linux-kernel - Re: [PATCH v3 02/23] mm/memblock: debug: don't free reserved array if !ARCH_DISCARD

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Date:	Tue, 10 Dec 2013 18:44:15 +0200
From:	Grygorii Strashko <grygorii.strashko@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Yinghai Lu <yinghai@...nel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v3 02/23] mm/memblock: debug: don't free reserved array
 if !ARCH_DISCARD_MEMBLOCK

Hi Andrew,

On 12/10/2013 02:11 AM, Andrew Morton wrote:
> On Mon, 9 Dec 2013 16:50:35 -0500 Santosh Shilimkar <santosh.shilimkar@...com> wrote:
> 
>> Now the Nobootmem allocator will always try to free memory allocated for
>> reserved memory regions (free_low_memory_core_early()) without taking
>> into to account current memblock debugging configuration
>> (CONFIG_ARCH_DISCARD_MEMBLOCK and CONFIG_DEBUG_FS state).
>> As result if:
>>   - CONFIG_DEBUG_FS defined
>>   - CONFIG_ARCH_DISCARD_MEMBLOCK not defined;
>> -  reserved memory regions array have been resized during boot
>>
>> then:
>> - memory allocated for reserved memory regions array will be freed to
>> buddy allocator;
>> - debug_fs entry "sys/kernel/debug/memblock/reserved" will show garbage
>> instead of state of memory reservations. like:
>>     0: 0x98393bc0..0x9a393bbf
>>     1: 0xff120000..0xff11ffff
>>     2: 0x00000000..0xffffffff
>>
>> Hence, do not free memory allocated for reserved memory regions if
>> defined(CONFIG_DEBUG_FS) && !defined(CONFIG_ARCH_DISCARD_MEMBLOCK).
> 
> Alternatives:
> 
> - disable /proc/sys/kernel/debug/memblock/reserved in this case
> 
> - disable defined(CONFIG_DEBUG_FS) &&
>    !defined(CONFIG_ARCH_DISCARD_MEMBLOCK) in Kconfig.

Yes. But this is debug information and it's useful to have it.

> 
> How much memory are we talking about here?  If it's more than "very
> little" then I think either of these would be better - most users will
> value the extra memory over an accurate
> /proc/sys/kernel/debug/memblock/reserved?
> 

Sorry, I have no real statistic information and I hit this issue while testing this series
by simulating huge amount of bootmem allocation during kernel boot.
The real number of entries i saw on Keystone & OMAP boards is no more than ~20.

Few digits below:
- size of static reserved memory regions array is 2048 bytes
- the size of array is doubled during each allocation

Regards,
-grygorii
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