RE: [Xen-devel] tmem and construct_dom0 memory allocation race

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Subject: Re: [Xen-devel] tmem and construct_dom0 memory allocation race
> 
> On 22/06/2010 08:17, "Dulloor" <dulloor@xxxxxxxxx> wrote:
> 
> > Hi Keir, You are right .. there is no race. I spent some time
> > debugging this. The problem is that a zero-order allocation (from
> > alloc_chunk, for the last dom0 page) fails with tmem on (in
> > alloc_heap_pages), even though there are pages available in the heap.
> > I don't think tmem really intends to get triggered so early. What do
> > you think ?
> 
> That's one for Dan to comment on.

Hmmm... the special casing in alloc_heap_pages to avoid fragmentation
need not be invoked if tmem doesn't hold any pages (as is the
case at dom0 boot)...

Does this patch fix the problem?  If so...

Signed-off-by: Dan Magenheimer

diff -r a24dbfcbdf69 xen/common/page_alloc.c
--- a/xen/common/page_alloc.c   Tue Jun 22 07:19:38 2010 +0100
+++ b/xen/common/page_alloc.c   Tue Jun 22 11:17:44 2010 -0600
@@ -316,11 +316,14 @@ static struct page_info *alloc_heap_page
     spin_lock(&heap_lock);
 
     /*
-     * TMEM: When available memory is scarce, allow only mid-size allocations
-     * to avoid worst of fragmentation issues. Others try TMEM pools then fail.
+     * TMEM: When available memory is scarce due to tmem absorbing it, allow
+     * only mid-size allocations to avoid worst of fragmentation issues.
+     * Others try tmem pools then fail.  This is a workaround until all
+     * post-dom0-creation-multi-page allocations can be eliminated.
      */
     if ( opt_tmem && ((order == 0) || (order >= 9)) &&
-         (total_avail_pages <= midsize_alloc_zone_pages) )
+         (total_avail_pages <= midsize_alloc_zone_pages) &&
+         tmem_freeable_pages() )
         goto try_tmem;
 
     /*
diff -r a24dbfcbdf69 xen/common/tmem.c
--- a/xen/common/tmem.c Tue Jun 22 07:19:38 2010 +0100
+++ b/xen/common/tmem.c Tue Jun 22 11:17:44 2010 -0600
@@ -2850,6 +2850,11 @@ EXPORT void *tmem_relinquish_pages(unsig
     return pfp;
 }
 
+EXPORT unsigned long tmem_freeable_pages(void)
+{
+    return tmh_freeable_pages();
+}
+
 /* called at hypervisor startup */
 static int __init init_tmem(void)
 {
diff -r a24dbfcbdf69 xen/include/xen/tmem.h
--- a/xen/include/xen/tmem.h    Tue Jun 22 07:19:38 2010 +0100
+++ b/xen/include/xen/tmem.h    Tue Jun 22 11:17:44 2010 -0600
@@ -11,6 +11,7 @@
 
 extern void tmem_destroy(void *);
 extern void *tmem_relinquish_pages(unsigned int, unsigned int);
+extern unsigned long tmem_freeable_pages(void);
 extern int  opt_tmem;
 
 #endif /* __XEN_TMEM_H__ */

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