linux-kernel - Re: [PATCH v3] xen/balloon: don't online new memory initially

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Message-ID: <4e4bcc27-f0c2-7aa6-ea4c-bcc1176b6052@suse.com>
Date:   Tue, 18 Jul 2017 18:12:58 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v3] xen/balloon: don't online new memory initially

On 18/07/17 18:08, Boris Ostrovsky wrote:
> On 07/10/2017 04:10 AM, Juergen Gross wrote:
>> When setting up the Xenstore watch for the memory target size the new
>> watch will fire at once. Don't try to reach the configured target size
>> by onlining new memory in this case, as the current memory size will
>> be smaller in almost all cases due to e.g. BIOS reserved pages.
>>
>> Onlining new memory will lead to more problems e.g. undesired conflicts
>> with NVMe devices meant to be operated as block devices.
>>
>> Instead remember the difference between target size and current size
>> when the watch fires for the first time and apply it to any further
>> size changes, too.
> 
> I don't think I understand how deferring setting the target until next
> watch firing will help with the original problem. Could you explain?

The difference of target and current size will be evaluated at the
time the watch fires for the first time. This difference is subtracted
from the target on later watch events.


Juergen

> 
> Thanks.
> -boris
> 
> 
>>
>> In order to avoid races between balloon.c and xen-balloon.c init calls
>> do the xen-balloon.c initialization from balloon.c.
>>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>>  drivers/xen/balloon.c     |  3 +++
>>  drivers/xen/xen-balloon.c | 22 ++++++++++++----------
>>  include/xen/balloon.h     |  8 ++++++++
>>  3 files changed, 23 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>> index 50dcb68d8070..ab609255a0f3 100644
>> --- a/drivers/xen/balloon.c
>> +++ b/drivers/xen/balloon.c
>> @@ -780,6 +780,9 @@ static int __init balloon_init(void)
>>  	}
>>  #endif
>>  
>> +	/* Init the xen-balloon driver. */
>> +	xen_balloon_init();
>> +
>>  	return 0;
>>  }
>>  subsys_initcall(balloon_init);
>> diff --git a/drivers/xen/xen-balloon.c b/drivers/xen/xen-balloon.c
>> index e7715cb62eef..e89136ab851e 100644
>> --- a/drivers/xen/xen-balloon.c
>> +++ b/drivers/xen/xen-balloon.c
>> @@ -59,6 +59,8 @@ static void watch_target(struct xenbus_watch *watch,
>>  {
>>  	unsigned long long new_target;
>>  	int err;
>> +	static bool watch_fired;
>> +	static long target_diff;
>>  
>>  	err = xenbus_scanf(XBT_NIL, "memory", "target", "%llu", &new_target);
>>  	if (err != 1) {
>> @@ -69,7 +71,14 @@ static void watch_target(struct xenbus_watch *watch,
>>  	/* The given memory/target value is in KiB, so it needs converting to
>>  	 * pages. PAGE_SHIFT converts bytes to pages, hence PAGE_SHIFT - 10.
>>  	 */
>> -	balloon_set_new_target(new_target >> (PAGE_SHIFT - 10));
>> +	new_target >>= PAGE_SHIFT - 10;
>> +	if (watch_fired) {
>> +		balloon_set_new_target(new_target - target_diff);
>> +		return;
>> +	}
>> +
>> +	watch_fired = true;
>> +	target_diff = new_target - balloon_stats.target_pages;
>>  }
>>  static struct xenbus_watch target_watch = {
>>  	.node = "memory/target",
>> @@ -94,22 +103,15 @@ static struct notifier_block xenstore_notifier = {
>>  	.notifier_call = balloon_init_watcher,
>>  };
>>  
>> -static int __init balloon_init(void)
>> +void xen_balloon_init(void)
>>  {
>> -	if (!xen_domain())
>> -		return -ENODEV;
>> -
>> -	pr_info("Initialising balloon driver\n");
>> -
>>  	register_balloon(&balloon_dev);
>>  
>>  	register_xen_selfballooning(&balloon_dev);
>>  
>>  	register_xenstore_notifier(&xenstore_notifier);
>> -
>> -	return 0;
>>  }
>> -subsys_initcall(balloon_init);
>> +EXPORT_SYMBOL_GPL(xen_balloon_init);
>>  
>>  #define BALLOON_SHOW(name, format, args...)				\
>>  	static ssize_t show_##name(struct device *dev,			\
>> diff --git a/include/xen/balloon.h b/include/xen/balloon.h
>> index d1767dfb0d95..8906361bb50c 100644
>> --- a/include/xen/balloon.h
>> +++ b/include/xen/balloon.h
>> @@ -35,3 +35,11 @@ static inline int register_xen_selfballooning(struct device *dev)
>>  	return -ENOSYS;
>>  }
>>  #endif
>> +
>> +#ifdef CONFIG_XEN_BALLOON
>> +void xen_balloon_init(void);
>> +#else
>> +static inline void xen_balloon_init(void)
>> +{
>> +}
>> +#endif
> 
>