[PATCH v6 0/2] xenbus/backend: Add a memory pressure handler callback

[PATCH v6 0/2] 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 use it to mitigate the problem in
'xen-blkback' (patch 2).  The third patch is a trivial cleanup of
variable names.

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_v6


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

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 (2):
  xenbus/backend: Add memory pressure handler callback
  xen/blkback: Squeeze page pools if a memory pressure is detected

 drivers/block/xen-blkback/blkback.c       | 23 +++++++++++++++--
 drivers/block/xen-blkback/common.h        |  1 +
 drivers/block/xen-blkback/xenbus.c        |  3 ++-
 drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 5 files changed, 56 insertions(+), 3 deletions(-)

-- 
2.17.1

[PATCH v6 1/3] xenbus/backend: Add 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 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>
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..aedbe2198de5 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 xenbus_backend_reclaim(struct device *dev, void *data)
+{
+	struct xenbus_driver *drv;
+
+	if (!dev->driver)
+		return 0;
+	drv = to_xenbus_driver(dev->driver);
+	if (drv && drv->reclaim)
+		drv->reclaim(to_xenbus_device(dev));
+	return 0;
+}
+
+/*
+ * Returns 0 always because we are using shrinker to only detect memory
+ * pressure.
+ */
+static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
+				struct shrink_control *sc)
+{
+	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
+			xenbus_backend_reclaim);
+	return 0;
+}
+
+static struct shrinker xenbus_backend_shrinker = {
+	.count_objects = xenbus_backend_shrink_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(&xenbus_backend_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..196260017666 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)(struct xenbus_device *dev);
 };
 
 static inline struct xenbus_driver *to_xenbus_driver(struct device_driver *drv)
-- 
2.17.1

[PATCH v6 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

Each `blkif` has a free pages pool for the grant mapping.  The size of
the pool starts from zero and be 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, if a system (maybe mistakenly) allows `blkfront` running
guests to attach a large number of devices, the guests could cause a
memory pressure in the `blkback` running guest by attaching a large
number of block devices and inducing I/O.  System administrators can
avoid such problematic situations 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 the 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 a 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 a 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.  We 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).

For the I/O performance measurement, I use a simple `dd` command.

Default Performance
-------------------

    [dom0]# echo 1024 > /sys/module/xen_blkback/parameters/max_buffer_pages
    [instance]$ for i in {1..5}; do dd if=/dev/zero of=file \
                                       bs=4k count=$((256*512)); sync; done
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8827 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8781 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8737 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8702 s, 38.7 MB/s

Worst-case Performance
----------------------

    [dom0]# echo 0 > /sys/module/xen_blkback/parameters/max_buffer_pages
    [instance]$ for i in {1..5}; do dd if=/dev/zero of=file \
                                       bs=4k count=$((256*512)); sync; done
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.878 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8746 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8786 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8749 s, 38.7 MB/s

In short, even worst case squeezing makes no visible performance
degradation on this test machine.  I think this is due to the slow speed
of the I/O devices I used.  In other words, the additional page
allocation overhead is hidden under the much slower I/O latency.
Nevertheless, pleaset note that this is just a very simple and minimal
test using a slow block device.  On systems using fast block devices
such as ramdisks or NVMe SSDs, the results could be very different.  If
you are in such cases, you should control the squeezing duration via the
module parameter.

Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/block/xen-blkback/blkback.c | 22 ++++++++++++++++++++--
 drivers/block/xen-blkback/common.h  |  1 +
 drivers/block/xen-blkback/xenbus.c  |  3 ++-
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index fd1e19f1a49f..b493c306e84f 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -142,6 +142,21 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
 		HZ * xen_blkif_pgrant_timeout);
 }
 
+/* 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");
+
+static unsigned long buffer_squeeze_end;
+
+void xen_blkbk_reclaim(struct xenbus_device *dev)
+{
+	buffer_squeeze_end = jiffies +
+		msecs_to_jiffies(buffer_squeeze_duration_ms);
+}
+
 static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
 {
 	unsigned long flags;
@@ -656,8 +671,11 @@ 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, 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..8a3195d2dca7 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -383,6 +383,7 @@ irqreturn_t xen_blkif_be_int(int irq, void *dev_id);
 int xen_blkif_schedule(void *arg);
 int xen_blkif_purge_persistent(void *arg);
 void xen_blkbk_free_caches(struct xen_blkif_ring *ring);
+void xen_blkbk_reclaim(struct xenbus_device *dev);
 
 int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
 			      struct backend_info *be, int state);
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index b90dbcd99c03..b596c6e8b006 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -1115,7 +1115,8 @@ 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 = xen_blkbk_reclaim,
 };
 
 int xen_blkif_xenbus_init(void)
-- 
2.17.1

[PATCH v6 3/3] xen/blkback: Remove unnecessary static variable name prefixes

[SeongJae Park]

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

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 b493c306e84f..f690373669b8 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);
 }
 
 /* Once a memory pressure is detected, squeeze free page pools for a while. */
@@ -249,7 +248,7 @@ 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;
@@ -412,14 +411,13 @@ 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);
@@ -614,8 +612,7 @@ 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;
@@ -675,7 +672,7 @@ int xen_blkif_schedule(void *arg)
 		if (time_before(jiffies, 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);
@@ -902,7 +899,7 @@ 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.
@@ -929,7 +926,7 @@ 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

Re: [PATCH v6 1/3] xenbus/backend: Add memory pressure handler callback

[Roger Pau Monné]

On Wed, Dec 11, 2019 at 04:26:57AM +0000, SeongJae Park wrote:
> 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
                                                              ^ s/the//
> 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
                       ^ s/a//
> every backend driver to volunarily release its memory.
> 
> Note that it would be able to improve the callback facility for more
                        ^ possible
> sophisticated handlings of general pressures.  For example, it would be
                ^ handling of resource starvation.
> 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>
> 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..aedbe2198de5 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 xenbus_backend_reclaim(struct device *dev, void *data)

No need for the xenbus_ prefix since it's a static function, ie:
backend_reclaim_memory should be fine IMO.

> +{
> +	struct xenbus_driver *drv;

I've asked for this variable to be constified in v5, is it not
possible to make it const?

> +
> +	if (!dev->driver)
> +		return 0;
> +	drv = to_xenbus_driver(dev->driver);
> +	if (drv && drv->reclaim)
> +		drv->reclaim(to_xenbus_device(dev));
> +	return 0;
> +}
> +
> +/*
> + * Returns 0 always because we are using shrinker to only detect memory
> + * pressure.
> + */
> +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> +				struct shrink_control *sc)
> +{
> +	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> +			xenbus_backend_reclaim);
> +	return 0;
> +}
> +
> +static struct shrinker xenbus_backend_shrinker = {

I would drop the xenbus prefix, and I think it's not possible to
constify this due to register_shrinker expecting a non-const
parameter?

> +	.count_objects = xenbus_backend_shrink_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(&xenbus_backend_shrinker))
> +		pr_warn("shrinker registration failed\n");

Can you add a xenbus prefix to the error message? Or else it's hard to
know which subsystem is complaining when you see such message on the
log. ie: "xenbus: shrinker ..."

> +
>  	return 0;
>  }
>  subsys_initcall(xenbus_probe_backend_init);
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index 869c816d5f8c..196260017666 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)(struct xenbus_device *dev);

reclaim_memory (if Juergen agrees).

Thanks, Roger.

Re: [PATCH v6 3/3] xen/blkback: Remove unnecessary static variable name prefixes

[Roger Pau Monné]

On Wed, Dec 11, 2019 at 04:27:33AM +0000, SeongJae Park wrote:
> A few of static variables in blkback have 'xen_blkif_' prefix, though it
> is unnecessary for static variables.  This commit removes such prefixes.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>

Thanks.

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Re: Re: [PATCH v6 1/3] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

On Wed, 11 Dec 2019 12:46:51 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> > 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
>                                                               ^ s/the//
> > 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
>                        ^ s/a//
> > every backend driver to volunarily release its memory.
> > 
> > Note that it would be able to improve the callback facility for more
>                         ^ possible
> > sophisticated handlings of general pressures.  For example, it would be
>                 ^ handling of resource starvation.
> > 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>
> > 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..aedbe2198de5 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 xenbus_backend_reclaim(struct device *dev, void *data)
> 
> No need for the xenbus_ prefix since it's a static function, ie:
> backend_reclaim_memory should be fine IMO.

Agreed, will change the name in the next version.

> 
> > +{
> > +	struct xenbus_driver *drv;
> 
> I've asked for this variable to be constified in v5, is it not
> possible to make it const?

Sorry, my mistake...  I was difinitely too hurry.

> 
> > +
> > +	if (!dev->driver)
> > +		return 0;
> > +	drv = to_xenbus_driver(dev->driver);
> > +	if (drv && drv->reclaim)
> > +		drv->reclaim(to_xenbus_device(dev));
> > +	return 0;
> > +}
> > +
> > +/*
> > + * Returns 0 always because we are using shrinker to only detect memory
> > + * pressure.
> > + */
> > +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> > +				struct shrink_control *sc)
> > +{
> > +	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> > +			xenbus_backend_reclaim);
> > +	return 0;
> > +}
> > +
> > +static struct shrinker xenbus_backend_shrinker = {
> 
> I would drop the xenbus prefix, and I think it's not possible to
> constify this due to register_shrinker expecting a non-const
> parameter?

Yes, constifying it results in another compile warning.  Will drop the prefix.

> 
> > +	.count_objects = xenbus_backend_shrink_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(&xenbus_backend_shrinker))
> > +		pr_warn("shrinker registration failed\n");
> 
> Can you add a xenbus prefix to the error message? Or else it's hard to
> know which subsystem is complaining when you see such message on the
> log. ie: "xenbus: shrinker ..."

Because we have #define `pr_fmt(fmt) KBUILD_MODNAME ": " fmt` in the beginning
of the file, the message will have a proper prefix.

> 
> > +
> >  	return 0;
> >  }
> >  subsys_initcall(xenbus_probe_backend_init);
> > diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> > index 869c816d5f8c..196260017666 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)(struct xenbus_device *dev);
> 
> reclaim_memory (if Juergen agrees).

Okay.


Thanks,
SeongJae Park

> 
> Thanks, Roger.
> 

Re: [PATCH v6 1/3] xenbus/backend: Add memory pressure handler callback

[Jürgen Groß]

On 11.12.19 12:46, Roger Pau Monné wrote:
> On Wed, Dec 11, 2019 at 04:26:57AM +0000, SeongJae Park wrote:
>> +
>>   	return 0;
>>   }
>>   subsys_initcall(xenbus_probe_backend_init);
>> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
>> index 869c816d5f8c..196260017666 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)(struct xenbus_device *dev);
> 
> reclaim_memory (if Juergen agrees).

I do agree.


Juergen