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Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected

On 17.12.19 14:15, SeongJae Park wrote:
On Tue, 17 Dec 2019 12:39:15 +0100 "Roger Pau Monné" <roger.pau@xxxxxxxxxx> 

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@xxxxxxxxxx> wrote:

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

From: SeongJae Park <sjpark@xxxxxxxxx>

--- 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;
+               buffer_squeeze_duration_ms, int, 0644);
+"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.


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