linux-kernel - Re: [PATCH 07/11] x86, memblock: Set lowest limit for memblock_alloc_base

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Date:	Wed, 04 Sep 2013 10:05:51 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	rjw@...k.pl, lenb@...nel.org, tglx@...utronix.de, mingo@...e.hu,
	hpa@...or.com, akpm@...ux-foundation.org, tj@...nel.org,
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	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,
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	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH 07/11] x86, memblock: Set lowest limit for memblock_alloc_base_nid().

On 09/04/2013 08:37 AM, Toshi Kani wrote:
> On Tue, 2013-08-27 at 17:37 +0800, Tang Chen wrote:
>> memblock_alloc_base_nid() is a common API of memblock. And it calls
>> memblock_find_in_range_node() with %start = 0, which means it has no
>> limit for the lowest address by default.
>>
>> 	memblock_find_in_range_node(0, max_addr, size, align, nid);
>>
>> Since we introduced current_limit_low to memblock, if we have no limit
>> for the lowest address or we are not sure, we should pass
>> MEMBLOCK_ALLOC_ACCESSIBLE to %start so that it will be limited by the
>> default low limit.
>>
>> dma_contiguous_reserve() and setup_log_buf() will eventually call
>> memblock_alloc_base_nid() to allocate memory. So if the allocation order
>> is from low to high, they will allocate memory from the lowest limit
>> to higher memory.
>
> This requires the callers to use MEMBLOCK_ALLOC_ACCESSIBLE instead of 0.
> Is there a good way to make sure that all callers will follow this rule
> going forward?  Perhaps, memblock_find_in_range_node() should emit some
> message if 0 is passed when current_order is low to high and the boot
> option is specified?

How about set this as the default rule:

	When using from low to high order, always allocate memory from
	current_limit_low.

So far, I think only movablenode boot option will use this order.

>
> Similarly, I wonder if we should have a check to the allocation size to
> make sure that all allocations will stay small in this case.
>

We can check the size. But what is the stragety after we found that the 
size
is too large ?  Do we refuse to allocate memory ?  I don't think so.

I think only relocate_initrd() and reserve_crachkernel() could allocate 
large
memory. reserve_crachkernel() is easy to reorder, but reordering 
relocate_initrd()
is difficult because acpi_initrd_override() need to access to it with va.

I think on most servers, we don't need to do relocate_initrd(). initrd 
will be
loaded to mapped memory in normal situation. Can we just leave it there ?

Thanks.
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