linux-kernel - Re: [PATCH v3 2/5] memblock: Improve memblock to support allocation from lower address.

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Message-ID: <52408351.8080400@gmail.com>
Date:	Tue, 24 Sep 2013 02:07:13 +0800
From:	Zhang Yanfei <zhangyanfei.yes@...il.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Tang Chen <tangchen@...fujitsu.com>, rjw@...k.pl, lenb@...nel.org,
	tglx@...utronix.de, mingo@...e.hu, hpa@...or.com,
	akpm@...ux-foundation.org, toshi.kani@...com,
	zhangyanfei@...fujitsu.com, liwanp@...ux.vnet.ibm.com,
	trenn@...e.de, yinghai@...nel.org, jiang.liu@...wei.com,
	wency@...fujitsu.com, laijs@...fujitsu.com,
	isimatu.yasuaki@...fujitsu.com, izumi.taku@...fujitsu.com,
	mgorman@...e.de, minchan@...nel.org, mina86@...a86.com,
	gong.chen@...ux.intel.com, vasilis.liaskovitis@...fitbricks.com,
	lwoodman@...hat.com, riel@...hat.com, jweiner@...hat.com,
	prarit@...hat.com, x86@...nel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH v3 2/5] memblock: Improve memblock to support allocation
 from lower address.

Hello tejun,

On 09/23/2013 11:50 PM, Tejun Heo wrote:
> Hello,
> 
> Please separate out factoring out of top-down allocation.  That change
> is an equivalent conversion which shouldn't involve any functional
> difference.  Mixing that with introduction of new feature isn't a good
> idea, so the patch split should be 1. split out top-down allocation
> from memblock_find_in_range_node() 2. introduce bottom-up flag and
> implement the feature.
> 
> On Fri, Sep 13, 2013 at 05:30:52PM +0800, Tang Chen wrote:
>> +/**
>>   * memblock_find_in_range_node - find free area in given range and node
>> - * @start: start of candidate range
>> + * @start: start of candidate range, can be %MEMBLOCK_ALLOC_ACCESSIBLE
> 
> The only reason @end has special ACCESSIBLE flag is because we don't
> know how high is actually accessible and it needs to be distinguished
> from ANYWHERE.  We assume that the lower addresses are always mapped,
> so using ACCESSIBLE for @start is weird.  I think it'd be clearer to
> make the memblock interface to set the direction explicitly state what
> it's doing - ie. something like set_memblock_alloc_above_kernel(bool
> enable).  We clearly don't want pure bottom-up allocation and the
> @start/@end params in memblock interface are used to impose extra
> limitations for each allocation, not the overall allocator behavior.

Forgot this one...

Yes, I am following your advice in principle but kind of confused by
something you said above. Where should the set_memblock_alloc_above_kernel
be used? IMO, the function is like:

find_in_range_node()
{
     if (ok) {
           /* bottom-up */
           ret = __memblock_find_in_range(max(start, _end_of_kernel), end...);
           if (!ret)
                 return ret;
     }

     /* top-down retry */
     return __memblock_find_in_range_rev(start, end...)
}

For bottom-up allocation, we always start from max(start, _end_of_kernel).

Thanks.

> 
>> @@ -100,8 +180,7 @@ phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t start,
>>  					phys_addr_t end, phys_addr_t size,
>>  					phys_addr_t align, int nid)
>>  {
>> -	phys_addr_t this_start, this_end, cand;
>> -	u64 i;
>> +	phys_addr_t ret;
>>  
>>  	/* pump up @end */
>>  	if (end == MEMBLOCK_ALLOC_ACCESSIBLE)
>> @@ -111,18 +190,22 @@ phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t start,
>>  	start = max_t(phys_addr_t, start, PAGE_SIZE);
>>  	end = max(start, end);
>>  
>> -	for_each_free_mem_range_reverse(i, nid, &this_start, &this_end, NULL) {
>> -		this_start = clamp(this_start, start, end);
>> -		this_end = clamp(this_end, start, end);
>> +	if (memblock_direction_bottom_up()) {
>> +		/*
>> +		 * MEMBLOCK_ALLOC_ACCESSIBLE is 0, which is less than the end
>> +		 * of kernel image. So callers specify MEMBLOCK_ALLOC_ACCESSIBLE
>> +		 * as @start is OK.
>> +		 */
>> +		start =	max(start, __pa_symbol(_end)); /* End of kernel image. */
>>  
>> -		if (this_end < size)
>> -			continue;
>> +		ret = __memblock_find_range(start, end, size, align, nid);
>> +		if (ret)
>> +			return ret;
>>  
>> -		cand = round_down(this_end - size, align);
>> -		if (cand >= this_start)
>> -			return cand;
>> +		pr_warn("memblock: Failed to allocate memory in bottom up direction. Now try top down direction.\n");
> 
> You probably wanna explain why retrying top-down allocation may
> succeed when bottom-up failed.
> 
> Thanks.
> 


-- 
Thanks.
Zhang Yanfei
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