[Xen-devel] [PATCH v6 2/4] mm: modify domain_adjust_tot_pages() to better handle a zero adjustment

[Paul Durrant <pdurrant@xxxxxxxxxx>]

Currently the function will pointlessly acquire and release the global
'heap_lock' in this case.

NOTE: No caller yet calls domain_adjust_tot_pages() with a zero 'pages'
      argument, but a subsequent patch will make this possible.

Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Julien Grall <julien@xxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>

v6:
 - Modify memory_exchange()

v5:
 - Split out from the subsequent 'make MEMF_no_refcount pages safe to
   assign' patch as requested by Jan
---
 xen/common/memory.c     | 3 +--
 xen/common/page_alloc.c | 3 +++
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/common/memory.c b/xen/common/memory.c
index c7d2bac452..a4a5374d26 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -727,8 +727,7 @@ static long 
memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
                              (j * (1UL << exch.out.extent_order)));
 
                 spin_lock(&d->page_alloc_lock);
-                drop_dom_ref = (dec_count &&
-                                !domain_adjust_tot_pages(d, -dec_count));
+                drop_dom_ref = !domain_adjust_tot_pages(d, -dec_count);
                 spin_unlock(&d->page_alloc_lock);
 
                 if ( drop_dom_ref )
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 919a270587..135e15bae0 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -460,6 +460,9 @@ unsigned long domain_adjust_tot_pages(struct domain *d, 
long pages)
 {
     long dom_before, dom_after, dom_claimed, sys_before, sys_after;
 
+    if ( !pages )
+        goto out;
+
     ASSERT(spin_is_locked(&d->page_alloc_lock));
     d->tot_pages += pages;
 
-- 
2.20.1


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