Messages in this thread |  | | From | "Hatayama, Daisuke" <> | Subject | RE: [PATCH] kdump, oldmem: support mmap on /dev/oldmem | Date | Wed, 6 Feb 2013 07:24:46 +0000 |
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From: Vivek Goyal <vgoyal@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
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