Re: [PATCH] makedumpfile: memset() in cyclic bitmap initialization introduce segment fault

[HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>]

(2013/12/20 18:00), Dave Young wrote:
> On 12/20/13 at 05:49pm, HATAYAMA Daisuke wrote:
>> (2013/12/20 11:17), Dave Young wrote:
>>>> Also, I'm interested in the memory map passed to from EFI in that
>>>>
>>>>> cat /proc/iomem:
>>>>> 00000000-00000fff : reserved
>>>>> 00001000-0009ffff : System RAM
>>>>> 000a0000-000bffff : PCI Bus 0000:00
>>>>> 000f0000-000fffff : System ROM
>>>>> 00100000-3d162017 : System RAM
>>>>>    01000000-015cab9b : Kernel code
>>>>>    015cab9c-019beb3f : Kernel data
>>>>>    01b4f000-01da9fff : Kernel bss
>>>>>    30000000-37ffffff : Crash kernel
>>>>> 3d162018-3d171e57 : System RAM
>>>>> 3d171e58-3d172017 : System RAM
>>>>> 3d172018-3d17ae57 : System RAM
>>>>> 3d17ae58-3dc10fff : System RAM
>>>>
>>>> this part is consecutive but somehow is divided into 4 entries.
>>>> You called your environment as ``EFI virtual machine'', could you tell
>>>> me precisely what it mean? qemu/KVM or VMware guest system? I do want
>>>> to understand how this kind of memory map was created. I think this
>>>> kind of memory mapping is odd and I guess this is caused by the fact
>>>> that the system is a virtual environment.
>>>
>>> This is not specific to EFI machine, it's the reserved setup_data regions
>>> They happened to be continous but they do not have to be continuous.
>>>
>>
>> Thanks for pointing out that. I've just read Documentation/x86/boot.txt
>> and parse_setup_data().
>>
>> But I don't understand well why these regions are divided as these.
>> I guess kernel divides the System RAM this way and the memory map first
>> passed to by EFI is all page aligned, right?
>
> setup_data are passed as a link list by boot loader, each node is a block
> of memory, there could be many different setup_data type.
>
>>
>> Also, looking at parse_setup_data(), currently handled data in setup_data
>> interface is extended e820 entries and dtb case only.
>>
>>                  switch (data_type) {
>>                  case SETUP_E820_EXT:
>>                          parse_e820_ext(pa_data, data_len);
>>                          break;
>>                  case SETUP_DTB:
>>                          add_dtb(pa_data);
>>                          break;
>>                  default:
>>                          break;
>>                  }
>>
>> Is it right that this kind of memory map doesn't occur as long as either
>> of information is passed to via setup_data? IOW, is this necessary
>> information?
>
> If bootloader does not pass it, there will be no such mem ranges in /proc/iomem.
>
>>
>>>>
>>>> And for Vivek, this case is a concrete example of multiple RAM entries
>>>> appearing in a single page I suspected in the mmap failure patch,
>>>> although these entries are consecutive in physical address and can be
>>>> represented by a single entry by merging them in a single entry. But
>>>> then it seems to me that there could be more odd case that multiple
>>>> RAM entries but not consecutive. I again think this should be addressed
>>>> in the patch for the mmap failure issue. How do you think?
>>>
>>> They are different problems, the previous mmap bug is for cross page regions
>>> with different page flags.
>>>
>>
>> I understand that. What I think problem here is the case where multiple
>> System RAM entries appear in a single page. In the above memory map, they
>> are 3d171000, 3d172000 and 3d17a000. My fixing patch is to copy fractional
>> pages in the 2nd kernel in order to make it possible to mmap without affecting
>> non-System RAM area as much as possible, and then if there is this kind of
>> System RAM entries, we need to use the same page in the 2nd kernel for
>> different System RAM entries that shares the same page in the 1st kernel. This
>> needs a little additional processing and we want to keep implementation as
>> simple as possible as long as there's no such system in real world. However,
>> I'm surprised to see the memory mapping above.
>
> These ranges are "system ram" of type E820_RESERVED_KERN, please see below:
> arch/x86/kernel/setup.c: e820_reserve_setup_data()
>

Thanks for explaining this. I was confused E820_RESERVED_KERN appeared in
/proc/iomem as Reserved...

-- 
Thanks.
HATAYAMA, Daisuke


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