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

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Date:	Wed, 27 Aug 2014 16:09:09 -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 4:05 PM, Andrew Morton wrote:
> On Wed, 27 Aug 2014 17:59:28 -0500 Mike Travis <travis@....com> wrote:
> 
>> Since the ioremap operation is verifying that the specified address range
>> is NOT RAM, it will search the entire ioresource list if the condition
>> is true.  To make matters worse, it does this one 4k page at a time.
>> For a 128M BAR region this is 32 passes to determine the entire region
>> does not contain any RAM addresses.
>>
>> This patch provides another resource lookup function, region_is_ram,
>> that searches for the entire region specified, verifying that it is
>> completely contained within the resource region.  If it is found, then
>> it is checked to be RAM or not, within a single pass.
>>
>> The return result reflects if it was found or not (-1), and whether it is
>> RAM (1) or not (0).  This allows the caller to fallback to the previous
>> page by page search if it was not found.
>>
>> ...
>>
>> --- 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?

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