linux-kernel - Re: [PATCH] x86/mm/init: respect memblock reserved regions when destroying mappings

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Message-ID: <4D5488D5.2020607@kernel.org>
Date:	Thu, 10 Feb 2011 16:54:45 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH] x86/mm/init: respect memblock reserved regions when destroying
 mappings

On 02/10/2011 04:35 PM, H. Peter Anvin wrote:
> On 02/10/2011 03:57 PM, Yinghai Lu wrote:
>> On 02/10/2011 03:48 PM, Jeremy Fitzhardinge wrote:
>>> On 02/08/2011 11:34 AM, H. Peter Anvin wrote:
>>>> On 02/07/2011 07:12 PM, Yinghai Lu wrote:
>>>>> why punishing native path with those checking?
>>>>>
>>>> What happens if you end up with a reserved range in an unfortunate place
>>>> on real hardware?
>>>
>>> Yes, exactly.  The reserved region code isn't very useful if you can't
>>> rely on it to reserve stuff.
>>
>> assume context is under:
>> moving cleanup_highmap() down after brk is concluded, and check memblock_reserved there.
>>
>> one case for that: native path, bootloader could put initrd under 512M. and it is with memblock reserved.
>> if we check those range with memblock_reserved, initial kernel mapping will not be cleaned up.
>>
>> or worse if we are checking if there is any range from __pa(_brk_end) to 512M is with memblock reserved to decide
>> if we need to clean-up highmap. it will skip for whole range.
>>
> 
> I'm afraid I simply can't parse the above.

1. we have patch that will move down cleanup_highmap, and it will clean initial mapping from _brk_end to 512M (before we have two steps: clear _end to 512M and then _brk_end to _end)

2. So checking memblock_reserved with _brk_end to 512M will cause problem:
   a. will check 256 times less.
   b. if bootloader put initrd ramdisk overlapped with [_brk_end++, 512M), and overlap range will make clean_highmap bail out early. because those range is memblock_reserved.

BTW: Do we really need to cleanup initial mapping between _brk_end to _end?

origin patch from jan:

commit 498343967613183611ac37dccb2846496d954c06
Author: Jan Beulich <jbeulich@...ell.com>
Date:   Wed May 6 13:06:47 2009 +0100

    x86-64: finish cleanup_highmaps()'s job wrt. _brk_end
    
    With the introduction of the .brk section, special care must be taken
    that no unused page table entries remain if _brk_end and _end are
    separated by a 2M page boundary. cleanup_highmap() runs very early and
    hence cannot take care of that, hence potential entries needing to be
    removed past _brk_end must be cleared once the brk allocator has done
    its job.
    
    [ Impact: avoids undesirable TLB aliases ]
    
    Signed-off-by: Jan Beulich <jbeulich@...ell.com>
    Signed-off-by: H. Peter Anvin <hpa@...or.com>

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index fd3da1d..ae4f7b5 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -7,6 +7,7 @@
 #include <asm/page.h>
 #include <asm/page_types.h>
 #include <asm/sections.h>
+#include <asm/setup.h>
 #include <asm/system.h>
 #include <asm/tlbflush.h>
 
@@ -304,8 +305,23 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
 #endif
 
 #ifdef CONFIG_X86_64
-       if (!after_bootmem)
+       if (!after_bootmem && !start) {
+               pud_t *pud;
+               pmd_t *pmd;
+
                mmu_cr4_features = read_cr4();
+
+               /*
+                * _brk_end cannot change anymore, but it and _end may be
+                * located on different 2M pages. cleanup_highmap(), however,
+                * can only consider _end when it runs, so destroy any
+                * mappings beyond _brk_end here.
+                */
+               pud = pud_offset(pgd_offset_k(_brk_end), _brk_end);
+               pmd = pmd_offset(pud, _brk_end - 1);
+               while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1))
+                       pmd_clear(pmd);
+       }
 #endif
        __flush_tlb_all();
 

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