[PATCH v11 0/6] xenbus/backend: Add a memory pressure handler callback

[PATCH v11 0/6] xenbus/backend: Add a memory pressure handler callback

[SeongJae Park]

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lack
flexibility.

To mitigate such problems, this patchset adds a memory reclaim callback
to 'xenbus_driver' (patch 1) and then introduce a lock for race
condition avoidance (patch 2).  Those two patches could be merged into
one patch if necessary.

The third patch applies the callback mechanism to mitigate the problem
in 'xen-blkback' (patch 3), but it lacks use of the race condition
mitigation.  Following change (patch 4) applies the race protection
mechanism to the blkback.  Patch 3 and patch 4 has seperated for only
review convenience.  Highly recommend to merge those into one patch as
patch 3 applied version might confuse bisecting.

The fifth and sixth patches are trivial cleanups; those fix nits we
found during the development of this patchset.

Note that patch 1, 3, 5, 6 are same with previous version.  I made the
changes in this version to different commits (only second and fourth
patches) to make review more comfortable.  Especially, the third and
fourth patches should be merged into one patch, as the third one alone
might make bisecting confuse.  Tthe next version of this patchset will
also merge those.


Base Version
------------

This patch is based on v5.4.  A complete tree is also available at my
public git repo:
https://github.com/sjp38/linux/tree/patches/blkback/buffer_squeeze/v11


Patch History
-------------

Changes from v10
(https://lore.kernel.org/xen-devel/20191216124527.30306-1-sjpark@amazon.com/)
 - Fix race condition (reported by SeongJae, suggested by Juergen)

Changes from v9
(https://lore.kernel.org/xen-devel/20191213153546.17425-1-sjpark@amazon.de/)
 - Add 'Reviewed-by' and 'Acked-by' from Roger Pau Monné
 - Update the commit message for overhead test of the 2nd path

Changes from v8
(https://lore.kernel.org/xen-devel/20191213130211.24011-1-sjpark@amazon.de/)
 - Drop 'Reviewed-by: Juergen' from the second patch
   (suggested by Roger Pau Monné)
 - Update contact of the new module param to SeongJae Park
   <sjpark@amazon.de>
   (suggested by Roger Pau Monné)
 - Wordsmith the description of the parameter
   (suggested by Roger Pau Monné)
 - Fix dumb bugs
   (suggested by Roger Pau Monné)
 - Move module param definition to xenbus.c and reduce the number of
   lines for this change
   (suggested by Roger Pau Monné)
 - Add a comment for the new callback, reclaim_memory, as other
   callbacks also have
 - Add another trivial cleanup of xenbus.c file (4th patch)

Changes from v7
(https://lore.kernel.org/xen-devel/20191211181016.14366-1-sjpark@amazon.de/)
 - Update sysfs-driver-xen-blkback for new parameter
   (suggested by Roger Pau Monné)
 - Use per-xen_blkif buffer_squeeze_end instead of global variable
   (suggested by Roger Pau Monné)

Changes from v6
(https://lore.kernel.org/linux-block/20191211042428.5961-1-sjpark@amazon.de/)
 - Remove more unnecessary prefixes (suggested by Roger Pau Monné)
 - Constify a variable (suggested by Roger Pau Monné)
 - Rename 'reclaim' into 'reclaim_memory' (suggested by Roger Pau Monné)
 - More wordsmith of the commit message (suggested by Roger Pau Monné)

Changes from v5
(https://lore.kernel.org/linux-block/20191210080628.5264-1-sjpark@amazon.de/)
 - Wordsmith the commit messages (suggested by Roger Pau Monné)
 - Change the reclaim callback return type (suggested by Roger Pau
   Monné)
 - Change the type of the blkback squeeze duration variable
   (suggested by Roger Pau Monné)
 - Add a patch for removal of unnecessary static variable name prefixes
   (suggested by Roger Pau Monné)
 - Fix checkpatch.pl warnings

Changes from v4
(https://lore.kernel.org/xen-devel/20191209194305.20828-1-sjpark@amazon.com/)
 - Remove domain id parameter from the callback (suggested by Juergen
   Gross)
 - Rename xen-blkback module parameter (suggested by Stefan Nuernburger)

Changes from v3
(https://lore.kernel.org/xen-devel/20191209085839.21215-1-sjpark@amazon.com/)
 - Add general callback in xen_driver and use it (suggested by Juergen
   Gross)

Changes from v2
(https://lore.kernel.org/linux-block/af195033-23d5-38ed-b73b-f6e2e3b34541@amazon.com)
 - Rename the module parameter and variables for brevity
   (aggressive shrinking -> squeezing)

Changes from v1
(https://lore.kernel.org/xen-devel/20191204113419.2298-1-sjpark@amazon.com/)
 - Adjust the description to not use the term, `arbitrarily`
   (suggested by Paul Durrant)
 - Specify time unit of the duration in the parameter description,
   (suggested by Maximilian Heyne)
 - Change default aggressive shrinking duration from 1ms to 10ms
 - Merge two patches into one single patch


SeongJae Park (6):
  xenbus/backend: Add memory pressure handler callback
  xenbus/backend: Protect xenbus callback with lock
  xen/blkback: Squeeze page pools if a memory pressure is detected
  xen/blkback: Protect 'reclaim_memory()' with 'reclaim_lock'
  xen/blkback: Remove unnecessary static variable name prefixes
  xen/blkback: Consistently insert one empty line between functions

 .../ABI/testing/sysfs-driver-xen-blkback      | 10 +++++
 drivers/block/xen-blkback/blkback.c           | 42 +++++++++----------
 drivers/block/xen-blkback/common.h            |  1 +
 drivers/block/xen-blkback/xenbus.c            | 37 +++++++++++++---
 drivers/xen/xenbus/xenbus_probe.c             |  1 +
 drivers/xen/xenbus/xenbus_probe_backend.c     | 38 +++++++++++++++++
 include/xen/xenbus.h                          |  3 ++
 7 files changed, 106 insertions(+), 26 deletions(-)

-- 
2.17.1

[PATCH v11 1/6] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lack
flexibility.

To mitigate such problems, this commit adds a memory reclaim callback to
'xenbus_driver'.  If a memory pressure is detected, 'xenbus' requests
every backend driver to volunarily release its memory.

Note that it would be able to improve the callback facility for more
sophisticated handlings of general pressures.  For example, it would be
possible to monitor the memory consumption of each device and issue the
release requests to only devices which causing the pressure.  Also, the
callback could be extended to handle not only memory, but general
resources.  Nevertheless, this version of the implementation defers such
sophisticated goals as a future work.

Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/xen/xenbus/xenbus_probe_backend.c | 32 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 2 files changed, 33 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index b0bed4faf44c..7e78ebef7c54 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -248,6 +248,35 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
 	return NOTIFY_DONE;
 }
 
+static int backend_reclaim_memory(struct device *dev, void *data)
+{
+	const struct xenbus_driver *drv;
+
+	if (!dev->driver)
+		return 0;
+	drv = to_xenbus_driver(dev->driver);
+	if (drv && drv->reclaim_memory)
+		drv->reclaim_memory(to_xenbus_device(dev));
+	return 0;
+}
+
+/*
+ * Returns 0 always because we are using shrinker to only detect memory
+ * pressure.
+ */
+static unsigned long backend_shrink_memory_count(struct shrinker *shrinker,
+				struct shrink_control *sc)
+{
+	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
+			backend_reclaim_memory);
+	return 0;
+}
+
+static struct shrinker backend_memory_shrinker = {
+	.count_objects = backend_shrink_memory_count,
+	.seeks = DEFAULT_SEEKS,
+};
+
 static int __init xenbus_probe_backend_init(void)
 {
 	static struct notifier_block xenstore_notifier = {
@@ -264,6 +293,9 @@ static int __init xenbus_probe_backend_init(void)
 
 	register_xenstore_notifier(&xenstore_notifier);
 
+	if (register_shrinker(&backend_memory_shrinker))
+		pr_warn("shrinker registration failed\n");
+
 	return 0;
 }
 subsys_initcall(xenbus_probe_backend_init);
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index 869c816d5f8c..c861cfb6f720 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -104,6 +104,7 @@ struct xenbus_driver {
 	struct device_driver driver;
 	int (*read_otherend_details)(struct xenbus_device *dev);
 	int (*is_ready)(struct xenbus_device *dev);
+	void (*reclaim_memory)(struct xenbus_device *dev);
 };
 
 static inline struct xenbus_driver *to_xenbus_driver(struct device_driver *drv)
-- 
2.17.1

[PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

'reclaim_memory' callback can race with a driver code as this callback
will be called from any memory pressure detected context.  To deal with
the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
'reclaim_memory' callback is called, the lock of the device which passed
to the callback as its argument is locked.  Thus, drivers registering
their 'reclaim_memory' callback should protect the data that might race
with the callback with the lock by themselves.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/xen/xenbus/xenbus_probe.c         |  1 +
 drivers/xen/xenbus/xenbus_probe_backend.c | 10 ++++++++--
 include/xen/xenbus.h                      |  2 ++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index 5b471889d723..b86393f172e6 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
 		goto fail;
 
 	dev_set_name(&xendev->dev, "%s", devname);
+	spin_lock_init(&xendev->reclaim_lock);
 
 	/* Register with generic device framework. */
 	err = device_register(&xendev->dev);
diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index 7e78ebef7c54..516aa64b9967 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -251,12 +251,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
 static int backend_reclaim_memory(struct device *dev, void *data)
 {
 	const struct xenbus_driver *drv;
+	struct xenbus_device *xdev;
+	unsigned long flags;
 
 	if (!dev->driver)
 		return 0;
 	drv = to_xenbus_driver(dev->driver);
-	if (drv && drv->reclaim_memory)
-		drv->reclaim_memory(to_xenbus_device(dev));
+	if (drv && drv->reclaim_memory) {
+		xdev = to_xenbus_device(dev);
+		spin_trylock_irqsave(&xdev->reclaim_lock, flags);
+		drv->reclaim_memory(xdev);
+		spin_unlock_irqrestore(&xdev->reclaim_lock, flags);
+	}
 	return 0;
 }
 
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index c861cfb6f720..d9468313061d 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -76,6 +76,8 @@ struct xenbus_device {
 	enum xenbus_state state;
 	struct completion down;
 	struct work_struct work;
+	/* 'reclaim_memory' callback is called while this lock is acquired */
+	spinlock_t reclaim_lock;
 };
 
 static inline struct xenbus_device *to_xenbus_device(struct device *dev)
-- 
2.17.1

[PATCH v11 3/6] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

Each `blkif` has a free pages pool for the grant mapping.  The size of
the pool starts from zero and is increased on demand while processing
the I/O requests.  If current I/O requests handling is finished or 100
milliseconds has passed since last I/O requests handling, it checks and
shrinks the pool to not exceed the size limit, `max_buffer_pages`.

Therefore, host administrators can cause memory pressure in blkback by
attaching a large number of block devices and inducing I/O.  Such
problematic situations can be avoided by limiting the maximum number of
devices that can be attached, but finding the optimal limit is not so
easy.  Improper set of the limit can results in memory pressure or a
resource underutilization.  This commit avoids such problematic
situations by squeezing the pools (returns every free page in the pool
to the system) for a while (users can set this duration via a module
parameter) if memory pressure is detected.

Discussions
===========

The `blkback`'s original shrinking mechanism returns only pages in the
pool which are not currently be used by `blkback` to the system.  In
other words, the pages that are not mapped with granted pages.  Because
this commit is changing only the shrink limit but still uses the same
freeing mechanism it does not touch pages which are currently mapping
grants.

Once memory pressure is detected, this commit keeps the squeezing limit
for a user-specified time duration.  The duration should be neither too
long nor too short.  If it is too long, the squeezing incurring overhead
can reduce the I/O performance.  If it is too short, `blkback` will not
free enough pages to reduce the memory pressure.  This commit sets the
value as `10 milliseconds` by default because it is a short time in
terms of I/O while it is a long time in terms of memory operations.
Also, as the original shrinking mechanism works for at least every 100
milliseconds, this could be a somewhat reasonable choice.  I also tested
other durations (refer to the below section for more details) and
confirmed that 10 milliseconds is the one that works best with the test.
That said, the proper duration depends on actual configurations and
workloads.  That's why this commit allows users to set the duration as a
module parameter.

Memory Pressure Test
====================

To show how this commit fixes the memory pressure situation well, I
configured a test environment on a xen-running virtualization system.
On the `blkfront` running guest instances, I attach a large number of
network-backed volume devices and induce I/O to those.  Meanwhile, I
measure the number of pages that swapped in (pswpin) and out (pswpout)
on the `blkback` running guest.  The test ran twice, once for the
`blkback` before this commit and once for that after this commit.  As
shown below, this commit has dramatically reduced the memory pressure:

                pswpin  pswpout
    before      76,672  185,799
    after          212    3,325

Optimal Aggressive Shrinking Duration
-------------------------------------

To find a best squeezing duration, I repeated the test with three
different durations (1ms, 10ms, and 100ms).  The results are as below:

    duration    pswpin  pswpout
    1           852     6,424
    10          212     3,325
    100         203     3,340

As expected, the memory pressure has decreased as the duration is
increased, but the reduction stopped from the `10ms`.  Based on this
results, I chose the default duration as 10ms.

Performance Overhead Test
=========================

This commit could incur I/O performance degradation under severe memory
pressure because the squeezing will require more page allocations per
I/O.  To show the overhead, I artificially made a worst-case squeezing
situation and measured the I/O performance of a `blkfront` running
guest.

For the artificial squeezing, I set the `blkback.max_buffer_pages` using
the `/sys/module/xen_blkback/parameters/max_buffer_pages` file.  In this
test, I set the value to `1024` and `0`.  The `1024` is the default
value.  Setting the value as `0` is same to a situation doing the
squeezing always (worst-case).

If the underlying block device is slow enough, the squeezing overhead
could be hidden.  For the reason, I use a fast block device, namely the
rbd[1]:

    # xl block-attach guest phy:/dev/ram0 xvdb w

For the I/O performance measurement, I run a simple `dd` command 5 times
directly to the device as below and collect the 'MB/s' results.

    $ for i in {1..5}; do dd if=/dev/zero of=/dev/xvdb \
                             bs=4k count=$((256*512)); sync; done

The results are as below.  'max_pgs' represents the value of the
`blkback.max_buffer_pages` parameter.

    max_pgs   Min       Max       Median     Avg    Stddev
    0         417       423       420        419.4  2.5099801
    1024      414       425       416        417.8  4.4384682
    No difference proven at 95.0% confidence

In short, even worst case squeezing on ramdisk based fast block device
makes no visible performance degradation.  Please note that this is just
a very simple and minimal test.  On systems using super-fast block
devices and a special I/O workload, the results might be different.  If
you have any doubt, test on your machine with your workload to find the
optimal squeezing duration for you.

[1] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 .../ABI/testing/sysfs-driver-xen-blkback      | 10 +++++++++
 drivers/block/xen-blkback/blkback.c           |  7 +++++--
 drivers/block/xen-blkback/common.h            |  1 +
 drivers/block/xen-blkback/xenbus.c            | 21 ++++++++++++++++++-
 4 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index 4e7babb3ba1f..f01224231f3f 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -25,3 +25,13 @@ Description:
                 allocated without being in use. The time is in
                 seconds, 0 means indefinitely long.
                 The default is 60 seconds.
+
+What:           /sys/module/xen_blkback/parameters/buffer_squeeze_duration_ms
+Date:           December 2019
+KernelVersion:  5.5
+Contact:        SeongJae Park <sjpark@amazon.de>
+Description:
+                When memory pressure is reported to blkback this option
+                controls the duration in milliseconds that blkback will not
+                cache any page not backed by a grant mapping.
+                The default is 10ms.
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index fd1e19f1a49f..79f677aeb5cc 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -656,8 +656,11 @@ int xen_blkif_schedule(void *arg)
 			ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL);
 		}
 
-		/* Shrink if we have more than xen_blkif_max_buffer_pages */
-		shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
+		/* Shrink the free pages pool if it is too large. */
+		if (time_before(jiffies, blkif->buffer_squeeze_end))
+			shrink_free_pagepool(ring, 0);
+		else
+			shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
 
 		if (log_stats && time_after(jiffies, ring->st_print))
 			print_stats(ring);
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 1d3002d773f7..536c84f61fed 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -319,6 +319,7 @@ struct xen_blkif {
 	/* All rings for this device. */
 	struct xen_blkif_ring	*rings;
 	unsigned int		nr_rings;
+	unsigned long		buffer_squeeze_end;
 };
 
 struct seg_buf {
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index b90dbcd99c03..4f6ea4feca79 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -824,6 +824,24 @@ static void frontend_changed(struct xenbus_device *dev,
 }
 
 
+/* Once a memory pressure is detected, squeeze free page pools for a while. */
+static unsigned int buffer_squeeze_duration_ms = 10;
+module_param_named(buffer_squeeze_duration_ms,
+		buffer_squeeze_duration_ms, int, 0644);
+MODULE_PARM_DESC(buffer_squeeze_duration_ms,
+"Duration in ms to squeeze pages buffer when a memory pressure is detected");
+
+/*
+ * Callback received when the memory pressure is detected.
+ */
+static void reclaim_memory(struct xenbus_device *dev)
+{
+	struct backend_info *be = dev_get_drvdata(&dev->dev);
+
+	be->blkif->buffer_squeeze_end = jiffies +
+		msecs_to_jiffies(buffer_squeeze_duration_ms);
+}
+
 /* ** Connection ** */
 
 
@@ -1115,7 +1133,8 @@ static struct xenbus_driver xen_blkbk_driver = {
 	.ids  = xen_blkbk_ids,
 	.probe = xen_blkbk_probe,
 	.remove = xen_blkbk_remove,
-	.otherend_changed = frontend_changed
+	.otherend_changed = frontend_changed,
+	.reclaim_memory = reclaim_memory,
 };
 
 int xen_blkif_xenbus_init(void)
-- 
2.17.1

[PATCH v11 4/6] xen/blkback: Protect 'reclaim_memory()' with 'reclaim_lock'

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

The 'reclaim_memory()' callback of blkback could race with
'xen_blkbk_probe()' and 'xen_blkbk_remove()'.  In the case, incompletely
linked 'backend_info' and 'blkif' might be exposed to the callback, thus
result in bad results including NULL dereference.  This commit fixes the
problem by applying the 'reclaim_lock' protection to those.

Note that this commit is separated for review purpose only.  As the
previous commit might result in race condition and might make bisect
confuse, please squash this commit into previous commit if possible.

Signed-off-by: SeongJae Park <sjpark@amazon.de>

---
 drivers/block/xen-blkback/xenbus.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 4f6ea4feca79..20045827a391 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -492,6 +492,7 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 static int xen_blkbk_remove(struct xenbus_device *dev)
 {
 	struct backend_info *be = dev_get_drvdata(&dev->dev);
+	unsigned long flags;
 
 	pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
 
@@ -504,6 +505,7 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
 		be->backend_watch.node = NULL;
 	}
 
+	spin_lock_irqsave(&dev->reclaim_lock, flags);
 	dev_set_drvdata(&dev->dev, NULL);
 
 	if (be->blkif) {
@@ -512,6 +514,7 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
 		/* Put the reference we set in xen_blkif_alloc(). */
 		xen_blkif_put(be->blkif);
 	}
+	spin_unlock_irqrestore(&dev->reclaim_lock, flags);
 
 	return 0;
 }
@@ -597,6 +600,7 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 	int err;
 	struct backend_info *be = kzalloc(sizeof(struct backend_info),
 					  GFP_KERNEL);
+	unsigned long flags;
 
 	/* match the pr_debug in xen_blkbk_remove */
 	pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
@@ -607,6 +611,7 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 		return -ENOMEM;
 	}
 	be->dev = dev;
+	spin_lock_irqsave(&dev->reclaim_lock, flags);
 	dev_set_drvdata(&dev->dev, be);
 
 	be->blkif = xen_blkif_alloc(dev->otherend_id);
@@ -614,8 +619,10 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 		err = PTR_ERR(be->blkif);
 		be->blkif = NULL;
 		xenbus_dev_fatal(dev, err, "creating block interface");
+		spin_unlock_irqrestore(&dev->reclaim_lock, flags);
 		goto fail;
 	}
+	spin_unlock_irqrestore(&dev->reclaim_lock, flags);
 
 	err = xenbus_printf(XBT_NIL, dev->nodename,
 			    "feature-max-indirect-segments", "%u",
@@ -838,6 +845,10 @@ static void reclaim_memory(struct xenbus_device *dev)
 {
 	struct backend_info *be = dev_get_drvdata(&dev->dev);
 
+	/* Device is registered but not probed yet */
+	if (!be)
+		return;
+
 	be->blkif->buffer_squeeze_end = jiffies +
 		msecs_to_jiffies(buffer_squeeze_duration_ms);
 }
-- 
2.17.1

[PATCH v11 5/6] xen/blkback: Remove unnecessary static variable name prefixes

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

A few of static variables in blkback have 'xen_blkif_' prefix, though it
is unnecessary for static variables.  This commit removes such prefixes.

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/block/xen-blkback/blkback.c | 37 +++++++++++++----------------
 1 file changed, 17 insertions(+), 20 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 79f677aeb5cc..fbd67f8e4e4e 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -62,8 +62,8 @@
  * IO workloads.
  */
 
-static int xen_blkif_max_buffer_pages = 1024;
-module_param_named(max_buffer_pages, xen_blkif_max_buffer_pages, int, 0644);
+static int max_buffer_pages = 1024;
+module_param_named(max_buffer_pages, max_buffer_pages, int, 0644);
 MODULE_PARM_DESC(max_buffer_pages,
 "Maximum number of free pages to keep in each block backend buffer");
 
@@ -78,8 +78,8 @@ MODULE_PARM_DESC(max_buffer_pages,
  * algorithm.
  */
 
-static int xen_blkif_max_pgrants = 1056;
-module_param_named(max_persistent_grants, xen_blkif_max_pgrants, int, 0644);
+static int max_pgrants = 1056;
+module_param_named(max_persistent_grants, max_pgrants, int, 0644);
 MODULE_PARM_DESC(max_persistent_grants,
                  "Maximum number of grants to map persistently");
 
@@ -88,8 +88,8 @@ MODULE_PARM_DESC(max_persistent_grants,
  * use. The time is in seconds, 0 means indefinitely long.
  */
 
-static unsigned int xen_blkif_pgrant_timeout = 60;
-module_param_named(persistent_grant_unused_seconds, xen_blkif_pgrant_timeout,
+static unsigned int pgrant_timeout = 60;
+module_param_named(persistent_grant_unused_seconds, pgrant_timeout,
 		   uint, 0644);
 MODULE_PARM_DESC(persistent_grant_unused_seconds,
 		 "Time in seconds an unused persistent grant is allowed to "
@@ -137,9 +137,8 @@ module_param(log_stats, int, 0644);
 
 static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
 {
-	return xen_blkif_pgrant_timeout &&
-	       (jiffies - persistent_gnt->last_used >=
-		HZ * xen_blkif_pgrant_timeout);
+	return pgrant_timeout && (jiffies - persistent_gnt->last_used >=
+			HZ * pgrant_timeout);
 }
 
 static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
@@ -234,7 +233,7 @@ static int add_persistent_gnt(struct xen_blkif_ring *ring,
 	struct persistent_gnt *this;
 	struct xen_blkif *blkif = ring->blkif;
 
-	if (ring->persistent_gnt_c >= xen_blkif_max_pgrants) {
+	if (ring->persistent_gnt_c >= max_pgrants) {
 		if (!blkif->vbd.overflow_max_grants)
 			blkif->vbd.overflow_max_grants = 1;
 		return -EBUSY;
@@ -397,14 +396,13 @@ static void purge_persistent_gnt(struct xen_blkif_ring *ring)
 		goto out;
 	}
 
-	if (ring->persistent_gnt_c < xen_blkif_max_pgrants ||
-	    (ring->persistent_gnt_c == xen_blkif_max_pgrants &&
+	if (ring->persistent_gnt_c < max_pgrants ||
+	    (ring->persistent_gnt_c == max_pgrants &&
 	    !ring->blkif->vbd.overflow_max_grants)) {
 		num_clean = 0;
 	} else {
-		num_clean = (xen_blkif_max_pgrants / 100) * LRU_PERCENT_CLEAN;
-		num_clean = ring->persistent_gnt_c - xen_blkif_max_pgrants +
-			    num_clean;
+		num_clean = (max_pgrants / 100) * LRU_PERCENT_CLEAN;
+		num_clean = ring->persistent_gnt_c - max_pgrants + num_clean;
 		num_clean = min(ring->persistent_gnt_c, num_clean);
 		pr_debug("Going to purge at least %u persistent grants\n",
 			 num_clean);
@@ -599,8 +597,7 @@ static void print_stats(struct xen_blkif_ring *ring)
 		 current->comm, ring->st_oo_req,
 		 ring->st_rd_req, ring->st_wr_req,
 		 ring->st_f_req, ring->st_ds_req,
-		 ring->persistent_gnt_c,
-		 xen_blkif_max_pgrants);
+		 ring->persistent_gnt_c, max_pgrants);
 	ring->st_print = jiffies + msecs_to_jiffies(10 * 1000);
 	ring->st_rd_req = 0;
 	ring->st_wr_req = 0;
@@ -660,7 +657,7 @@ int xen_blkif_schedule(void *arg)
 		if (time_before(jiffies, blkif->buffer_squeeze_end))
 			shrink_free_pagepool(ring, 0);
 		else
-			shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
+			shrink_free_pagepool(ring, max_buffer_pages);
 
 		if (log_stats && time_after(jiffies, ring->st_print))
 			print_stats(ring);
@@ -887,7 +884,7 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
 			continue;
 		}
 		if (use_persistent_gnts &&
-		    ring->persistent_gnt_c < xen_blkif_max_pgrants) {
+		    ring->persistent_gnt_c < max_pgrants) {
 			/*
 			 * We are using persistent grants, the grant is
 			 * not mapped but we might have room for it.
@@ -914,7 +911,7 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
 			pages[seg_idx]->persistent_gnt = persistent_gnt;
 			pr_debug("grant %u added to the tree of persistent grants, using %u/%u\n",
 				 persistent_gnt->gnt, ring->persistent_gnt_c,
-				 xen_blkif_max_pgrants);
+				 max_pgrants);
 			goto next;
 		}
 		if (use_persistent_gnts && !blkif->vbd.overflow_max_grants) {
-- 
2.17.1

[PATCH v11 6/6] xen/blkback: Consistently insert one empty line between functions

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

The number of empty lines between functions in the xenbus.c is
inconsistent.  This trivial style cleanup commit fixes the file to
consistently place only one empty line.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/block/xen-blkback/xenbus.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 20045827a391..453f97dd533d 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -432,7 +432,6 @@ static void xenvbd_sysfs_delif(struct xenbus_device *dev)
 	device_remove_file(&dev->dev, &dev_attr_physical_device);
 }
 
-
 static void xen_vbd_free(struct xen_vbd *vbd)
 {
 	if (vbd->bdev)
@@ -489,6 +488,7 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 		handle, blkif->domid);
 	return 0;
 }
+
 static int xen_blkbk_remove(struct xenbus_device *dev)
 {
 	struct backend_info *be = dev_get_drvdata(&dev->dev);
@@ -575,6 +575,7 @@ static void xen_blkbk_discard(struct xenbus_transaction xbt, struct backend_info
 	if (err)
 		dev_warn(&dev->dev, "writing feature-discard (%d)", err);
 }
+
 int xen_blkbk_barrier(struct xenbus_transaction xbt,
 		      struct backend_info *be, int state)
 {
@@ -663,7 +664,6 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 	return err;
 }
 
-
 /*
  * Callback received when the hotplug scripts have placed the physical-device
  * node.  Read it and the mode node, and create a vbd.  If the frontend is
@@ -755,7 +755,6 @@ static void backend_changed(struct xenbus_watch *watch,
 	}
 }
 
-
 /*
  * Callback received when the frontend's state changes.
  */
@@ -830,7 +829,6 @@ static void frontend_changed(struct xenbus_device *dev,
 	}
 }
 
-
 /* Once a memory pressure is detected, squeeze free page pools for a while. */
 static unsigned int buffer_squeeze_duration_ms = 10;
 module_param_named(buffer_squeeze_duration_ms,
@@ -855,7 +853,6 @@ static void reclaim_memory(struct xenbus_device *dev)
 
 /* ** Connection ** */
 
-
 /*
  * Write the physical details regarding the block device to the store, and
  * switch to Connected state.
-- 
2.17.1

Re: [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

[Jürgen Groß]

On 17.12.19 17:07, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> 'reclaim_memory' callback can race with a driver code as this callback
> will be called from any memory pressure detected context.  To deal with
> the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
> 'reclaim_memory' callback is called, the lock of the device which passed
> to the callback as its argument is locked.  Thus, drivers registering
> their 'reclaim_memory' callback should protect the data that might race
> with the callback with the lock by themselves.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>   drivers/xen/xenbus/xenbus_probe.c         |  1 +
>   drivers/xen/xenbus/xenbus_probe_backend.c | 10 ++++++++--
>   include/xen/xenbus.h                      |  2 ++
>   3 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
> index 5b471889d723..b86393f172e6 100644
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
>   		goto fail;
>   
>   	dev_set_name(&xendev->dev, "%s", devname);
> +	spin_lock_init(&xendev->reclaim_lock);
>   
>   	/* Register with generic device framework. */
>   	err = device_register(&xendev->dev);
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index 7e78ebef7c54..516aa64b9967 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -251,12 +251,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>   static int backend_reclaim_memory(struct device *dev, void *data)
>   {
>   	const struct xenbus_driver *drv;
> +	struct xenbus_device *xdev;
> +	unsigned long flags;
>   
>   	if (!dev->driver)
>   		return 0;
>   	drv = to_xenbus_driver(dev->driver);
> -	if (drv && drv->reclaim_memory)
> -		drv->reclaim_memory(to_xenbus_device(dev));
> +	if (drv && drv->reclaim_memory) {
> +		xdev = to_xenbus_device(dev);
> +		spin_trylock_irqsave(&xdev->reclaim_lock, flags);

You need spin_lock_irqsave() here. Or maybe spin_lock() would be fine,
too? I can't see a reason why you'd want to disable irqs here.

> +		drv->reclaim_memory(xdev);
> +		spin_unlock_irqrestore(&xdev->reclaim_lock, flags);
> +	}
>   	return 0;
>   }
>   
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index c861cfb6f720..d9468313061d 100644
> --- a/include/xen/xenbus.h
> +++ b/include/xen/xenbus.h
> @@ -76,6 +76,8 @@ struct xenbus_device {
>   	enum xenbus_state state;
>   	struct completion down;
>   	struct work_struct work;
> +	/* 'reclaim_memory' callback is called while this lock is acquired */
> +	spinlock_t reclaim_lock;
>   };
>   
>   static inline struct xenbus_device *to_xenbus_device(struct device *dev)
> 


Juergen

Re: [PATCH v11 4/6] xen/blkback: Protect 'reclaim_memory()' with 'reclaim_lock'

[Jürgen Groß]

On 17.12.19 17:07, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> The 'reclaim_memory()' callback of blkback could race with
> 'xen_blkbk_probe()' and 'xen_blkbk_remove()'.  In the case, incompletely
> linked 'backend_info' and 'blkif' might be exposed to the callback, thus
> result in bad results including NULL dereference.  This commit fixes the
> problem by applying the 'reclaim_lock' protection to those.
> 
> Note that this commit is separated for review purpose only.  As the
> previous commit might result in race condition and might make bisect
> confuse, please squash this commit into previous commit if possible.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>

Please merge this patch into patch 2.


Juergen

Re: Re: [Xen-devel] [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

[SeongJae Park]

On Tue, 17 Dec 2019 17:13:42 +0100 "Jürgen Groß" <jgross@suse.com> wrote:

> On 17.12.19 17:07, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> > 
> > 'reclaim_memory' callback can race with a driver code as this callback
> > will be called from any memory pressure detected context.  To deal with
> > the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
> > 'reclaim_memory' callback is called, the lock of the device which passed
> > to the callback as its argument is locked.  Thus, drivers registering
> > their 'reclaim_memory' callback should protect the data that might race
> > with the callback with the lock by themselves.
> > 
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >   drivers/xen/xenbus/xenbus_probe.c         |  1 +
> >   drivers/xen/xenbus/xenbus_probe_backend.c | 10 ++++++++--
> >   include/xen/xenbus.h                      |  2 ++
> >   3 files changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
> > index 5b471889d723..b86393f172e6 100644
> > --- a/drivers/xen/xenbus/xenbus_probe.c
> > +++ b/drivers/xen/xenbus/xenbus_probe.c
> > @@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
> >   		goto fail;
> >   
> >   	dev_set_name(&xendev->dev, "%s", devname);
> > +	spin_lock_init(&xendev->reclaim_lock);
> >   
> >   	/* Register with generic device framework. */
> >   	err = device_register(&xendev->dev);
> > diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> > index 7e78ebef7c54..516aa64b9967 100644
> > --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> > +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> > @@ -251,12 +251,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
> >   static int backend_reclaim_memory(struct device *dev, void *data)
> >   {
> >   	const struct xenbus_driver *drv;
> > +	struct xenbus_device *xdev;
> > +	unsigned long flags;
> >   
> >   	if (!dev->driver)
> >   		return 0;
> >   	drv = to_xenbus_driver(dev->driver);
> > -	if (drv && drv->reclaim_memory)
> > -		drv->reclaim_memory(to_xenbus_device(dev));
> > +	if (drv && drv->reclaim_memory) {
> > +		xdev = to_xenbus_device(dev);
> > +		spin_trylock_irqsave(&xdev->reclaim_lock, flags);
> 
> You need spin_lock_irqsave() here. Or maybe spin_lock() would be fine,
> too? I can't see a reason why you'd want to disable irqs here.

I needed to diable irq here as this is called from the memory shrinker context.

Also, used 'trylock' because the 'probe()' and 'remove()' code of the driver
might include memory allocation.  And the xen-blkback actually does.  If the
allocation shows a memory pressure during the allocation, it will trigger this
shrinker callback again and then deadlock.


Thanks,
SeongJae Park

> 
> > +		drv->reclaim_memory(xdev);
> > +		spin_unlock_irqrestore(&xdev->reclaim_lock, flags);
> > +	}
> >   	return 0;
> >   }
> >   
> > diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> > index c861cfb6f720..d9468313061d 100644
> > --- a/include/xen/xenbus.h
> > +++ b/include/xen/xenbus.h
> > @@ -76,6 +76,8 @@ struct xenbus_device {
> >   	enum xenbus_state state;
> >   	struct completion down;
> >   	struct work_struct work;
> > +	/* 'reclaim_memory' callback is called while this lock is acquired */
> > +	spinlock_t reclaim_lock;
> >   };
> >   
> >   static inline struct xenbus_device *to_xenbus_device(struct device *dev)
> > 
> 
> 
> Juergen
> 

[PATCH 1/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

I though it would be better to review separated patches, but seems it
was my mistake.  As Juergen asked, merged them again and post here.
Also, dropped Roger's reviewed-by.


Thanks,
SeongJae Park


------------------------ >8 -------------------------------------------
Subject: [PATCH 1/3] xen/blkback: Squeeze page pools if a memory pressure is
 detected

Each `blkif` has a free pages pool for the grant mapping.  The size of
the pool starts from zero and is increased on demand while processing
the I/O requests.  If current I/O requests handling is finished or 100
milliseconds has passed since last I/O requests handling, it checks and
shrinks the pool to not exceed the size limit, `max_buffer_pages`.

Therefore, host administrators can cause memory pressure in blkback by
attaching a large number of block devices and inducing I/O.  Such
problematic situations can be avoided by limiting the maximum number of
devices that can be attached, but finding the optimal limit is not so
easy.  Improper set of the limit can results in memory pressure or a
resource underutilization.  This commit avoids such problematic
situations by squeezing the pools (returns every free page in the pool
to the system) for a while (users can set this duration via a module
parameter) if memory pressure is detected.

Discussions
===========

The `blkback`'s original shrinking mechanism returns only pages in the
pool which are not currently be used by `blkback` to the system.  In
other words, the pages that are not mapped with granted pages.  Because
this commit is changing only the shrink limit but still uses the same
freeing mechanism it does not touch pages which are currently mapping
grants.

Once memory pressure is detected, this commit keeps the squeezing limit
for a user-specified time duration.  The duration should be neither too
long nor too short.  If it is too long, the squeezing incurring overhead
can reduce the I/O performance.  If it is too short, `blkback` will not
free enough pages to reduce the memory pressure.  This commit sets the
value as `10 milliseconds` by default because it is a short time in
terms of I/O while it is a long time in terms of memory operations.
Also, as the original shrinking mechanism works for at least every 100
milliseconds, this could be a somewhat reasonable choice.  I also tested
other durations (refer to the below section for more details) and
confirmed that 10 milliseconds is the one that works best with the test.
That said, the proper duration depends on actual configurations and
workloads.  That's why this commit allows users to set the duration as a
module parameter.

Memory Pressure Test
====================

To show how this commit fixes the memory pressure situation well, I
configured a test environment on a xen-running virtualization system.
On the `blkfront` running guest instances, I attach a large number of
network-backed volume devices and induce I/O to those.  Meanwhile, I
measure the number of pages that swapped in (pswpin) and out (pswpout)
on the `blkback` running guest.  The test ran twice, once for the
`blkback` before this commit and once for that after this commit.  As
shown below, this commit has dramatically reduced the memory pressure:

                pswpin  pswpout
    before      76,672  185,799
    after          212    3,325

Optimal Aggressive Shrinking Duration
-------------------------------------

To find a best squeezing duration, I repeated the test with three
different durations (1ms, 10ms, and 100ms).  The results are as below:

    duration    pswpin  pswpout
    1           852     6,424
    10          212     3,325
    100         203     3,340

As expected, the memory pressure has decreased as the duration is
increased, but the reduction stopped from the `10ms`.  Based on this
results, I chose the default duration as 10ms.

Performance Overhead Test
=========================

This commit could incur I/O performance degradation under severe memory
pressure because the squeezing will require more page allocations per
I/O.  To show the overhead, I artificially made a worst-case squeezing
situation and measured the I/O performance of a `blkfront` running
guest.

For the artificial squeezing, I set the `blkback.max_buffer_pages` using
the `/sys/module/xen_blkback/parameters/max_buffer_pages` file.  In this
test, I set the value to `1024` and `0`.  The `1024` is the default
value.  Setting the value as `0` is same to a situation doing the
squeezing always (worst-case).

If the underlying block device is slow enough, the squeezing overhead
could be hidden.  For the reason, I use a fast block device, namely the
rbd[1]:

    # xl block-attach guest phy:/dev/ram0 xvdb w

For the I/O performance measurement, I run a simple `dd` command 5 times
directly to the device as below and collect the 'MB/s' results.

    $ for i in {1..5}; do dd if=/dev/zero of=/dev/xvdb \
                             bs=4k count=$((256*512)); sync; done

The results are as below.  'max_pgs' represents the value of the
`blkback.max_buffer_pages` parameter.

    max_pgs   Min       Max       Median     Avg    Stddev
    0         417       423       420        419.4  2.5099801
    1024      414       425       416        417.8  4.4384682
    No difference proven at 95.0% confidence

In short, even worst case squeezing on ramdisk based fast block device
makes no visible performance degradation.  Please note that this is just
a very simple and minimal test.  On systems using super-fast block
devices and a special I/O workload, the results might be different.  If
you have any doubt, test on your machine with your workload to find the
optimal squeezing duration for you.

[1] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 .../ABI/testing/sysfs-driver-xen-blkback      | 10 ++++++
 drivers/block/xen-blkback/blkback.c           |  7 ++--
 drivers/block/xen-blkback/common.h            |  1 +
 drivers/block/xen-blkback/xenbus.c            | 32 ++++++++++++++++++-
 4 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index 4e7babb3ba1f..f01224231f3f 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -25,3 +25,13 @@ Description:
                 allocated without being in use. The time is in
                 seconds, 0 means indefinitely long.
                 The default is 60 seconds.
+
+What:           /sys/module/xen_blkback/parameters/buffer_squeeze_duration_ms
+Date:           December 2019
+KernelVersion:  5.5
+Contact:        SeongJae Park <sjpark@amazon.de>
+Description:
+                When memory pressure is reported to blkback this option
+                controls the duration in milliseconds that blkback will not
+                cache any page not backed by a grant mapping.
+                The default is 10ms.
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index fd1e19f1a49f..79f677aeb5cc 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -656,8 +656,11 @@ int xen_blkif_schedule(void *arg)
 			ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL);
 		}
 
-		/* Shrink if we have more than xen_blkif_max_buffer_pages */
-		shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
+		/* Shrink the free pages pool if it is too large. */
+		if (time_before(jiffies, blkif->buffer_squeeze_end))
+			shrink_free_pagepool(ring, 0);
+		else
+			shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
 
 		if (log_stats && time_after(jiffies, ring->st_print))
 			print_stats(ring);
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 1d3002d773f7..536c84f61fed 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -319,6 +319,7 @@ struct xen_blkif {
 	/* All rings for this device. */
 	struct xen_blkif_ring	*rings;
 	unsigned int		nr_rings;
+	unsigned long		buffer_squeeze_end;
 };
 
 struct seg_buf {
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index b90dbcd99c03..20045827a391 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -492,6 +492,7 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 static int xen_blkbk_remove(struct xenbus_device *dev)
 {
 	struct backend_info *be = dev_get_drvdata(&dev->dev);
+	unsigned long flags;
 
 	pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
 
@@ -504,6 +505,7 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
 		be->backend_watch.node = NULL;
 	}
 
+	spin_lock_irqsave(&dev->reclaim_lock, flags);
 	dev_set_drvdata(&dev->dev, NULL);
 
 	if (be->blkif) {
@@ -512,6 +514,7 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
 		/* Put the reference we set in xen_blkif_alloc(). */
 		xen_blkif_put(be->blkif);
 	}
+	spin_unlock_irqrestore(&dev->reclaim_lock, flags);
 
 	return 0;
 }
@@ -597,6 +600,7 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 	int err;
 	struct backend_info *be = kzalloc(sizeof(struct backend_info),
 					  GFP_KERNEL);
+	unsigned long flags;
 
 	/* match the pr_debug in xen_blkbk_remove */
 	pr_debug("%s %p %d\n", __func__, dev, dev->otherend_id);
@@ -607,6 +611,7 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 		return -ENOMEM;
 	}
 	be->dev = dev;
+	spin_lock_irqsave(&dev->reclaim_lock, flags);
 	dev_set_drvdata(&dev->dev, be);
 
 	be->blkif = xen_blkif_alloc(dev->otherend_id);
@@ -614,8 +619,10 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 		err = PTR_ERR(be->blkif);
 		be->blkif = NULL;
 		xenbus_dev_fatal(dev, err, "creating block interface");
+		spin_unlock_irqrestore(&dev->reclaim_lock, flags);
 		goto fail;
 	}
+	spin_unlock_irqrestore(&dev->reclaim_lock, flags);
 
 	err = xenbus_printf(XBT_NIL, dev->nodename,
 			    "feature-max-indirect-segments", "%u",
@@ -824,6 +831,28 @@ static void frontend_changed(struct xenbus_device *dev,
 }
 
 
+/* Once a memory pressure is detected, squeeze free page pools for a while. */
+static unsigned int buffer_squeeze_duration_ms = 10;
+module_param_named(buffer_squeeze_duration_ms,
+		buffer_squeeze_duration_ms, int, 0644);
+MODULE_PARM_DESC(buffer_squeeze_duration_ms,
+"Duration in ms to squeeze pages buffer when a memory pressure is detected");
+
+/*
+ * Callback received when the memory pressure is detected.
+ */
+static void reclaim_memory(struct xenbus_device *dev)
+{
+	struct backend_info *be = dev_get_drvdata(&dev->dev);
+
+	/* Device is registered but not probed yet */
+	if (!be)
+		return;
+
+	be->blkif->buffer_squeeze_end = jiffies +
+		msecs_to_jiffies(buffer_squeeze_duration_ms);
+}
+
 /* ** Connection ** */
 
 
@@ -1115,7 +1144,8 @@ static struct xenbus_driver xen_blkbk_driver = {
 	.ids  = xen_blkbk_ids,
 	.probe = xen_blkbk_probe,
 	.remove = xen_blkbk_remove,
-	.otherend_changed = frontend_changed
+	.otherend_changed = frontend_changed,
+	.reclaim_memory = reclaim_memory,
 };
 
 int xen_blkif_xenbus_init(void)
-- 
2.17.1

Re: [Xen-devel] [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

[Jürgen Groß]

On 17.12.19 17:24, SeongJae Park wrote:
> On Tue, 17 Dec 2019 17:13:42 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
> 
>> On 17.12.19 17:07, SeongJae Park wrote:
>>> From: SeongJae Park <sjpark@amazon.de>
>>>
>>> 'reclaim_memory' callback can race with a driver code as this callback
>>> will be called from any memory pressure detected context.  To deal with
>>> the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
>>> 'reclaim_memory' callback is called, the lock of the device which passed
>>> to the callback as its argument is locked.  Thus, drivers registering
>>> their 'reclaim_memory' callback should protect the data that might race
>>> with the callback with the lock by themselves.
>>>
>>> Signed-off-by: SeongJae Park <sjpark@amazon.de>
>>> ---
>>>    drivers/xen/xenbus/xenbus_probe.c         |  1 +
>>>    drivers/xen/xenbus/xenbus_probe_backend.c | 10 ++++++++--
>>>    include/xen/xenbus.h                      |  2 ++
>>>    3 files changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
>>> index 5b471889d723..b86393f172e6 100644
>>> --- a/drivers/xen/xenbus/xenbus_probe.c
>>> +++ b/drivers/xen/xenbus/xenbus_probe.c
>>> @@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
>>>    		goto fail;
>>>    
>>>    	dev_set_name(&xendev->dev, "%s", devname);
>>> +	spin_lock_init(&xendev->reclaim_lock);
>>>    
>>>    	/* Register with generic device framework. */
>>>    	err = device_register(&xendev->dev);
>>> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
>>> index 7e78ebef7c54..516aa64b9967 100644
>>> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
>>> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
>>> @@ -251,12 +251,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>>>    static int backend_reclaim_memory(struct device *dev, void *data)
>>>    {
>>>    	const struct xenbus_driver *drv;
>>> +	struct xenbus_device *xdev;
>>> +	unsigned long flags;
>>>    
>>>    	if (!dev->driver)
>>>    		return 0;
>>>    	drv = to_xenbus_driver(dev->driver);
>>> -	if (drv && drv->reclaim_memory)
>>> -		drv->reclaim_memory(to_xenbus_device(dev));
>>> +	if (drv && drv->reclaim_memory) {
>>> +		xdev = to_xenbus_device(dev);
>>> +		spin_trylock_irqsave(&xdev->reclaim_lock, flags);
>>
>> You need spin_lock_irqsave() here. Or maybe spin_lock() would be fine,
>> too? I can't see a reason why you'd want to disable irqs here.
> 
> I needed to diable irq here as this is called from the memory shrinker context.

Okay.

> 
> Also, used 'trylock' because the 'probe()' and 'remove()' code of the driver
> might include memory allocation.  And the xen-blkback actually does.  If the
> allocation shows a memory pressure during the allocation, it will trigger this
> shrinker callback again and then deadlock.

In that case you need to either return when you didn't get the lock or

- when obtaining the lock during probe() and remove() set a variable
   containing the current cpu number
- and reset that to e.g NR_CPUS before releasing the lock again
- in the shrinker callback do trylock, and if you didn't get the lock
   test whether the cpu-variable above is set to your current cpu and
   continue only if yes; if not, redo the the trylock


Juergen

Re: Re: [Xen-devel] [PATCH v11 2/6] xenbus/backend: Protect xenbus callback with lock

[SeongJae Park]

On Tue, 17 Dec 2019 18:10:19 +0100 "Jürgen Groß" <jgross@suse.com> wrote:

> On 17.12.19 17:24, SeongJae Park wrote:
> > On Tue, 17 Dec 2019 17:13:42 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
> > 
> >> On 17.12.19 17:07, SeongJae Park wrote:
> >>> From: SeongJae Park <sjpark@amazon.de>
> >>>
> >>> 'reclaim_memory' callback can race with a driver code as this callback
> >>> will be called from any memory pressure detected context.  To deal with
> >>> the case, this commit adds a spinlock in the 'xenbus_device'.  Whenever
> >>> 'reclaim_memory' callback is called, the lock of the device which passed
> >>> to the callback as its argument is locked.  Thus, drivers registering
> >>> their 'reclaim_memory' callback should protect the data that might race
> >>> with the callback with the lock by themselves.
> >>>
> >>> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> >>> ---
> >>>    drivers/xen/xenbus/xenbus_probe.c         |  1 +
> >>>    drivers/xen/xenbus/xenbus_probe_backend.c | 10 ++++++++--
> >>>    include/xen/xenbus.h                      |  2 ++
> >>>    3 files changed, 11 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
> >>> index 5b471889d723..b86393f172e6 100644
> >>> --- a/drivers/xen/xenbus/xenbus_probe.c
> >>> +++ b/drivers/xen/xenbus/xenbus_probe.c
> >>> @@ -472,6 +472,7 @@ int xenbus_probe_node(struct xen_bus_type *bus,
> >>>    		goto fail;
> >>>    
> >>>    	dev_set_name(&xendev->dev, "%s", devname);
> >>> +	spin_lock_init(&xendev->reclaim_lock);
> >>>    
> >>>    	/* Register with generic device framework. */
> >>>    	err = device_register(&xendev->dev);
> >>> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> >>> index 7e78ebef7c54..516aa64b9967 100644
> >>> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> >>> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> >>> @@ -251,12 +251,18 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
> >>>    static int backend_reclaim_memory(struct device *dev, void *data)
> >>>    {
> >>>    	const struct xenbus_driver *drv;
> >>> +	struct xenbus_device *xdev;
> >>> +	unsigned long flags;
> >>>    
> >>>    	if (!dev->driver)
> >>>    		return 0;
> >>>    	drv = to_xenbus_driver(dev->driver);
> >>> -	if (drv && drv->reclaim_memory)
> >>> -		drv->reclaim_memory(to_xenbus_device(dev));
> >>> +	if (drv && drv->reclaim_memory) {
> >>> +		xdev = to_xenbus_device(dev);
> >>> +		spin_trylock_irqsave(&xdev->reclaim_lock, flags);
> >>
> >> You need spin_lock_irqsave() here. Or maybe spin_lock() would be fine,
> >> too? I can't see a reason why you'd want to disable irqs here.
> > 
> > I needed to diable irq here as this is called from the memory shrinker context.
> 
> Okay.
> 
> > 
> > Also, used 'trylock' because the 'probe()' and 'remove()' code of the driver
> > might include memory allocation.  And the xen-blkback actually does.  If the
> > allocation shows a memory pressure during the allocation, it will trigger this
> > shrinker callback again and then deadlock.
> 
> In that case you need to either return when you didn't get the lock or

Yes, it should.  Cannot believe how I posted this code.  Seems I made some
terrible mistake while formatting patches.  Anyway, will return if fail to
acquire the lock, in the next version.


Thanks,
SeongJae Park

> 
> - when obtaining the lock during probe() and remove() set a variable
>    containing the current cpu number
> - and reset that to e.g NR_CPUS before releasing the lock again
> - in the shrinker callback do trylock, and if you didn't get the lock
>    test whether the cpu-variable above is set to your current cpu and
>    continue only if yes; if not, redo the the trylock
> 
> 
> Juergen