linux-kernel - RE: [PATCH - resend] Memory-Hotplug: Fix the bug on interface /dev/mem for 64-bit kernel(v1)

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Message-ID: <DA586906BA1FFC4384FCFD6429ECE860316C002A@shzsmsx502.ccr.corp.intel.com>
Date:	Tue, 12 Jan 2010 13:51:15 +0800
From:	"Zheng, Shaohui" <shaohui.zheng@...el.com>
To:	"Wu, Fengguang" <fengguang.wu@...el.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"y-goto@...fujitsu.com" <y-goto@...fujitsu.com>,
	Dave Hansen <haveblue@...ibm.com>,
	"x86@...nel.org" <x86@...nel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: RE: [PATCH - resend] Memory-Hotplug: Fix the bug on interface
	/dev/mem for 64-bit kernel(v1)

> > +	/* if add to low memory, update max_low_pfn */
> > +	if (unlikely(start_pfn < limit_low_pfn)) {
> > +		if (end_pfn <= limit_low_pfn)
> > +			max_low_pfn = end_pfn;
> > +		else
> > +			max_low_pfn = limit_low_pfn;
> 
> X86_64 actually always set max_low_pfn=max_pfn, in setup_arch():
> [Zheng, Shaohui] there should be some misunderstanding, I read the
> code carefully, if the total memory is under 4G, it always
> max_low_pfn=max_pfn. If the total memory is larger than 4G,
> max_low_pfn means the end of low ram. It set

> max_low_pfn = e820_end_of_low_ram_pfn();.

The above line is very misleading.. In setup_arch(), it will be
overrode by the following block.
[Zheng, Shaohui] yes, I misunderstand it because of this code. It seems that max_low_pfn == max_pfn is always true on x86_32 and x86_64.  Thanks fengguang to point it out.

>  899 #ifdef CONFIG_X86_64
>  900         if (max_pfn > max_low_pfn) {
>  901                 max_pfn_mapped = init_memory_mapping(1UL<<32,
>  902                                                      max_pfn<<PAGE_SHIFT);
>  903                 /* can we preseve max_low_pfn ?*/
>  904                 max_low_pfn = max_pfn;
>  905         }
>  906 #endif
 
Thanks,
Fengguang
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