Message-ID: <55AC6A94.5070700@intel.com>
Date: Mon, 20 Jul 2015 11:27:16 +0800
From: Pan Xinhui <xinhuix.pan@intel.com>
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To: Thomas Gleixner <tglx@linutronix.de>
CC: linux-kernel@vger.kernel.org, mingo@redhat.com, hpa@zytor.com,
        x86@kernel.org, bp@suse.de, jgross@suse.com, mcgrof@suse.com,
        decui@microsoft.com, ross.zwisler@linux.intel.com,
        sfr@canb.auug.org.au, toshi.kani@hp.com,
        "mnipxh@163.com" <mnipxh@163.com>
Subject: Re: [PATCH] x86/mm/pat: let level meaningful even NULL return in,
 lookup_address_in_pgd
References: <55A5048E.3090403@intel.com> <alpine.DEB.2.11.1507171643200.18576@nanos>
In-Reply-To: <alpine.DEB.2.11.1507171643200.18576@nanos>
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hi, tglx
	thanks for your reply.

On 2015年07月17日 22:50, Thomas Gleixner wrote:
> On Tue, 14 Jul 2015, Pan Xinhui wrote:
>> If pmd or pud is not set, we may set a wrong page mapping level.
> 
> No. The behaviour is simply undefined, if the return value of the
> function is NULL.
> 
> So what you are trying to do is to make the level information accurate
> even for the failure case.
>  
yes. it's good to report level information. then we can handle some errors.


>> We know *address* belongs to *pud*, however for some reasons *pmd* is
>> NULL. For example, this address has no physical pages mapped. What we
>> could benefit from this patch are below:
>> 1) We can walk memory range easier.
>> If addressA passed to lookup_address(), and NULL returned. We can pass
>> addressA + level_to_size(level) to lookup_address() in next loop.
>> ...
>> if (!pte) {
>> 	/* level_to_size has not been implemented in upstream*/
>> 	address += level_to_size(level);
>> 	continue;
>> }
> 
> This example is completely useless because we do not see how the loop
> itself looks like and how that improves anything. The proper way to do
> this is to post:
> 
>      - the patch which changes the function
>      - another patch which makes use of the change 
> 
> But so far I cannot see any reason why we want to change it.
> 
sorry for that. There are some debug patches protected. I will try to make a simple example in other mails.

>> ...
>> 2) keep same behavior because level is set to PG_LEVEL_4K even when pte
>> is NULL.
> 
> And what's the actual benefit of #2? Keeping the same behaviour is a
> requirement if you don't want to break any users of that function.
> 
agree with you. :)
I did not explain it in correct ways.
When pte is NULL, lookup_address will return NULL on failure. however the level is correct and set to PG_LEVEL_4K.
So what I am trying to do is that if lookup_address return NULL on *pud* or *pmd* NULL failure, level is still correct or more correct.
A correct level information is very useful when we walk a large range of memory.

thanks
xinhui

> Thanks,
> 
> 	tglx
> 
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