linux-kernel - Re: [Xen-devel] [PATCHv1 6/8] xen/balloon: only hotplug additional memory if required

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Message-ID: <558D13BF.9030907@citrix.com>
Date:	Fri, 26 Jun 2015 09:56:31 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Daniel Kiper <daniel.kiper@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	<linux-mm@...ck.org>, <xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCHv1 6/8] xen/balloon: only hotplug additional
 memory if required

On 25/06/15 22:18, Daniel Kiper wrote:
> On Thu, Jun 25, 2015 at 06:11:01PM +0100, David Vrabel wrote:
>> Now that we track the total number of pages (included hotplugged
>> regions), it is easy to determine if more memory needs to be
>> hotplugged.
>>
>> Signed-off-by: David Vrabel <david.vrabel@...rix.com>
>> ---
>>  drivers/xen/balloon.c |   16 +++++++++++++---
>>  1 file changed, 13 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
>> index 960ac79..dd41da8 100644
>> --- a/drivers/xen/balloon.c
>> +++ b/drivers/xen/balloon.c
>> @@ -241,12 +241,22 @@ static void release_memory_resource(struct resource *resource)
>>   * bit set). Real size of added memory is established at page onlining stage.
>>   */
>>
>> -static enum bp_state reserve_additional_memory(long credit)
>> +static enum bp_state reserve_additional_memory(void)
>>  {
>> +	long credit;
>>  	struct resource *resource;
>>  	int nid, rc;
>>  	unsigned long balloon_hotplug;
>>
>> +	credit = balloon_stats.target_pages - balloon_stats.total_pages;
>> +
>> +	/*
>> +	 * Already hotplugged enough pages?  Wait for them to be
>> +	 * onlined.
>> +	 */
> 
> Comment is wrong or at least misleading. Both values does not depend on onlining.

If we get here and credit <=0 then the balloon is empty and we have
already hotplugged enough sections to reach target.  We need to wait for
userspace to online the sections that already exist.

>> +	if (credit <= 0)
>> +		return BP_EAGAIN;
> 
> Not BP_EAGAIN for sure. It should be BP_DONE but then balloon_process() will go
> into loop until memory is onlined at least up to balloon_stats.target_pages.
> BP_ECANCELED does work but it is misleading because it is not an error. So, maybe
> we should introduce BP_STOP (or something like that) which works like BP_ECANCELED
> and is not BP_ECANCELED.

We don't want to spin while waiting for userspace to online a new
section so BP_EAGAIN is correct here as it causes the balloon process to
be rescheduled at a later time.

David
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