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

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Date:	Fri, 10 May 2013 07:57:47 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
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

On Fri, May 10, 2013 at 3:28 AM, Zhang Yanfei
<zhangyanfei@...fujitsu.com> wrote:
> 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?

the comment says: /* will get exact value next */

For x86 32bit path, and xen set bigger max_pfn_mapped before.

Yinghai
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