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

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Date:	Mon, 9 Dec 2013 16:11:34 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
Cc:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Grygorii Strashko <grygorii.strashko@...com>,
	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

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.

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?
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