> 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
