paging) < 0 )
+            return;
 
     /* Fix p2m mapping */
     gfn_lock(p2m, gfn, 0);
@@ -1042,10 +1046,18 @@ void p2m_mem_paging_populate(struct doma
     if ( p2mt != p2m_ram_paging_out && p2mt != p2m_ram_paged )
     {
         /* gfn is already on its way back and vcpu is not paused */
-        mem_event_cancel_slot(d, &d->mem_event->paging);
+        if ( d != current->domain )
+            mem_event_cancel_slot(d, &d->mem_event->paging);
         return;
     }
 
+    /* Now local users should claim a slot. */
+    if ( d == current->domain ) 
+    {
+        rc = mem_event_claim_slot(d, &d->mem_event->paging);
+        ASSERT(rc == 0);
+    }
+
     /* Send request to pager */
     req.p2mt = p2mt;
     req.vcpu_id = v->vcpu_id;

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