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

[Hatayama, Daisuke]

> From: Vivek Goyal [mailto:vgoyal at redhat.com]
> Sent: Friday, February 08, 2013 12:06 AM

> On Wed, Feb 06, 2013 at 07:24:46AM +0000, Hatayama, Daisuke wrote:
> > 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.
> 
> Ok, got it. So everything needs to be page aligned and if size is not
> sufficient then we need a way to pad memory areas to make next object
> page aligned.
> 
> To begin with supporting mmap on /dev/oldmem is fine with me. Once that
> gets in, it will be good to look at how to make all the individual items
> page aligned  so that mmap can be supported on /proc/vmcore.

I'm already beginning with making the patch set. At the time when I was writing /dev/oldmem patch, I was confused that remap_pfn_range must have been rewritten. But the complication is in fact simpler. I would post it early next week.

By the way, the third argument pfn of remap_pfn_range is defined as unsigned long, of 4 bytes on 32-bit x86. On PAE paging 32-bit linear addresses are converted maximally to 52-bit physical addresses (4 PiB). We need 40 bits to fully represent all page frame numbers over 52-bit physical memory space, so 4 bytes is not enough. But it seems to me unlikely that there's users who want to use huge memory with 32-bit kernel. I'll not support 32-bit x86 at least on the next patch.

Thanks.
HATAYAMA, Daisuke