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Re: [Xen-devel] [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

On 09.12.19 20:43, SeongJae Park wrote:
From: SeongJae Park <sjpark@xxxxxxxxx>

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 lacks a

To mitigate such problems, this commit adds a memory reclaim callback to
'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
a memory pressure and request specific domains of specific backend
drivers which causing the given pressure to voluntarily release its

That said, this commit simply requests every callback registered driver
to release its memory for every domain, rather than issueing the
requests to the drivers and domain in charge.  Such things would be a
future work.  Also, this commit focuses on memory only.  However, it
would be ablt to be extended for general resources.

Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
  drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
  include/xen/xenbus.h                      |  1 +
  2 files changed, 32 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c 
index b0bed4faf44c..cd5fd1cd8de3 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block 
        return NOTIFY_DONE;
+static int xenbus_backend_reclaim(struct device *dev, void *data)
+       struct xenbus_driver *drv;
+       if (!dev->driver)
+               return -ENOENT;
+       drv = to_xenbus_driver(dev->driver);
+       if (drv && drv->reclaim)
+               drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);

Oh, sorry for first requesting you to add the domid as a parameter,
but now I realize this could be handled in the xenbus driver, as
struct xenbus_device already contains the otherend_id.

Would you mind dropping the parameter again, please?


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