From: Vivek Goyal <vgoyal@xxxxxxxxxx> 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
