Re: [PATCH] xen/tmem: Don't over-write tmem_frontswap_poolid after tmem_frontswap_init set it.

[Bob Liu <bob.liu@oracle.com>]

On 06/08/2013 03:45 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jun 07, 2013 at 12:29:15PM -0700, Konrad Rzeszutek Wilk wrote:
> 
> CC-ing Bob.
> 
>> Commit 10a7a0771399a57a297fca9615450dbb3f88081a ("xen: tmem: enable Xen
>> tmem shim to be built/loaded as a module") allows the tmem module
>> to be loaded any time. For this work the frontswap API had to
>> be able to asynchronously to call tmem_frontswap_init before
>> or after the swap image had been set. That was added in git
>> commit 905cd0e1bf9ffe82d6906a01fd974ea0f70be97a
>> ("mm: frontswap: lazy initialization to allow tmem backends to build/run as modules").
>>
>> Which means we could do this (The common case):
>>
>>  modprobe tmem		[so calls frontswap_register_ops, no ->init]
>> 			 modifies tmem_frontswap_poolid = -1
>>  swapon /dev/xvda1	[__frontswap_init, calls -> init, tmem_frontswap_poolid is
>> 			 < 0 so tmem hypercall done]
>>
>> Or the failing one:
>>
>>  swapon /dev/xvda1	[calls __frontswap_init, sets the need_init bitmap]
>>  modprobe tmem		[calls frontswap_register_ops, -->init calls, finds out
>> 			tmem_frontswap_poolid is 0, does not make a hypercall.
>> 			Later in the module_init, sets tmem_frontswap_poolid=-1]
>>
>> Which meant that in the failing case we would not call the hypercall
>> to initialize the pool and never be able to make any frontswap
>> backend calls.
>>
>> Moving the frontswap_register_ops after setting the tmem_frontswap_poolid
>> fixes it.
>>
>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Nice catch!
Reviewed-by: Bob Liu <bob.liu@oracle.com>

>> ---
>>  drivers/xen/tmem.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
>> index cc072c6..0f0493c 100644
>> --- a/drivers/xen/tmem.c
>> +++ b/drivers/xen/tmem.c
>> @@ -379,10 +379,10 @@ static int xen_tmem_init(void)
>>  #ifdef CONFIG_FRONTSWAP
>>  	if (tmem_enabled && frontswap) {
>>  		char *s = "";
>> -		struct frontswap_ops *old_ops =
>> -			frontswap_register_ops(&tmem_frontswap_ops);
>> +		struct frontswap_ops *old_ops;
>>  
>>  		tmem_frontswap_poolid = -1;
>> +		old_ops = frontswap_register_ops(&tmem_frontswap_ops);
>>  		if (IS_ERR(old_ops) || old_ops) {
>>  			if (IS_ERR(old_ops))
>>  				return PTR_ERR(old_ops);
>> -- 
>> 1.8.1.4
>>

-- 
Regards,
-Bob