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

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Message-ID: <20130511073638.GA24435@gmail.com>
Date:	Sat, 11 May 2013 09:36:38 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"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


* Yinghai Lu <yinghai@...nel.org> wrote:

> On Fri, May 10, 2013 at 3:28 AM, Zhang Yanfei
> <zhangyanfei@...fujitsu.com> wrote:
> > ?? 2013??05??10?? 17:27, Yinghai Lu ????:
> >> 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 */

And what does that comment mean??

next where? Which call? How? What's the logic? Where is it described 
accurately so that people who haven't seen the code (for a while) can 
understand it?

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

and what does this mean? Perhaps:

 "32-bit x86 and Xen set a bigger value for max_pfn_mapped before this 
  call."

?

And where and why does it matter that we set a bigger max_pfn_mapped 
before?

The dependencies in this code are incestous.

Thanks,

	Ingo
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