[PATCH] kdump, oldmem: support mmap on /dev/oldmem

[Hatayama, Daisuke]

From: Vivek Goyal <vgoyal at redhat.com>
Subject: Re: [PATCH] kdump, oldmem: support mmap on /dev/oldmem
Date: Tue, 5 Feb 2013 10:12:56 -0500

> On Mon, Feb 04, 2013 at 04:59:35AM +0000, Hatayama, Daisuke wrote:

> [..]
>> For design decision, I didn't support mmap() on /proc/vmcore because
>> it abstracts old memory as ELF format, so there's range consequtive on
>> /proc/vmcore but not consequtive on the actual old memory. For
>> example, consider ELF headers on the 2nd kernel and the note objects,
>> memory chunks corresponding to PT_LOAD entries on the first kernel.
>> They are not consequtive on the old memory. So reampping them so
>> /proc/vmcore appears consequtive using existing remap_pfn_range() needs
>> some complicated work.
> 
> Can't we call remap_pfn_range() multiple times. Once for each sequential
> range of memory. /proc/vmcore already has list of contiguous memory areas.
> So we can parse user passed file offset and size and map into respective
> physical chunks and call rempa_pfn_range() on all these chunks.
> 
> I think supporting mmap() both on /dev/oldmem as well as /proc/vmcore will
> be nice.
> 
> Agreed that supporting mmap() on /proc/vmcore is more work as compared to
> /dev/oldmem but should be doable.
> 

The complication to support mmap() on /proc/vmcore lies in kdump
side. Objects exported from /proc/vmcore needs to be page-size aligned
on /proc/vmcore. This comes from the restriction of mmap() that
requires user-space address and physical address to be page-size
aligned.

As I said in the description, objects implicitly referened by
/proc/vmcore are

  - ELF headers,
  - NOTE objects (NT_PRSTATUS entries x cpus, VMCOREINFO), and
  - memory chunks x (the number of PT_LOAD entries).

Note objects are scattered on old memory. They are exported as a
single NOTE entry from program headers, so they need to be gathered at
the same location in the 2nd kernel starting from the page-size
aligned address.

VMCOREINFO is about 1.5KB on 2.6.32 kernel. One NT_PRSTATUS is 355
bytes. Recent limit of NR_CPUS is 5120 on x86_64. So less than about 2
MB is enough even on the worst case.

Note that the format of /proc/vmcore need to change since offset of
each object need to be page-size aligned.

Thanks.
HATAYAMA, Daisuke