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* [PATCH v10 0/4] xenbus/backend: Add a memory pressure handler callback
@ 2019-12-16 12:45 SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 1/4] xenbus/backend: Add " SeongJae Park
                   ` (3 more replies)
  0 siblings, 4 replies; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 12:45 UTC (permalink / raw)
  To: jgross, axboe, konrad.wilk, roger.pau
  Cc: SeongJae Park, pdurrant, sj38.park, xen-devel, linux-block, linux-kernel

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 use it to mitigate the problem in
'xen-blkback' (patch 2).  The third and fourth patches are trivial
cleanups.

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/blkback_squeezing_v10


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

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 (4):
  xenbus/backend: Add memory pressure handler callback
  xen/blkback: Squeeze page pools if a memory pressure is detected
  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            | 26 +++++++++---
 drivers/xen/xenbus/xenbus_probe_backend.c     | 32 ++++++++++++++
 include/xen/xenbus.h                          |  1 +
 6 files changed, 86 insertions(+), 26 deletions(-)

-- 
2.17.1


^ permalink raw reply	[flat|nested] 17+ messages in thread

* [PATCH v10 1/4] xenbus/backend: Add memory pressure handler callback
  2019-12-16 12:45 [PATCH v10 0/4] xenbus/backend: Add a memory pressure handler callback SeongJae Park
@ 2019-12-16 12:45 ` SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected SeongJae Park
                   ` (2 subsequent siblings)
  3 siblings, 0 replies; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 12:45 UTC (permalink / raw)
  To: jgross, axboe, konrad.wilk, roger.pau
  Cc: SeongJae Park, pdurrant, sjpark, sj38.park, xen-devel,
	linux-block, linux-kernel

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


^ permalink raw reply related	[flat|nested] 17+ messages in thread

* [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-16 12:45 [PATCH v10 0/4] xenbus/backend: Add a memory pressure handler callback SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 1/4] xenbus/backend: Add " SeongJae Park
@ 2019-12-16 12:45 ` SeongJae Park
  2019-12-16 14:37   ` [Xen-devel] " SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 3/4] xen/blkback: Remove unnecessary static variable name prefixes SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 4/4] xen/blkback: Consistently insert one empty line between functions SeongJae Park
  3 siblings, 1 reply; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 12:45 UTC (permalink / raw)
  To: jgross, axboe, konrad.wilk, roger.pau
  Cc: SeongJae Park, pdurrant, sjpark, sj38.park, xen-devel,
	linux-block, linux-kernel

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


^ permalink raw reply related	[flat|nested] 17+ messages in thread

* [PATCH v10 3/4] xen/blkback: Remove unnecessary static variable name prefixes
  2019-12-16 12:45 [PATCH v10 0/4] xenbus/backend: Add a memory pressure handler callback SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 1/4] xenbus/backend: Add " SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected SeongJae Park
@ 2019-12-16 12:45 ` SeongJae Park
  2019-12-16 12:45 ` [PATCH v10 4/4] xen/blkback: Consistently insert one empty line between functions SeongJae Park
  3 siblings, 0 replies; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 12:45 UTC (permalink / raw)
  To: jgross, axboe, konrad.wilk, roger.pau
  Cc: SeongJae Park, pdurrant, sjpark, sj38.park, xen-devel,
	linux-block, linux-kernel

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


^ permalink raw reply related	[flat|nested] 17+ messages in thread

* [PATCH v10 4/4] xen/blkback: Consistently insert one empty line between functions
  2019-12-16 12:45 [PATCH v10 0/4] xenbus/backend: Add a memory pressure handler callback SeongJae Park
                   ` (2 preceding siblings ...)
  2019-12-16 12:45 ` [PATCH v10 3/4] xen/blkback: Remove unnecessary static variable name prefixes SeongJae Park
@ 2019-12-16 12:45 ` SeongJae Park
  3 siblings, 0 replies; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 12:45 UTC (permalink / raw)
  To: jgross, axboe, konrad.wilk, roger.pau
  Cc: SeongJae Park, pdurrant, sjpark, sj38.park, xen-devel,
	linux-block, linux-kernel

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 4f6ea4feca79..dc0ea123c74c 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);
@@ -572,6 +572,7 @@ 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)
 {
@@ -656,7 +657,6 @@ 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
@@ -748,7 +748,6 @@ static void backend_changed(struct xenbus_watch *watch,
 	}
 }
 
-
 /*
  * Callback received when the frontend's state changes.
  */
@@ -823,7 +822,6 @@ 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,
@@ -844,7 +842,6 @@ 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


^ permalink raw reply related	[flat|nested] 17+ messages in thread

* Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-16 12:45 ` [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected SeongJae Park
@ 2019-12-16 14:37   ` SeongJae Park
  2019-12-16 16:15     ` SeongJae Park
  0 siblings, 1 reply; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 14:37 UTC (permalink / raw)
  To: jgross, axboe, konrad.wilk, roger.pau
  Cc: linux-block, sjpark, pdurrant, SeongJae Park, linux-kernel,
	sj38.park, xen-devel

On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:

> 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);

This callback might race with 'xen_blkbk_probe()'.  The race could result in
__NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
'be' to the 'dev'.  Please _don't merge_ this patch now!

I will do more test and share results.  Meanwhile, if you have any opinion,
please let me know.


Thanks,
SeongJae Park

> +}
> +
>  /* ** 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
> 

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-16 14:37   ` [Xen-devel] " SeongJae Park
@ 2019-12-16 16:15     ` SeongJae Park
  2019-12-16 16:23       ` Jürgen Groß
  0 siblings, 1 reply; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 16:15 UTC (permalink / raw)
  To: jgross, axboe, konrad.wilk, roger.pau
  Cc: pdurrant, linux-kernel, linux-block, xen-devel, sj38.park, sjpark

On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:

> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> 
> > From: SeongJae Park <sjpark@amazon.de>
> > 
[...]
> > --- 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);
> 
> This callback might race with 'xen_blkbk_probe()'.  The race could result in
> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
> 'be' to the 'dev'.  Please _don't merge_ this patch now!
> 
> I will do more test and share results.  Meanwhile, if you have any opinion,
> please let me know.

Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
concurrency issues could be in other drivers in their way, I suggest to change
the reclaim callback ('->reclaim_memory') to be called for each driver instead
of each device.  Then, each driver could be able to deal with its concurrency
issues by itself.

For blkback, we could reuse the global variable based approach, as similar to
the v7[1] of this patchset.  As the callback is called for each driver instead
of each device now, the duplicated set of the timeout will not happen.


Thanks,
SeongJae Park

[1] https://lore.kernel.org/xen-devel/20191211181016.14366-1-sjpark@amazon.de/

> 
> 
> Thanks,
> SeongJae Park
> 
> > +}
> > +
> >  /* ** 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
> > 
> 

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-16 16:15     ` SeongJae Park
@ 2019-12-16 16:23       ` Jürgen Groß
  2019-12-16 19:48         ` SeongJae Park
  0 siblings, 1 reply; 17+ messages in thread
From: Jürgen Groß @ 2019-12-16 16:23 UTC (permalink / raw)
  To: SeongJae Park, axboe, konrad.wilk, roger.pau
  Cc: pdurrant, linux-kernel, linux-block, xen-devel, sj38.park

On 16.12.19 17:15, SeongJae Park wrote:
> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> 
>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
>>
>>> From: SeongJae Park <sjpark@amazon.de>
>>>
> [...]
>>> --- 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);
>>
>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
>>
>> I will do more test and share results.  Meanwhile, if you have any opinion,
>> please let me know.
> 
> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
> concurrency issues could be in other drivers in their way, I suggest to change
> the reclaim callback ('->reclaim_memory') to be called for each driver instead
> of each device.  Then, each driver could be able to deal with its concurrency
> issues by itself.

Hmm, I don't like that. This would need to be changed back in case we
add per-guest quota.

Wouldn't a get_device() before calling the callback and a put_device()
afterwards avoid that problem?


Juergen

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-16 16:23       ` Jürgen Groß
@ 2019-12-16 19:48         ` SeongJae Park
  2019-12-17  6:23           ` Jürgen Groß
  2019-12-17 11:39           ` Roger Pau Monné
  0 siblings, 2 replies; 17+ messages in thread
From: SeongJae Park @ 2019-12-16 19:48 UTC (permalink / raw)
  To: jgross
  Cc: axboe, konrad.wilk, roger.pau, linux-block, xen-devel, pdurrant,
	sj38.park, linux-kernel

On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:

> On 16.12.19 17:15, SeongJae Park wrote:
> > On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > 
> >> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> >>
> >>> From: SeongJae Park <sjpark@amazon.de>
> >>>
> > [...]
> >>> --- 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);
> >>
> >> This callback might race with 'xen_blkbk_probe()'.  The race could result in
> >> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
> >> 'be' to the 'dev'.  Please _don't merge_ this patch now!
> >>
> >> I will do more test and share results.  Meanwhile, if you have any opinion,
> >> please let me know.

I reduced system memory and attached bunch of devices in short time so that
memory pressure occurs while device attachments are ongoing.  Under this
circumstance, I was able to see the race.

> > 
> > Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
> > concurrency issues could be in other drivers in their way, I suggest to change
> > the reclaim callback ('->reclaim_memory') to be called for each driver instead
> > of each device.  Then, each driver could be able to deal with its concurrency
> > issues by itself.
> 
> Hmm, I don't like that. This would need to be changed back in case we
> add per-guest quota.

Extending this callback in that way would be still not too hard.  We could use
the argument to the callback.  I would keep the argument of the callback to
'struct device *' as is, and will add a comment saying 'NULL' value of the
argument means every devices.  As an example, xenbus would pass NULL-ending
array of the device pointers that need to free its resources.

After seeing this race, I am now also thinking it could be better to delegate
detailed control of each device to its driver, as some drivers have some
complicated and unique relation with its devices.

> 
> Wouldn't a get_device() before calling the callback and a put_device()
> afterwards avoid that problem?

I didn't used the reference count manipulation operations because other similar
parts also didn't.  But, if there is no implicit reference count guarantee, it
seems those operations are indeed necessary.

That said, as get/put operations only adjust the reference count, those will
not make the callback to wait until the linking of the 'backend' and 'blkif' to
the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
Or, am I missing something?

I also modified the code to do 'get_device()' and 'put_device()' as you
suggested and did test, but the race was still reproducible.


Thanks,
SeongJae Park

> 
> 
> Juergen

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-16 19:48         ` SeongJae Park
@ 2019-12-17  6:23           ` Jürgen Groß
  2019-12-17  7:59             ` SeongJae Park
  2019-12-17 11:39           ` Roger Pau Monné
  1 sibling, 1 reply; 17+ messages in thread
From: Jürgen Groß @ 2019-12-17  6:23 UTC (permalink / raw)
  To: SeongJae Park
  Cc: axboe, konrad.wilk, roger.pau, linux-block, xen-devel, pdurrant,
	linux-kernel

On 16.12.19 20:48, SeongJae Park wrote:
> On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
> 
>> On 16.12.19 17:15, SeongJae Park wrote:
>>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
>>>
>>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
>>>>
>>>>> From: SeongJae Park <sjpark@amazon.de>
>>>>>
>>> [...]
>>>>> --- 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);
>>>>
>>>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
>>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
>>>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
>>>>
>>>> I will do more test and share results.  Meanwhile, if you have any opinion,
>>>> please let me know.
> 
> I reduced system memory and attached bunch of devices in short time so that
> memory pressure occurs while device attachments are ongoing.  Under this
> circumstance, I was able to see the race.
> 
>>>
>>> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
>>> concurrency issues could be in other drivers in their way, I suggest to change
>>> the reclaim callback ('->reclaim_memory') to be called for each driver instead
>>> of each device.  Then, each driver could be able to deal with its concurrency
>>> issues by itself.
>>
>> Hmm, I don't like that. This would need to be changed back in case we
>> add per-guest quota.
> 
> Extending this callback in that way would be still not too hard.  We could use
> the argument to the callback.  I would keep the argument of the callback to
> 'struct device *' as is, and will add a comment saying 'NULL' value of the
> argument means every devices.  As an example, xenbus would pass NULL-ending
> array of the device pointers that need to free its resources.
> 
> After seeing this race, I am now also thinking it could be better to delegate
> detailed control of each device to its driver, as some drivers have some
> complicated and unique relation with its devices.
> 
>>
>> Wouldn't a get_device() before calling the callback and a put_device()
>> afterwards avoid that problem?
> 
> I didn't used the reference count manipulation operations because other similar
> parts also didn't.  But, if there is no implicit reference count guarantee, it
> seems those operations are indeed necessary.
> 
> That said, as get/put operations only adjust the reference count, those will
> not make the callback to wait until the linking of the 'backend' and 'blkif' to
> the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
> Or, am I missing something?

No, I think we need a xenbus lock per device which will need to be
taken in xen_blkbk_probe(), xenbus_dev_remove() and while calling the
callback.


Juergen

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-17  6:23           ` Jürgen Groß
@ 2019-12-17  7:59             ` SeongJae Park
  2019-12-17  8:16               ` Jürgen Groß
  0 siblings, 1 reply; 17+ messages in thread
From: SeongJae Park @ 2019-12-17  7:59 UTC (permalink / raw)
  To: Jürgen Groß
  Cc: SeongJae Park, axboe, konrad.wilk, pdurrant, linux-kernel,
	linux-block, xen-devel, roger.pau

On Tue, 17 Dec 2019 07:23:12 +0100 "Jürgen Groß" <jgross@suse.com> wrote:

> On 16.12.19 20:48, SeongJae Park wrote:
> > On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
> > 
> >> On 16.12.19 17:15, SeongJae Park wrote:
> >>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> >>>
> >>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> >>>>
> >>>>> From: SeongJae Park <sjpark@amazon.de>
> >>>>>
> >>> [...]
> >>>>> --- 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);
> >>>>
> >>>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
> >>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
> >>>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
> >>>>
> >>>> I will do more test and share results.  Meanwhile, if you have any opinion,
> >>>> please let me know.
> > 
> > I reduced system memory and attached bunch of devices in short time so that
> > memory pressure occurs while device attachments are ongoing.  Under this
> > circumstance, I was able to see the race.
> > 
> >>>
> >>> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
> >>> concurrency issues could be in other drivers in their way, I suggest to change
> >>> the reclaim callback ('->reclaim_memory') to be called for each driver instead
> >>> of each device.  Then, each driver could be able to deal with its concurrency
> >>> issues by itself.
> >>
> >> Hmm, I don't like that. This would need to be changed back in case we
> >> add per-guest quota.
> > 
> > Extending this callback in that way would be still not too hard.  We could use
> > the argument to the callback.  I would keep the argument of the callback to
> > 'struct device *' as is, and will add a comment saying 'NULL' value of the
> > argument means every devices.  As an example, xenbus would pass NULL-ending
> > array of the device pointers that need to free its resources.
> > 
> > After seeing this race, I am now also thinking it could be better to delegate
> > detailed control of each device to its driver, as some drivers have some
> > complicated and unique relation with its devices.
> > 
> >>
> >> Wouldn't a get_device() before calling the callback and a put_device()
> >> afterwards avoid that problem?
> > 
> > I didn't used the reference count manipulation operations because other similar
> > parts also didn't.  But, if there is no implicit reference count guarantee, it
> > seems those operations are indeed necessary.
> > 
> > That said, as get/put operations only adjust the reference count, those will
> > not make the callback to wait until the linking of the 'backend' and 'blkif' to
> > the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
> > Or, am I missing something?
> 
> No, I think we need a xenbus lock per device which will need to be
> taken in xen_blkbk_probe(), xenbus_dev_remove() and while calling the
> callback.

I also agree that locking should be used at last.  But, as each driver manages
its devices and resources in their way, it could have its unique race
conditions.  And, each unique race condition might have its unique efficient
way to synchronize it.  Therefore, I think the synchronization should be done
by each driver, not by xenbus and thus we should make the callback to be called
per-driver.


Thanks,
SeongJae Park

> 
> 
> Juergen
> 

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-17  7:59             ` SeongJae Park
@ 2019-12-17  8:16               ` Jürgen Groß
  2019-12-17  8:30                 ` SeongJae Park
  0 siblings, 1 reply; 17+ messages in thread
From: Jürgen Groß @ 2019-12-17  8:16 UTC (permalink / raw)
  To: SeongJae Park
  Cc: SeongJae Park, axboe, konrad.wilk, pdurrant, linux-kernel,
	linux-block, xen-devel, roger.pau

On 17.12.19 08:59, SeongJae Park wrote:
> On Tue, 17 Dec 2019 07:23:12 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
> 
>> On 16.12.19 20:48, SeongJae Park wrote:
>>> On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
>>>
>>>> On 16.12.19 17:15, SeongJae Park wrote:
>>>>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
>>>>>
>>>>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
>>>>>>
>>>>>>> From: SeongJae Park <sjpark@amazon.de>
>>>>>>>
>>>>> [...]
>>>>>>> --- 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);
>>>>>>
>>>>>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
>>>>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
>>>>>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
>>>>>>
>>>>>> I will do more test and share results.  Meanwhile, if you have any opinion,
>>>>>> please let me know.
>>>
>>> I reduced system memory and attached bunch of devices in short time so that
>>> memory pressure occurs while device attachments are ongoing.  Under this
>>> circumstance, I was able to see the race.
>>>
>>>>>
>>>>> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
>>>>> concurrency issues could be in other drivers in their way, I suggest to change
>>>>> the reclaim callback ('->reclaim_memory') to be called for each driver instead
>>>>> of each device.  Then, each driver could be able to deal with its concurrency
>>>>> issues by itself.
>>>>
>>>> Hmm, I don't like that. This would need to be changed back in case we
>>>> add per-guest quota.
>>>
>>> Extending this callback in that way would be still not too hard.  We could use
>>> the argument to the callback.  I would keep the argument of the callback to
>>> 'struct device *' as is, and will add a comment saying 'NULL' value of the
>>> argument means every devices.  As an example, xenbus would pass NULL-ending
>>> array of the device pointers that need to free its resources.
>>>
>>> After seeing this race, I am now also thinking it could be better to delegate
>>> detailed control of each device to its driver, as some drivers have some
>>> complicated and unique relation with its devices.
>>>
>>>>
>>>> Wouldn't a get_device() before calling the callback and a put_device()
>>>> afterwards avoid that problem?
>>>
>>> I didn't used the reference count manipulation operations because other similar
>>> parts also didn't.  But, if there is no implicit reference count guarantee, it
>>> seems those operations are indeed necessary.
>>>
>>> That said, as get/put operations only adjust the reference count, those will
>>> not make the callback to wait until the linking of the 'backend' and 'blkif' to
>>> the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
>>> Or, am I missing something?
>>
>> No, I think we need a xenbus lock per device which will need to be
>> taken in xen_blkbk_probe(), xenbus_dev_remove() and while calling the
>> callback.
> 
> I also agree that locking should be used at last.  But, as each driver manages
> its devices and resources in their way, it could have its unique race
> conditions.  And, each unique race condition might have its unique efficient
> way to synchronize it.  Therefore, I think the synchronization should be done
> by each driver, not by xenbus and thus we should make the callback to be called
> per-driver.

xenbus controls creation and removing of devices, so applying locking
at xenbus level is the right thing to do in order to avoid races with
device removal.

In case a backend has further synchronization requirements those have to
be handled at backend level, of course.

In the end you'll need the xenbus level locking anyway in order to avoid
a race when the last backend specific device is just being removed when
the callback is about to be called for that device. Or you'd need to
call try_get_module() before calling into each backend...


Juergen

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-17  8:16               ` Jürgen Groß
@ 2019-12-17  8:30                 ` SeongJae Park
  2019-12-17 16:17                   ` SeongJae Park
  0 siblings, 1 reply; 17+ messages in thread
From: SeongJae Park @ 2019-12-17  8:30 UTC (permalink / raw)
  To: Jürgen Groß
  Cc: SeongJae Park, axboe, linux-block, konrad.wilk, pdurrant,
	linux-kernel, SeongJae Park, xen-devel, roger.pau

On Tue, 17 Dec 2019 09:16:47 +0100 "Jürgen Groß" <jgross@suse.com> wrote:

> On 17.12.19 08:59, SeongJae Park wrote:
> > On Tue, 17 Dec 2019 07:23:12 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
> > 
> >> On 16.12.19 20:48, SeongJae Park wrote:
> >>> On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
> >>>
> >>>> On 16.12.19 17:15, SeongJae Park wrote:
> >>>>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> >>>>>
> >>>>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> >>>>>>
> >>>>>>> From: SeongJae Park <sjpark@amazon.de>
> >>>>>>>
> >>>>> [...]
> >>>>>>> --- 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);
> >>>>>>
> >>>>>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
> >>>>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
> >>>>>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
> >>>>>>
> >>>>>> I will do more test and share results.  Meanwhile, if you have any opinion,
> >>>>>> please let me know.
> >>>
> >>> I reduced system memory and attached bunch of devices in short time so that
> >>> memory pressure occurs while device attachments are ongoing.  Under this
> >>> circumstance, I was able to see the race.
> >>>
> >>>>>
> >>>>> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
> >>>>> concurrency issues could be in other drivers in their way, I suggest to change
> >>>>> the reclaim callback ('->reclaim_memory') to be called for each driver instead
> >>>>> of each device.  Then, each driver could be able to deal with its concurrency
> >>>>> issues by itself.
> >>>>
> >>>> Hmm, I don't like that. This would need to be changed back in case we
> >>>> add per-guest quota.
> >>>
> >>> Extending this callback in that way would be still not too hard.  We could use
> >>> the argument to the callback.  I would keep the argument of the callback to
> >>> 'struct device *' as is, and will add a comment saying 'NULL' value of the
> >>> argument means every devices.  As an example, xenbus would pass NULL-ending
> >>> array of the device pointers that need to free its resources.
> >>>
> >>> After seeing this race, I am now also thinking it could be better to delegate
> >>> detailed control of each device to its driver, as some drivers have some
> >>> complicated and unique relation with its devices.
> >>>
> >>>>
> >>>> Wouldn't a get_device() before calling the callback and a put_device()
> >>>> afterwards avoid that problem?
> >>>
> >>> I didn't used the reference count manipulation operations because other similar
> >>> parts also didn't.  But, if there is no implicit reference count guarantee, it
> >>> seems those operations are indeed necessary.
> >>>
> >>> That said, as get/put operations only adjust the reference count, those will
> >>> not make the callback to wait until the linking of the 'backend' and 'blkif' to
> >>> the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
> >>> Or, am I missing something?
> >>
> >> No, I think we need a xenbus lock per device which will need to be
> >> taken in xen_blkbk_probe(), xenbus_dev_remove() and while calling the
> >> callback.
> > 
> > I also agree that locking should be used at last.  But, as each driver manages
> > its devices and resources in their way, it could have its unique race
> > conditions.  And, each unique race condition might have its unique efficient
> > way to synchronize it.  Therefore, I think the synchronization should be done
> > by each driver, not by xenbus and thus we should make the callback to be called
> > per-driver.
> 
> xenbus controls creation and removing of devices, so applying locking
> at xenbus level is the right thing to do in order to avoid races with
> device removal.
> 
> In case a backend has further synchronization requirements those have to
> be handled at backend level, of course.
> 
> In the end you'll need the xenbus level locking anyway in order to avoid
> a race when the last backend specific device is just being removed when
> the callback is about to be called for that device. Or you'd need to
> call try_get_module() before calling into each backend...

Agreed.  Thank you for your kind explanation of your concerns.


Thanks,
SeongJae Park

> 
> 
> Juergen

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-16 19:48         ` SeongJae Park
  2019-12-17  6:23           ` Jürgen Groß
@ 2019-12-17 11:39           ` Roger Pau Monné
  2019-12-17 13:15             ` SeongJae Park
  1 sibling, 1 reply; 17+ messages in thread
From: Roger Pau Monné @ 2019-12-17 11:39 UTC (permalink / raw)
  To: SeongJae Park
  Cc: jgross, axboe, konrad.wilk, linux-block, xen-devel, pdurrant,
	linux-kernel

On Mon, Dec 16, 2019 at 08:48:03PM +0100, SeongJae Park wrote:
> On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
> 
> > On 16.12.19 17:15, SeongJae Park wrote:
> > > On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > > 
> > >> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > >>
> > >>> From: SeongJae Park <sjpark@amazon.de>
> > >>>
> > > [...]
> > >>> --- 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);
> > >>
> > >> This callback might race with 'xen_blkbk_probe()'.  The race could result in
> > >> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
> > >> 'be' to the 'dev'.  Please _don't merge_ this patch now!
> > >>
> > >> I will do more test and share results.  Meanwhile, if you have any opinion,
> > >> please let me know.
> 
> I reduced system memory and attached bunch of devices in short time so that
> memory pressure occurs while device attachments are ongoing.  Under this
> circumstance, I was able to see the race.
> 
> > > 
> > > Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
> > > concurrency issues could be in other drivers in their way, I suggest to change
> > > the reclaim callback ('->reclaim_memory') to be called for each driver instead
> > > of each device.  Then, each driver could be able to deal with its concurrency
> > > issues by itself.
> > 
> > Hmm, I don't like that. This would need to be changed back in case we
> > add per-guest quota.
> 
> Extending this callback in that way would be still not too hard.  We could use
> the argument to the callback.  I would keep the argument of the callback to
> 'struct device *' as is, and will add a comment saying 'NULL' value of the
> argument means every devices.  As an example, xenbus would pass NULL-ending
> array of the device pointers that need to free its resources.
> 
> After seeing this race, I am now also thinking it could be better to delegate
> detailed control of each device to its driver, as some drivers have some
> complicated and unique relation with its devices.
> 
> > 
> > Wouldn't a get_device() before calling the callback and a put_device()
> > afterwards avoid that problem?
> 
> I didn't used the reference count manipulation operations because other similar
> parts also didn't.  But, if there is no implicit reference count guarantee, it
> seems those operations are indeed necessary.
> 
> That said, as get/put operations only adjust the reference count, those will
> not make the callback to wait until the linking of the 'backend' and 'blkif' to
> the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
> Or, am I missing something?

I would expect the device is not added to the list of backend devices
until the probe hook has finished with a non-error return code. Ie:
bus_for_each_dev should _not_ iterate over devices for which the probe
function hasn't been run to competition without errors.

The same way I would expect the remove hook to first remove the device
from the list of backend devices and then run the remove hook.

blkback uses an ad-hoc reference counting mechanism, but if the above
assumptions are true I think it would be enough to take an extra
reference in xen_blkbk_probe and drop it in xen_blkbk_remove.

Additionally it might be interesting to switch the ad-hoc reference
counting to use get_device/put_device (in a separate patch), but I'm
not sure how feasible that is.

Roger.

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-17 11:39           ` Roger Pau Monné
@ 2019-12-17 13:15             ` SeongJae Park
  2019-12-17 13:51               ` Jürgen Groß
  0 siblings, 1 reply; 17+ messages in thread
From: SeongJae Park @ 2019-12-17 13:15 UTC (permalink / raw)
  To: Roger Pau Monné, jgross
  Cc: SeongJae Park, sjpark, axboe, konrad.wilk, pdurrant,
	linux-kernel, linux-block, xen-devel

On Tue, 17 Dec 2019 12:39:15 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> On Mon, Dec 16, 2019 at 08:48:03PM +0100, SeongJae Park wrote:
> > On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
> > 
> > > On 16.12.19 17:15, SeongJae Park wrote:
> > > > On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > > > 
> > > >> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > > >>
> > > >>> From: SeongJae Park <sjpark@amazon.de>
> > > >>>
> > > > [...]
> > > >>> --- 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);
> > > >>
> > > >> This callback might race with 'xen_blkbk_probe()'.  The race could result in
> > > >> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
> > > >> 'be' to the 'dev'.  Please _don't merge_ this patch now!
> > > >>
> > > >> I will do more test and share results.  Meanwhile, if you have any opinion,
> > > >> please let me know.
> > 
> > I reduced system memory and attached bunch of devices in short time so that
> > memory pressure occurs while device attachments are ongoing.  Under this
> > circumstance, I was able to see the race.
> > 
> > > > 
> > > > Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
> > > > concurrency issues could be in other drivers in their way, I suggest to change
> > > > the reclaim callback ('->reclaim_memory') to be called for each driver instead
> > > > of each device.  Then, each driver could be able to deal with its concurrency
> > > > issues by itself.
> > > 
> > > Hmm, I don't like that. This would need to be changed back in case we
> > > add per-guest quota.
> > 
> > Extending this callback in that way would be still not too hard.  We could use
> > the argument to the callback.  I would keep the argument of the callback to
> > 'struct device *' as is, and will add a comment saying 'NULL' value of the
> > argument means every devices.  As an example, xenbus would pass NULL-ending
> > array of the device pointers that need to free its resources.
> > 
> > After seeing this race, I am now also thinking it could be better to delegate
> > detailed control of each device to its driver, as some drivers have some
> > complicated and unique relation with its devices.
> > 
> > > 
> > > Wouldn't a get_device() before calling the callback and a put_device()
> > > afterwards avoid that problem?
> > 
> > I didn't used the reference count manipulation operations because other similar
> > parts also didn't.  But, if there is no implicit reference count guarantee, it
> > seems those operations are indeed necessary.
> > 
> > That said, as get/put operations only adjust the reference count, those will
> > not make the callback to wait until the linking of the 'backend' and 'blkif' to
> > the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
> > Or, am I missing something?
> 
> I would expect the device is not added to the list of backend devices
> until the probe hook has finished with a non-error return code. Ie:
> bus_for_each_dev should _not_ iterate over devices for which the probe
> function hasn't been run to competition without errors.
> 
> The same way I would expect the remove hook to first remove the device
> from the list of backend devices and then run the remove hook.
> 
> blkback uses an ad-hoc reference counting mechanism, but if the above
> assumptions are true I think it would be enough to take an extra
> reference in xen_blkbk_probe and drop it in xen_blkbk_remove.

Well, if the assumption is true, wouldn't the Juergen's approach solved the
problem?  As previously said, I tried the approach but failed to solve this
race.  The assumption is wrong or I missed something.  I think Juergen also
think the assumption is not true as he suggested use of locking but not sure.
Juergen, if I misunderstood, please let me know.


Thanks,
SeongJae Park

> 
> Additionally it might be interesting to switch the ad-hoc reference
> counting to use get_device/put_device (in a separate patch), but I'm
> not sure how feasible that is.
> 
> Roger.

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-17 13:15             ` SeongJae Park
@ 2019-12-17 13:51               ` Jürgen Groß
  0 siblings, 0 replies; 17+ messages in thread
From: Jürgen Groß @ 2019-12-17 13:51 UTC (permalink / raw)
  To: SeongJae Park, Roger Pau Monné
  Cc: SeongJae Park, sjpark, axboe, konrad.wilk, pdurrant,
	linux-kernel, linux-block, xen-devel

On 17.12.19 14:15, SeongJae Park wrote:
> On Tue, 17 Dec 2019 12:39:15 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
> 
>> On Mon, Dec 16, 2019 at 08:48:03PM +0100, SeongJae Park wrote:
>>> On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
>>>
>>>> On 16.12.19 17:15, SeongJae Park wrote:
>>>>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
>>>>>
>>>>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
>>>>>>
>>>>>>> From: SeongJae Park <sjpark@amazon.de>
>>>>>>>
>>>>> [...]
>>>>>>> --- 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);
>>>>>>
>>>>>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
>>>>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
>>>>>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
>>>>>>
>>>>>> I will do more test and share results.  Meanwhile, if you have any opinion,
>>>>>> please let me know.
>>>
>>> I reduced system memory and attached bunch of devices in short time so that
>>> memory pressure occurs while device attachments are ongoing.  Under this
>>> circumstance, I was able to see the race.
>>>
>>>>>
>>>>> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
>>>>> concurrency issues could be in other drivers in their way, I suggest to change
>>>>> the reclaim callback ('->reclaim_memory') to be called for each driver instead
>>>>> of each device.  Then, each driver could be able to deal with its concurrency
>>>>> issues by itself.
>>>>
>>>> Hmm, I don't like that. This would need to be changed back in case we
>>>> add per-guest quota.
>>>
>>> Extending this callback in that way would be still not too hard.  We could use
>>> the argument to the callback.  I would keep the argument of the callback to
>>> 'struct device *' as is, and will add a comment saying 'NULL' value of the
>>> argument means every devices.  As an example, xenbus would pass NULL-ending
>>> array of the device pointers that need to free its resources.
>>>
>>> After seeing this race, I am now also thinking it could be better to delegate
>>> detailed control of each device to its driver, as some drivers have some
>>> complicated and unique relation with its devices.
>>>
>>>>
>>>> Wouldn't a get_device() before calling the callback and a put_device()
>>>> afterwards avoid that problem?
>>>
>>> I didn't used the reference count manipulation operations because other similar
>>> parts also didn't.  But, if there is no implicit reference count guarantee, it
>>> seems those operations are indeed necessary.
>>>
>>> That said, as get/put operations only adjust the reference count, those will
>>> not make the callback to wait until the linking of the 'backend' and 'blkif' to
>>> the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
>>> Or, am I missing something?
>>
>> I would expect the device is not added to the list of backend devices
>> until the probe hook has finished with a non-error return code. Ie:
>> bus_for_each_dev should _not_ iterate over devices for which the probe
>> function hasn't been run to competition without errors.
>>
>> The same way I would expect the remove hook to first remove the device
>> from the list of backend devices and then run the remove hook.
>>
>> blkback uses an ad-hoc reference counting mechanism, but if the above
>> assumptions are true I think it would be enough to take an extra
>> reference in xen_blkbk_probe and drop it in xen_blkbk_remove.
> 
> Well, if the assumption is true, wouldn't the Juergen's approach solved the
> problem?  As previously said, I tried the approach but failed to solve this
> race.  The assumption is wrong or I missed something.  I think Juergen also
> think the assumption is not true as he suggested use of locking but not sure.
> Juergen, if I misunderstood, please let me know.

bus_for_each_dev() does no locking at all. All it does is
taking krefs on the iterated objects in order to avoid them
to be freed under its feet.


Juergen

^ permalink raw reply	[flat|nested] 17+ messages in thread

* Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
  2019-12-17  8:30                 ` SeongJae Park
@ 2019-12-17 16:17                   ` SeongJae Park
  0 siblings, 0 replies; 17+ messages in thread
From: SeongJae Park @ 2019-12-17 16:17 UTC (permalink / raw)
  To: SeongJae Park
  Cc: Jürgen Groß,
	axboe, SeongJae Park, konrad.wilk, pdurrant, linux-kernel,
	linux-block, xen-devel, roger.pau

On Tue, 17 Dec 2019 09:30:32 +0100 SeongJae Park <sjpark@amazon.com> wrote:

> On Tue, 17 Dec 2019 09:16:47 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
> 
> > On 17.12.19 08:59, SeongJae Park wrote:
> > > On Tue, 17 Dec 2019 07:23:12 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
> > > 
> > >> On 16.12.19 20:48, SeongJae Park wrote:
> > >>> On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
> > >>>
> > >>>> On 16.12.19 17:15, SeongJae Park wrote:
> > >>>>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > >>>>>
> > >>>>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > >>>>>>
> > >>>>>>> From: SeongJae Park <sjpark@amazon.de>
> > >>>>>>>
> > >>>>> [...]
> > >>>>>>> --- 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);
> > >>>>>>
> > >>>>>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
> > >>>>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
> > >>>>>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
> > >>>>>>
> > >>>>>> I will do more test and share results.  Meanwhile, if you have any opinion,
> > >>>>>> please let me know.
> > >>>
> > >>> I reduced system memory and attached bunch of devices in short time so that
> > >>> memory pressure occurs while device attachments are ongoing.  Under this
> > >>> circumstance, I was able to see the race.
> > >>>
> > >>>>>
> > >>>>> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
> > >>>>> concurrency issues could be in other drivers in their way, I suggest to change
> > >>>>> the reclaim callback ('->reclaim_memory') to be called for each driver instead
> > >>>>> of each device.  Then, each driver could be able to deal with its concurrency
> > >>>>> issues by itself.
> > >>>>
> > >>>> Hmm, I don't like that. This would need to be changed back in case we
> > >>>> add per-guest quota.
> > >>>
> > >>> Extending this callback in that way would be still not too hard.  We could use
> > >>> the argument to the callback.  I would keep the argument of the callback to
> > >>> 'struct device *' as is, and will add a comment saying 'NULL' value of the
> > >>> argument means every devices.  As an example, xenbus would pass NULL-ending
> > >>> array of the device pointers that need to free its resources.
> > >>>
> > >>> After seeing this race, I am now also thinking it could be better to delegate
> > >>> detailed control of each device to its driver, as some drivers have some
> > >>> complicated and unique relation with its devices.
> > >>>
> > >>>>
> > >>>> Wouldn't a get_device() before calling the callback and a put_device()
> > >>>> afterwards avoid that problem?
> > >>>
> > >>> I didn't used the reference count manipulation operations because other similar
> > >>> parts also didn't.  But, if there is no implicit reference count guarantee, it
> > >>> seems those operations are indeed necessary.
> > >>>
> > >>> That said, as get/put operations only adjust the reference count, those will
> > >>> not make the callback to wait until the linking of the 'backend' and 'blkif' to
> > >>> the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
> > >>> Or, am I missing something?
> > >>
> > >> No, I think we need a xenbus lock per device which will need to be
> > >> taken in xen_blkbk_probe(), xenbus_dev_remove() and while calling the
> > >> callback.
> > > 
> > > I also agree that locking should be used at last.  But, as each driver manages
> > > its devices and resources in their way, it could have its unique race
> > > conditions.  And, each unique race condition might have its unique efficient
> > > way to synchronize it.  Therefore, I think the synchronization should be done
> > > by each driver, not by xenbus and thus we should make the callback to be called
> > > per-driver.
> > 
> > xenbus controls creation and removing of devices, so applying locking
> > at xenbus level is the right thing to do in order to avoid races with
> > device removal.
> > 
> > In case a backend has further synchronization requirements those have to
> > be handled at backend level, of course.
> > 
> > In the end you'll need the xenbus level locking anyway in order to avoid
> > a race when the last backend specific device is just being removed when
> > the callback is about to be called for that device. Or you'd need to
> > call try_get_module() before calling into each backend...
> 
> Agreed.  Thank you for your kind explanation of your concerns.

Just posted the v11 patchset[1], which is based on your idea and passed my
test.

[1] https://lore.kernel.org/xen-devel/20191217160748.693-1-sjpark@amazon.com/


Thanks,
SeongJae Park

> 
> 
> Thanks,
> SeongJae Park
> 
> > 
> > 
> > Juergen

^ permalink raw reply	[flat|nested] 17+ messages in thread

end of thread, other threads:[~2019-12-17 16:18 UTC | newest]

Thread overview: 17+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-12-16 12:45 [PATCH v10 0/4] xenbus/backend: Add a memory pressure handler callback SeongJae Park
2019-12-16 12:45 ` [PATCH v10 1/4] xenbus/backend: Add " SeongJae Park
2019-12-16 12:45 ` [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected SeongJae Park
2019-12-16 14:37   ` [Xen-devel] " SeongJae Park
2019-12-16 16:15     ` SeongJae Park
2019-12-16 16:23       ` Jürgen Groß
2019-12-16 19:48         ` SeongJae Park
2019-12-17  6:23           ` Jürgen Groß
2019-12-17  7:59             ` SeongJae Park
2019-12-17  8:16               ` Jürgen Groß
2019-12-17  8:30                 ` SeongJae Park
2019-12-17 16:17                   ` SeongJae Park
2019-12-17 11:39           ` Roger Pau Monné
2019-12-17 13:15             ` SeongJae Park
2019-12-17 13:51               ` Jürgen Groß
2019-12-16 12:45 ` [PATCH v10 3/4] xen/blkback: Remove unnecessary static variable name prefixes SeongJae Park
2019-12-16 12:45 ` [PATCH v10 4/4] xen/blkback: Consistently insert one empty line between functions SeongJae Park

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