linux-kernel - Re: [PATCH] x86: mm: Remove unnecessary assignment for max_pfn

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Message-ID: <518CCBB2.5010308@cn.fujitsu.com>
Date:	Fri, 10 May 2013 18:28:02 +0800
From:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: mm: Remove unnecessary assignment for max_pfn_mapped:q

于 2013年05月10日 17:27, Yinghai Lu 写道:
> On Fri, May 10, 2013 at 2:01 AM, Zhang Yanfei
> <zhangyanfei@...fujitsu.com> wrote:
>> init_memory_mapping will set max_pfn_mapped:
>> int_memory_mapping
>>   --> add_pfn_range_mapped
>>     --> max_pfn_mapped = max(max_pfn_mapped, end_pfn)
>>
>> In init_mem_mapping, before we set max_pfn_mapped to 0, we
>> have already called init_memory_mapping to setup pagetable
>> for [0, ISA_END_ADDRESS], and that sets max_pfn_mapped. So
>> the assignment to 0 is not necessary, remove it.
> 
> NAK.
> 
> for 32bit or Xen, max_pfn_mapped is set way before in head_32.S and
> xen-enlighen.

Hi Yinghai

I might be wrong, but just from the code in init_mem_mapping only:

 410         /* the ISA range is always mapped regardless of memory holes */
 411         init_memory_mapping(0, ISA_END_ADDRESS);
 412 
 413         /* xen has big range in reserved near end of ram, skip it at first.*/
 414         addr = memblock_find_in_range(ISA_END_ADDRESS, end, PMD_SIZE, PMD_SIZE);
 415         real_end = addr + PMD_SIZE;
 416 
 417         /* step_size need to be small so pgt_buf from BRK could cover it */
 418         step_size = PMD_SIZE;
 419         max_pfn_mapped = 0; /* will get exact value next */

Line 411 set max_pfn_mapped, and then line 419 set it to zero again, so
why keep the later assignment?

Thanks
Zhang
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