linux-kernel - Re: [PATCH] xen/tmem: Don't over-write tmem_frontswap_poolid after tmem_frontswap

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Message-ID: <20130607194534.GB21543@phenom.dumpdata.com>
Date:	Fri, 7 Jun 2013 15:45:34 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	bob.liu@...cle.com
Cc:	dan.magenheimer@...cle.com
Subject: Re: [PATCH] xen/tmem: Don't over-write tmem_frontswap_poolid after
 tmem_frontswap_init set it.

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@...cle.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
> 
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