linux-kernel - Re: [PATCH 1/2] x86: Optimize resource lookups for ioremap

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Date:	Wed, 27 Aug 2014 16:25:24 -0700
From:	Mike Travis <travis@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
	msalter@...hat.com, dyoung@...hat.com, riel@...hat.com,
	peterz@...radead.org, mgorman@...e.de,
	linux-kernel@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
	Alex Thorlton <athorlton@....com>
Subject: Re: [PATCH 1/2] x86: Optimize resource lookups for ioremap



On 8/27/2014 4:18 PM, Andrew Morton wrote:
> On Wed, 27 Aug 2014 16:09:09 -0700 Mike Travis <travis@....com> wrote:
> 
>>
>>>>
>>>> ...
>>>>
>>>> --- linux.orig/kernel/resource.c
>>>> +++ linux/kernel/resource.c
>>>> @@ -494,6 +494,43 @@ int __weak page_is_ram(unsigned long pfn
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(page_is_ram);
>>>>  
>>>> +/*
>>>> + * Search for a resouce entry that fully contains the specified region.
>>>> + * If found, return 1 if it is RAM, 0 if not.
>>>> + * If not found, or region is not fully contained, return -1
>>>> + *
>>>> + * Used by the ioremap functions to insure user not remapping RAM and is as
>>>> + * vast speed up over walking through the resource table page by page.
>>>> + */
>>>> +int __weak region_is_ram(resource_size_t start, unsigned long size)
>>>> +{
>>>> +	struct resource *p;
>>>> +	resource_size_t end = start + size - 1;
>>>> +	int flags = IORESOURCE_MEM | IORESOURCE_BUSY;
>>>> +	const char *name = "System RAM";
>>>> +	int ret = -1;
>>>> +
>>>> +	read_lock(&resource_lock);
>>>> +	for (p = iomem_resource.child; p ; p = p->sibling) {
>>>> +		if (end < p->start)
>>>> +			continue;
>>>> +
>>>> +		if (p->start <= start && end <= p->end) {
>>>> +			/* resource fully contains region */
>>>> +			if ((p->flags != flags) || strcmp(p->name, name))
>>>> +				ret = 0;
>>>> +			else
>>>> +				ret = 1;
>>>> +			break;
>>>> +		}
>>>> +		if (p->end < start)
>>>> +			break;	/* not found */
>>>> +	}
>>>> +	read_unlock(&resource_lock);
>>>> +	return ret;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(region_is_ram);
>>>
>>> Exporting a __weak symbol is strange.  I guess it works, but neither
>>> the __weak nor the export are actually needed?
>>>
>>
>> I mainly used 'weak' and export because that was what the page_is_ram
>> function was using.  Most likely this won't be used anywhere else but
>> I wasn't sure.  I can certainly remove the weak and export, at least
>> until it's actually needed?
> 
> Several architectures implement custom page_is_ram(), so they need the
> __weak.  region_is_ram() needs neither so yes, they should be removed.

Okay.
> 
> <looks at the code>
> 
> Doing strcmp("System RAM") is rather a hack.  Is there nothing in
> resource.flags which can be used?  Or added otherwise?

I agree except this mimics the page_is_ram function:

        while ((res.start < res.end) &&
                (find_next_iomem_res(&res, "System RAM", true) >= 0)) {

So it passes the same literal string which then find_next does the
same strcmp on it:

                if (p->flags != res->flags)
                        continue;
                if (name && strcmp(p->name, name))
                        continue;

I should add back in the check to insure name is not NULL.

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