linux-kernel - Re: [RFC PATCH 4/4] x86/mm: remove bottom-up allocation style for x86

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Date:   Tue, 8 Jan 2019 07:37:19 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Pingfan Liu <kernelfans@...il.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     x86@...nel.org, linux-acpi@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] x86/mm: remove bottom-up allocation style for
 x86_64

On 08/01/2019 07:13, Pingfan Liu wrote:
> On Tue, Jan 8, 2019 at 1:42 AM Dave Hansen <dave.hansen@...el.com> wrote:
>>
>> On 1/7/19 12:24 AM, Pingfan Liu wrote:
>>> There are two acheivements by this patch.
>>> -1st. keep the subtree of pgtable away from movable node.
>>> Background about the defect of the current bottom-up allocation style, take
>>> the following scenario:
>>>   |  unmovable node |     movable node                           |
>>>      | kaslr-kernel |subtree of pgtable for phy<->virt |
>>
>>
>>
>>> Although kaslr-kernel can avoid to stain the movable node. [1] But the
>>> pgtable can still stain the movable node. That is a probability problem,
>>> with low probability, but still exist. This patch tries to eliminate the
>>> probability. With the previous patch, at the point of init_mem_mapping(),
>>> memblock allocator can work with the knowledge of acpi memory hotmovable
>>> info, and avoid to stain the movable node. As a result,
>>> memory_map_bottom_up() is not needed any more.
>>>
>>> -2nd. simplify the logic of memory_map_top_down()
>>> Thanks to the help of early_make_pgtable(), x86_64 can directly set up the
>>> subtree of pgtable at any place, hence the careful iteration in
>>> memory_map_top_down() can be discard.
>>
>>>  void __init init_mem_mapping(void)
>>>  {
>>>       unsigned long end;
>>> @@ -663,6 +540,7 @@ void __init init_mem_mapping(void)
>>>
>>>  #ifdef CONFIG_X86_64
>>>       end = max_pfn << PAGE_SHIFT;
>>> +     set_alloc_range(0x100000, end);
>>>  #else
>>
>> Why is this 0x100000 open-coded?  Why is this needed *now*?
>>
> 
> Memory under 1MB should be used by BIOS. For x86_64, after

Xen PV- and PVH-guests don't have that BIOS restriction.


Juergen