linux-kernel - Re: [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve

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Message-ID: <55AF5CFC.1080303@intel.com>
Date:	Wed, 22 Jul 2015 17:06:04 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, bp@...e.de, toshi.kani@...com,
	jgross@...e.com, mcgrof@...e.com,
	"mnipxh@....com" <mnipxh@....com>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH V3] x86/mm/pat: Do a small optimization and fix in reserve_memtype

hi, Borislav
	thanks for your kind reply. :)

On 2015年07月22日 15:46, Borislav Petkov wrote:
> On Wed, Jul 22, 2015 at 01:38:48PM +0800, Pan Xinhui wrote:
>> From: Pan Xinhui <xinhuix.pan@...el.com>
>>
>> It's more reasonable to unlock memtype_lock right after
>> rbt_memtype_check_insert. memtype_lock protects all data stored in
>> rb-tree from multiple access. It's not cool to call kfree, pr_info, etc
>> with this lock held. So move spin_unlock a little ahead.
>>
>> If *new* succeed to be stored into the rb-tree, we might hit panic.
>> Because we access *new* in dprintk "cattr_name(new->type)". Data stored
>> in the rb-tree might be freed at any possbile time. It's abviously wrong
>> to access such data without lock held. As new->type might be changed in
>> rbt_memtype_check_insert, so save new->type to actual_type, then use
>> actual_type in dprintk.
>>
>> Signed-off-by: Pan Xinhui <xinhuix.pan@...el.com>
>> ---
>> change from v2:
>> 	update comments.
>> change from V1:
>> 	fix an access of *new* without memtype_lock held.
>> ---
>>  arch/x86/mm/pat.c | 15 +++++++++------
>>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> This patch still doesn't update the comments over memtype_lock.
> 
sorry for that.
how about:
memtype_lock protects the rb-tree root and the rb-nodes which is a field of memtype from delete/add/lookup in a race.

>>
>> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
>> index 188e3e0..894a096 100644
>> --- a/arch/x86/mm/pat.c
>> +++ b/arch/x86/mm/pat.c
>> @@ -538,22 +538,25 @@ int reserve_memtype(u64 start, u64 end, enum page_cache_mode req_type,
>>  	new->type	= actual_type;
>>  
>>  	spin_lock(&memtype_lock);
>> -
>>  	err = rbt_memtype_check_insert(new, new_type);
>> +	/*
>> +	 * new->type might be changed in rbt_memtype_check_insert.
>> +	 * So save new->type to actual_type as dprintk uses it.
>> +	 * We are not allowed to touch new after unlocking memtype_lock.
>> +	 */
>> +	actual_type = new->type;
> 
> We already assign actual_type to new->type above. I think the dprintk
> needs actual_type and not what new->type has been changed to as that is
> in new_type.
> 
Actually I have same questions. I find these output logs are added in commit: 6997ab4982a29925e79f72c3a59823cf944c3529(x86: add PAT related debug prints)
In the past, *new_type == actual_type == new->type on success. codes are below. author use actual_type there.
376     if (ret_type) {
377         printk(
378     "reserve_memtype added 0x%Lx-0x%Lx, track %s, req %s, ret %s\n",
379             start, end, cattr_name(actual_type),
380             cattr_name(req_type), cattr_name(*ret_type));
381     } else {
382         printk(
383     "reserve_memtype added 0x%Lx-0x%Lx, track %s, req %s\n",
384             start, end, cattr_name(actual_type),
385             cattr_name(req_type));
386     }

But after reserve_memtype reworked, only new->type == *new_type on success. actual_type is not synced with the them. So someone use new->type instead of actual_type in dprintk.
I am not very clear why author need these debug information. So to avoid any misunderstanding, I keep the same behavior of this dprintk. Keep what the dpinrk does in the past.

If someone really think this debug information need change, maybe it's better to send a new patch to fix it.

because *new_type is equal to new->type or new->type just did not change when new_type is NULL. perhaps we can assign actual_type in such way below.
+	actual_type = new_type ? *new_type : actual_type;


thanks
xinxhui
>> +	spin_unlock(&memtype_lock);
>> +
>>  	if (err) {
>>  		pr_info("x86/PAT: reserve_memtype failed [mem %#010Lx-%#010Lx], track %s, req %s\n",
>>  			start, end - 1,
>>  			cattr_name(new->type), cattr_name(req_type));
>>  		kfree(new);
>> -		spin_unlock(&memtype_lock);
>> -
>>  		return err;
>>  	}
>>  
>> -	spin_unlock(&memtype_lock);
>> -
>>  	dprintk("reserve_memtype added [mem %#010Lx-%#010Lx], track %s, req %s, ret %s\n",
>> -		start, end - 1, cattr_name(new->type), cattr_name(req_type),
>> +		start, end - 1, cattr_name(actual_type), cattr_name(req_type),
>>  		new_type ? cattr_name(*new_type) : "-");
>>  
>>  	return err;
>> -- 
>> 1.9.1
> 
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