Re: [PATCH][v2.6.29][XEN] Return unused memory to hypervisor

[Miroslav Rezanina <mrezanin@redhat.com>]

----- Original Message -----
From: "Jeremy Fitzhardinge" <jeremy@goop.org>
To: "Miroslav Rezanina" <mrezanin@redhat.com>
Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xensource.com, "Gianluca Guida" <gianluca.guida@citrix.com>
Sent: Wednesday, August 19, 2009 6:16:33 PM GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna
Subject: Re: [PATCH][v2.6.29][XEN] Return unused memory to hypervisor

>>On 08/19/09 06:05, Miroslav Rezanina wrote:
>> when running linux as XEN guest and use boot parameter mem= to set
>> memory lower then is assigned to guest, not used memory should be 
>> returned to hypervisor as free. This is working with kernel 
>> available on xen.org pages, but is not working with kernel 2.6.29.
>> Comparing both kernels I found code for returning unused memory to 
>> hypervisor is missing. Following patch add this functionality to 
>>2.6.29 kernel.
>>  
>
> The idea is sound, but I think it might be better to walk the e820
> table, and remove any memory ranges which aren't marked as E820_RAM. 
> That makes it possible to carve holes in the address space as well as
> simply truncate it.

Hi Jeremy,
there is handled e820 map in guest. However, this patch informs 
hypervisor, that guest uses less memory than was assigned to it. 
If hypervisor is not informed, memory is reserved for guest that
do not need it. If hypervisor is informed, he decrease memory
reservation for guest and unused memory is marked as free 
for use by other guests.

Mirek

> Also, something appears to have smashed your indentation.
>
>    J

Oh, something goes wrong. Resending patch:

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 6a8811a..fd6b0e7 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -118,6 +118,10 @@ struct boot_params __initdata boot_params;
 struct boot_params boot_params;
 #endif
 
+#ifdef CONFIG_XEN
+void __init xen_return_unused_mem(void);
+#endif
+
 /*
  * Machine setup..
  */
@@ -920,6 +924,9 @@ void __init setup_arch(char **cmdline_p)
 	paging_init();
 	paravirt_pagetable_setup_done(swapper_pg_dir);
 	paravirt_post_allocator_init();
+#ifdef CONFIG_XEN
+	xen_return_unused_mem();
+#endif
 
 #ifdef CONFIG_X86_64
 	map_vsyscall();
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 15c6c68..bc5d2bc 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -20,6 +20,7 @@
 #include <xen/page.h>
 #include <xen/interface/callback.h>
 #include <xen/interface/physdev.h>
+#include <xen/interface/memory.h>
 #include <xen/features.h>
 
 #include "xen-ops.h"
@@ -34,6 +35,36 @@ extern void xen_syscall32_target(void);
 
 
 /**
+ * Author: Miroslav Rezanina <mrezanin@redhat.com>
+ * Function retuns unused memory to hypevisor
+ **/
+void __init xen_return_unused_mem(void)
+{
+	if (xen_start_info->nr_pages > max_pfn) {
+        /*
+	* the max_pfn was shrunk (probably by mem=
+	* kernel parameter); shrink reservation with the HV
+	*/
+	struct xen_memory_reservation reservation = {
+	.address_bits = 0,
+	.extent_order = 0,
+	.domid = DOMID_SELF
+	};
+	unsigned int difference;
+	int ret;
+
+	difference = xen_start_info->nr_pages - max_pfn;
+
+	set_xen_guest_handle(reservation.extent_start,
+		((unsigned long *)xen_start_info->mfn_list) + max_pfn);
+	reservation.nr_extents = difference;
+	ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation,
+		&reservation);
+	BUG_ON (ret != difference);
+	}
+}
+
+/**
  * machine_specific_memory_setup - Hook for machine specific memory setup.
  **/