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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH v4] [linux-3.0+ for xen] tmem: self-ballooning and frontswap-selfshrinking
Hi Konrad (and other reviewers) --
Here is version 4 of the Xen tmem selfballooning/frontswap-selfshrinking
patch. V4 includes the last round of syntactic feedback from Daniel Kiper.
I hope V4 is ready for merging as I'm hoping to get this merged for
the upcoming 3.1 window.
If it is satisfactory, you may wish to check the commit in my tree:
git://git.kernel.org/pub/scm/linux/kernel/git/djm/tmem.git
#xen-tmem-selfballoon-v4
If all looks good and there are no issues raised in the next
day or two, you may wish to pull this without a new git-pull
request, but I will send another one if you wish; please let
me know.
A reminder that this patch has a slight build dependency on a previous commit:
afec6e04922d0c8c7e244be2e544bac5e7e36294 xen: prepare tmem shim to handle
frontswap
in your devel/next-3.0 branch (because tmem_enabled is EXPORT_SYM'ed
in that commit and extern'ed in a file in this commit). I don't
think this is a problem as you've already pushed that commit
to linux-next, but am noting the dependency in case some kind
of commit reordering were to occur.
---
>From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: xen: tmem: self-ballooning and frontswap-selfshrinking
This patch introduces two in-kernel drivers for Xen transcendent memory
("tmem") functionality that complement cleancache and frontswap. Both
use control theory to dynamically adjust and optimize memory utilization.
Selfballooning controls the in-kernel Xen balloon driver, targeting a goal
value (vm_committed_as), thus pushing less frequently used clean
page cache pages (through the cleancache code) into Xen tmem where
Xen can balance needs across all VMs residing on the physical machine.
Frontswap-selfshrinking controls the number of pages in frontswap,
driving it towards zero (effectively doing a partial swapoff) when
in-kernel memory pressure subsides, freeing up RAM for other VMs.
More detail is provided in the header comment of xen-selfballooning.c.
Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
[v4: dkiper@xxxxxxxxxxxx: use strict_strtoul plus a few syntactic nits]
[v3: konrad.wilk@xxxxxxxxxx: fix potential divides-by-zero]
[v3: konrad.wilk@xxxxxxxxxx: add many more comments, fix nits]
[v2: rebased to linux-3.0-rc1]
[v2: Ian.Campbell@xxxxxxxxxx: reorganize as new file (xen-selfballoon.c)]
[v2: dkiper@xxxxxxxxxxxx: proper access to vm_committed_as]
[v2: dkiper@xxxxxxxxxxxx: accounting fixes]
Cc: Jan Beulich <JBeulich@xxxxxxxxxx>
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Cc: <xen-devel@xxxxxxxxxxxxxxxxxxx>
---
Diffstat:
drivers/xen/Kconfig | 12
drivers/xen/Makefile | 1
drivers/xen/xen-balloon.c | 2
drivers/xen/xen-selfballoon.c | 439 +++++++++++++++++++++++++++++++
include/xen/balloon.h | 10
include/xen/tmem.h | 5
6 files changed, 469 insertions(+)
diff -Napur linux-3.0-rc1-frontswap/drivers/xen/Kconfig
linux-3.0-rc1-frontswap-selfballoon/drivers/xen/Kconfig
--- linux-3.0-rc1-frontswap/drivers/xen/Kconfig 2011-05-31 17:09:07.606910896
-0600
+++ linux-3.0-rc1-frontswap-selfballoon/drivers/xen/Kconfig 2011-06-29
10:52:39.834899532 -0600
@@ -9,6 +9,18 @@ config XEN_BALLOON
the system to expand the domain's memory allocation, or alternatively
return unneeded memory to the system.
+config XEN_SELFBALLOONING
+ bool "dynamically self-balloon kernel memory to target"
+ depends on XEN && XEN_BALLOON && CLEANCACHE
+ default n
+ help
+ Self-ballooning dynamically balloons available kernel memory driven
+ by the current usage of anonymous memory ("committed AS") and
+ controlled by various sysfs-settable parameters. May be overridden
+ by the noselfballooning kernel boot parameter. If FRONTSWAP
+ is configured, also enables frontswap-selfshrinking, which can be
+ overridden by the noselfshrink kernel boot paramater.
+
config XEN_SCRUB_PAGES
bool "Scrub pages before returning them to system"
depends on XEN_BALLOON
diff -Napur -X linux-3.0-rc1/Documentation/dontdiff
linux-3.0-rc1-frontswap/drivers/xen/Makefile
linux-3.0-rc1-frontswap-selfballoon/drivers/xen/Makefile
--- linux-3.0-rc1-frontswap/drivers/xen/Makefile 2011-05-31
17:09:57.006875306 -0600
+++ linux-3.0-rc1-frontswap-selfballoon/drivers/xen/Makefile 2011-06-16
11:39:06.123852289 -0600
@@ -8,6 +8,7 @@ obj-$(CONFIG_BLOCK) += biomerge.o
obj-$(CONFIG_HOTPLUG_CPU) += cpu_hotplug.o
obj-$(CONFIG_XEN_XENCOMM) += xencomm.o
obj-$(CONFIG_XEN_BALLOON) += xen-balloon.o
+obj-$(CONFIG_XEN_SELFBALLOONING) += xen-selfballoon.o
obj-$(CONFIG_XEN_DEV_EVTCHN) += xen-evtchn.o
obj-$(CONFIG_XEN_GNTDEV) += xen-gntdev.o
obj-$(CONFIG_XEN_GRANT_DEV_ALLOC) += xen-gntalloc.o
diff -Napur -X linux-3.0-rc1/Documentation/dontdiff
linux-3.0-rc1-frontswap/drivers/xen/xen-balloon.c
linux-3.0-rc1-frontswap-selfballoon/drivers/xen/xen-balloon.c
--- linux-3.0-rc1-frontswap/drivers/xen/xen-balloon.c 2011-05-29
18:43:36.000000000 -0600
+++ linux-3.0-rc1-frontswap-selfballoon/drivers/xen/xen-balloon.c
2011-06-20 15:37:30.405798859 -0600
@@ -98,6 +98,8 @@ static int __init balloon_init(void)
register_balloon(&balloon_sysdev);
+ register_xen_selfballooning(&balloon_sysdev);
+
target_watch.callback = watch_target;
xenstore_notifier.notifier_call = balloon_init_watcher;
diff -Napur -X linux-3.0-rc1/Documentation/dontdiff
linux-3.0-rc1-frontswap/drivers/xen/xen-selfballoon.c
linux-3.0-rc1-frontswap-selfballoon/drivers/xen/xen-selfballoon.c
--- linux-3.0-rc1-frontswap/drivers/xen/xen-selfballoon.c 1969-12-31
17:00:00.000000000 -0700
+++ linux-3.0-rc1-frontswap-selfballoon/drivers/xen/xen-selfballoon.c
2011-07-05 08:30:20.262835552 -0600
@@ -0,0 +1,439 @@
+/******************************************************************************
+ * Xen selfballoon driver (and optional frontswap self-shrinking driver)
+ *
+ * Copyright (c) 2009-2011, Dan Magenheimer, Oracle Corp.
+ *
+ * This code complements the cleancache and frontswap patchsets to optimize
+ * support for Xen Transcendent Memory ("tmem"). The policy it implements
+ * is rudimentary and will likely improve over time, but it does work well
+ * enough today.
+ *
+ * Two functionalities are implemented here which both use "control theory"
+ * (feedback) to optimize memory utilization. In a virtualized environment
+ * such as Xen, RAM is often a scarce resource and we would like to ensure
+ * that each of a possibly large number of virtual machines is using RAM
+ * efficiently, i.e. using as little as possible when under light load
+ * and obtaining as much as possible when memory demands are high.
+ * Since RAM needs vary highly dynamically and sometimes dramatically,
+ * "hysteresis" is used, that is, memory target is determined not just
+ * on current data but also on past data stored in the system.
+ *
+ * "Selfballooning" creates memory pressure by managing the Xen balloon
+ * driver to decrease and increase available kernel memory, driven
+ * largely by the target value of "Committed_AS" (see /proc/meminfo).
+ * Since Committed_AS does not account for clean mapped pages (i.e. pages
+ * in RAM that are identical to pages on disk), selfballooning has the
+ * affect of pushing less frequently used clean pagecache pages out of
+ * kernel RAM and, presumably using cleancache, into Xen tmem where
+ * Xen can more efficiently optimize RAM utilization for such pages.
+ *
+ * When kernel memory demand unexpectedly increases faster than Xen, via
+ * the selfballoon driver, is able to (or chooses to) provide usable RAM,
+ * the kernel may invoke swapping. In most cases, frontswap is able
+ * to absorb this swapping into Xen tmem. However, due to the fact
+ * that the kernel swap subsystem assumes swapping occurs to a disk,
+ * swapped pages may sit on the disk for a very long time; even if
+ * the kernel knows the page will never be used again. This is because
+ * the disk space costs very little and can be overwritten when
+ * necessary. When such stale pages are in frontswap, however, they
+ * are taking up valuable real estate. "Frontswap selfshrinking" works
+ * to resolve this: When frontswap activity is otherwise stable
+ * and the guest kernel is not under memory pressure, the frontswap
+ * selfshrinking" accounts for this by providing pressure to remove some
+ * pages from frontswap and return them to kernel memory.
+ *
+ * For both "selfballooning" and "frontswap-selfshrinking", worker
+ * threads are used and sysfs tunables are provided to adjust the frequency
+ * and rate of adjustments to achieve the goal, as well as to disable one
+ * or both functions independently.
+ *
+ * While some argue that this functionality can and should be implemented
+ * in userspace, it has been observed that bad things happen (e.g. OOMs).
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/mman.h>
+
+#include <xen/balloon.h>
+
+#ifdef CONFIG_FRONTSWAP
+#include <linux/frontswap.h>
+#endif
+#include <xen/tmem.h>
+
+/* enable/disable with sysfs */
+static int xen_selfballooning_enabled __read_mostly;
+
+/* enable/disable with kernel boot option */
+static bool use_selfballooning __initdata = true;
+
+/*
+ * controls rate at which memory target (this iteration) approaches
+ * ultimate goal when memory need is increasing (up-hysteresis) or
+ * decreasing (down-hysteresis). higher values of hysteresis cause
+ * slower increases/decreases
+ */
+static unsigned int selfballoon_downhysteresis __read_mostly;
+static unsigned int selfballoon_uphysteresis __read_mostly;
+
+/* in HZ, controls frequency of worker invocation */
+static unsigned int selfballoon_interval __read_mostly;
+
+static void selfballoon_process(struct work_struct *work);
+static DECLARE_DELAYED_WORK(selfballoon_worker, selfballoon_process);
+
+#ifdef CONFIG_FRONTSWAP
+/* enable/disable with sysfs */
+static bool frontswap_selfshrinking __read_mostly;
+
+/* enable/disable with kernel boot option */
+static bool use_frontswap_selfshrink __initdata = true;
+
+/* control rate for frontswap shrinking. higher hysteresis is slower */
+static unsigned int frontswap_hysteresis __read_mostly;
+
+/*
+ * number of selfballoon worker invocations to wait before observing that
+ * frontswap selfshrinking should commence. (Note that selfshrinking does
+ * not use a separate worker thread.)
+ */
+static unsigned int frontswap_inertia __read_mostly;
+
+/* countdown to next invocation of frontswap_shrink() */
+static unsigned long frontswap_inertia_counter;
+
+/*
+ * Invoked by the selfballoon worker thread, uses current number of pages
+ * in frontswap (frontswap_curr_pages()), previous status, and control
+ * values (hysteresis and inertia) to determine if frontswap should be
+ * shrunk and what the new frontswap size should be. Note that
+ * frontswap_shrink is essentially a partial swapoff that immediately
+ * transfers pages from the "swap device" (frontswap) back into kernel
+ * RAM; despite the name, frontswap "shrinking" is very different from
+ * the "shrinker" interface used by the kernel MM subsystem to reclaim
+ * memory.
+ */
+static void frontswap_selfshrink(void)
+{
+ static unsigned long cur_frontswap_pages;
+ static unsigned long last_frontswap_pages;
+ static unsigned long tgt_frontswap_pages;
+
+ if (!frontswap_selfshrinking)
+ return;
+ last_frontswap_pages = cur_frontswap_pages;
+ cur_frontswap_pages = frontswap_curr_pages();
+ if (!cur_frontswap_pages ||
+ (cur_frontswap_pages > last_frontswap_pages)) {
+ frontswap_inertia_counter = frontswap_inertia;
+ return;
+ }
+ if (frontswap_inertia_counter && --frontswap_inertia_counter)
+ return;
+ if (cur_frontswap_pages <= frontswap_hysteresis)
+ tgt_frontswap_pages = 0;
+ else
+ tgt_frontswap_pages = cur_frontswap_pages -
+ (cur_frontswap_pages / frontswap_hysteresis);
+ frontswap_shrink(tgt_frontswap_pages);
+}
+
+static int __init no_frontswap_selfshrink_setup(char *s)
+{
+ use_frontswap_selfshrink = false;
+ return 1;
+}
+
+__setup("noselfshrink", no_frontswap_selfshrink_setup);
+#endif
+
+/*
+ * use current balloon size, the goal (vm_committed_as), and hysteresis
+ * parameters to set a new target balloon size
+ */
+static void selfballoon_process(struct work_struct *work)
+{
+ unsigned long cur_pages, goal_pages, tgt_pages;
+ bool reset_timer = false;
+
+ if (xen_selfballooning_enabled) {
+ cur_pages = balloon_stats.current_pages;
+ tgt_pages = cur_pages; /* default is no change */
+ goal_pages = percpu_counter_read_positive(&vm_committed_as) +
+ balloon_stats.current_pages - totalram_pages;
+ if (cur_pages > goal_pages)
+ tgt_pages = cur_pages -
+ ((cur_pages - goal_pages) /
+ selfballoon_downhysteresis);
+ else if (cur_pages < goal_pages)
+ tgt_pages = cur_pages +
+ ((goal_pages - cur_pages) /
+ selfballoon_uphysteresis);
+ /* else if cur_pages == goal_pages, no change */
+ balloon_set_new_target(tgt_pages);
+ reset_timer = true;
+ }
+#ifdef CONFIG_FRONTSWAP
+ if (frontswap_selfshrinking) {
+ frontswap_selfshrink();
+ reset_timer = true;
+ }
+#endif
+ if (reset_timer)
+ schedule_delayed_work(&selfballoon_worker,
+ selfballoon_interval * HZ);
+}
+
+#ifdef CONFIG_SYSFS
+
+#include <linux/sysdev.h>
+#include <linux/capability.h>
+
+#define SELFBALLOON_SHOW(name, format, args...)
\
+ static ssize_t show_##name(struct sys_device *dev, \
+ struct sysdev_attribute *attr, \
+ char *buf) \
+ { \
+ return sprintf(buf, format, ##args); \
+ }
+
+SELFBALLOON_SHOW(selfballooning, "%d\n", xen_selfballooning_enabled);
+
+static ssize_t store_selfballooning(struct sys_device *dev,
+ struct sysdev_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ bool was_enabled = xen_selfballooning_enabled;
+ unsigned long tmp;
+ int err;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+
+ err = strict_strtoul(buf, 10, &tmp);
+ if (err || ((tmp != 0) && (tmp != 1)))
+ return -EINVAL;
+
+ xen_selfballooning_enabled = !!tmp;
+ if (!was_enabled && xen_selfballooning_enabled)
+ schedule_delayed_work(&selfballoon_worker,
+ selfballoon_interval * HZ);
+
+ return count;
+}
+
+static SYSDEV_ATTR(selfballooning, S_IRUGO | S_IWUSR,
+ show_selfballooning, store_selfballooning);
+
+SELFBALLOON_SHOW(selfballoon_interval, "%d\n", selfballoon_interval);
+
+static ssize_t store_selfballoon_interval(struct sys_device *dev,
+ struct sysdev_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ unsigned long val;
+ int err;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ err = strict_strtoul(buf, 10, &val);
+ if (err || val == 0)
+ return -EINVAL;
+ selfballoon_interval = val;
+ return count;
+}
+
+static SYSDEV_ATTR(selfballoon_interval, S_IRUGO | S_IWUSR,
+ show_selfballoon_interval, store_selfballoon_interval);
+
+SELFBALLOON_SHOW(selfballoon_downhys, "%d\n", selfballoon_downhysteresis);
+
+static ssize_t store_selfballoon_downhys(struct sys_device *dev,
+ struct sysdev_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ unsigned long val;
+ int err;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ err = strict_strtoul(buf, 10, &val);
+ if (err || val == 0)
+ return -EINVAL;
+ selfballoon_downhysteresis = val;
+ return count;
+}
+
+static SYSDEV_ATTR(selfballoon_downhysteresis, S_IRUGO | S_IWUSR,
+ show_selfballoon_downhys, store_selfballoon_downhys);
+
+
+SELFBALLOON_SHOW(selfballoon_uphys, "%d\n", selfballoon_uphysteresis);
+
+static ssize_t store_selfballoon_uphys(struct sys_device *dev,
+ struct sysdev_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ unsigned long val;
+ int err;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ err = strict_strtoul(buf, 10, &val);
+ if (err || val == 0)
+ return -EINVAL;
+ selfballoon_uphysteresis = val;
+ return count;
+}
+
+static SYSDEV_ATTR(selfballoon_uphysteresis, S_IRUGO | S_IWUSR,
+ show_selfballoon_uphys, store_selfballoon_uphys);
+
+#ifdef CONFIG_FRONTSWAP
+SELFBALLOON_SHOW(frontswap_selfshrinking, "%d\n", frontswap_selfshrinking);
+
+static ssize_t store_frontswap_selfshrinking(struct sys_device *dev,
+ struct sysdev_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ bool was_enabled = frontswap_selfshrinking;
+ unsigned long tmp;
+ int err;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ err = strict_strtoul(buf, 10, &tmp);
+ if (err || ((tmp != 0) && (tmp != 1)))
+ return -EINVAL;
+ frontswap_selfshrinking = !!tmp;
+ if (!was_enabled && !xen_selfballooning_enabled &&
+ frontswap_selfshrinking)
+ schedule_delayed_work(&selfballoon_worker,
+ selfballoon_interval * HZ);
+
+ return count;
+}
+
+static SYSDEV_ATTR(frontswap_selfshrinking, S_IRUGO | S_IWUSR,
+ show_frontswap_selfshrinking, store_frontswap_selfshrinking);
+
+SELFBALLOON_SHOW(frontswap_inertia, "%d\n", frontswap_inertia);
+
+static ssize_t store_frontswap_inertia(struct sys_device *dev,
+ struct sysdev_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ unsigned long val;
+ int err;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ err = strict_strtoul(buf, 10, &val);
+ if (err || val == 0)
+ return -EINVAL;
+ frontswap_inertia = val;
+ frontswap_inertia_counter = val;
+ return count;
+}
+
+static SYSDEV_ATTR(frontswap_inertia, S_IRUGO | S_IWUSR,
+ show_frontswap_inertia, store_frontswap_inertia);
+
+SELFBALLOON_SHOW(frontswap_hysteresis, "%d\n", frontswap_hysteresis);
+
+static ssize_t store_frontswap_hysteresis(struct sys_device *dev,
+ struct sysdev_attribute *attr,
+ const char *buf,
+ size_t count)
+{
+ unsigned long val;
+ int err;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ err = strict_strtoul(buf, 10, &val);
+ if (err || val == 0)
+ return -EINVAL;
+ frontswap_hysteresis = val;
+ return count;
+}
+
+static SYSDEV_ATTR(frontswap_hysteresis, S_IRUGO | S_IWUSR,
+ show_frontswap_hysteresis, store_frontswap_hysteresis);
+
+#endif /* CONFIG_FRONTSWAP */
+
+static struct attribute *selfballoon_attrs[] = {
+ &attr_selfballooning.attr,
+ &attr_selfballoon_interval.attr,
+ &attr_selfballoon_downhysteresis.attr,
+ &attr_selfballoon_uphysteresis.attr,
+#ifdef CONFIG_FRONTSWAP
+ &attr_frontswap_selfshrinking.attr,
+ &attr_frontswap_hysteresis.attr,
+ &attr_frontswap_inertia.attr,
+#endif
+};
+
+static struct attribute_group selfballoon_group = {
+ .name = "selfballoon",
+ .attrs = selfballoon_attrs
+};
+#endif
+
+int register_xen_selfballooning(struct sys_device *sysdev)
+{
+ int error = -1;
+
+#ifdef CONFIG_SYSFS
+ error = sysfs_create_group(&sysdev->kobj, &selfballoon_group);
+#endif
+ return error;
+}
+EXPORT_SYMBOL(register_xen_selfballooning);
+
+static int __init noselfballooning_setup(char *s)
+{
+ use_selfballooning = false;
+ return 1;
+}
+
+__setup("noselfballooning", noselfballooning_setup);
+
+/*
+ * the default values for the various parameters were deemed reasonable
+ * by experimentation, may be workload-dependent, and can all be
+ * adjusted via sysfs
+ */
+static int __init xen_selfballoon_init(void)
+{
+ if (!xen_domain())
+ return -ENODEV;
+
+ pr_info("xen/balloon: Initializing Xen selfballooning driver.\n");
+ xen_selfballooning_enabled = tmem_enabled && use_selfballooning;
+ selfballoon_interval = 5;
+ selfballoon_downhysteresis = 8;
+ selfballoon_uphysteresis = 1;
+#ifdef CONFIG_FRONTSWAP
+ pr_info("xen/balloon: Initializing frontswap selfshrinking driver.\n");
+ frontswap_selfshrinking = use_frontswap_selfshrink && frontswap_enabled;
+ frontswap_hysteresis = 20;
+ frontswap_inertia = 3;
+#endif
+ schedule_delayed_work(&selfballoon_worker, selfballoon_interval * HZ);
+
+ return 0;
+}
+
+subsys_initcall(xen_selfballoon_init);
+
+MODULE_LICENSE("GPL");
diff -Napur -X linux-3.0-rc1/Documentation/dontdiff
linux-3.0-rc1-frontswap/include/xen/balloon.h
linux-3.0-rc1-frontswap-selfballoon/include/xen/balloon.h
--- linux-3.0-rc1-frontswap/include/xen/balloon.h 2011-05-29
18:43:36.000000000 -0600
+++ linux-3.0-rc1-frontswap-selfballoon/include/xen/balloon.h 2011-07-04
13:50:35.406866454 -0600
@@ -23,3 +23,13 @@ void balloon_set_new_target(unsigned lon
int alloc_xenballooned_pages(int nr_pages, struct page** pages);
void free_xenballooned_pages(int nr_pages, struct page** pages);
+
+struct sys_device;
+#ifdef CONFIG_XEN_SELFBALLOONING
+extern int register_xen_selfballooning(struct sys_device *sysdev);
+#else
+static inline int register_xen_selfballooning(struct sys_device *sysdev)
+{
+ return -ENOSYS;
+}
+#endif
diff -Napur -X linux-3.0-rc1/Documentation/dontdiff
linux-3.0-rc1-frontswap/include/xen/tmem.h
linux-3.0-rc1-frontswap-selfballoon/include/xen/tmem.h
--- linux-3.0-rc1-frontswap/include/xen/tmem.h 1969-12-31 17:00:00.000000000
-0700
+++ linux-3.0-rc1-frontswap-selfballoon/include/xen/tmem.h 2011-06-29
10:30:11.113929299 -0600
@@ -0,0 +1,5 @@
+#ifndef _XEN_TMEM_H
+#define _XEN_TMEM_H
+/* defined in drivers/xen/tmem.c */
+extern int tmem_enabled;
+#endif /* _XEN_TMEM_H */
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