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    SubjectRe: [PATCH -next] Documentation: Improve crashkernel= description
    On Sun, Feb 06, 2011 at 08:25:20PM -0600, Rob Landley wrote:
    > On 02/06/2011 03:57 PM, Simon Horman wrote:
    > > On Sun, Feb 06, 2011 at 05:41:08PM +0200, Ahmed S. Darwish wrote:
    > >> (Also applicable over 2.6.38-rc3)
    > >>
    > >> Had to explore two C code files to make sense of the 'crashkernel='
    > >> kernel parameter values. Improve the situation.
    > >>
    > >> Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
    > >> ---
    > >>
    > >> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
    > >> index 89835a4..8f26b42 100644
    > >> --- a/Documentation/kernel-parameters.txt
    > >> +++ b/Documentation/kernel-parameters.txt
    > >> @@ -545,9 +545,12 @@ and is between 256 and 4096 characters. It is defined in the file
    > >> Format:
    > >> <first_slot>,<last_slot>,<port>,<enum_bit>[,<debug>]
    > >>
    > >> - crashkernel=nn[KMG]@ss[KMG]
    > >> - [KNL] Reserve a chunk of physical memory to
    > >> - hold a kernel to switch to with kexec on panic.
    > >> + crashkernel=size[KMG][@offset[KMG]]
    > >> + [KNL] Using kexec, Linux can switch to a 'crash kernel'
    > >> + upon panic. This parameter reserves the physical
    > >> + memory region [offset, offset + size] for that kernel
    > >> + image. If the '@offset' part was ignored, Linux finds
    > >> + a suitable crash image start address automatically.
    > >
    > > I think this would be further improved as:
    > >
    > > ... If '@offset' is omitted then a suitable
    > > offset is selected automatically.
    >
    > Suitable offset as in parses a known image type (ELF, bzImage, etc) to
    > find the start address? Or just assumes the entry point and load
    > address are the same?

    As in it finds a large enough area of contiguous free memory.
    I believe that originally it searched from the bottom of memory,
    it may be slightly smarter now.

    Suitable is perhaps too strong a term.

    In terms of the start address. On x86 at least it used to be
    important to make sure that the start address matched but
    these days using a relocatable kernel is an easier option.

    > Is this the size for just the kernel image, or also for the physical
    > memory it uses so it won't overwrite the existing kernel's stuff on the
    > way up? (If a compressed kernel wants to decompress itself or extract
    > an initramfs for itself, what happens?)

    It is the memory that is used by the crash-kernel to avoid overwriting
    the first-kernel's stuff. It is not the memory where the (usually
    compressed) second-kernel is stored before it is kexeced.

    If a compressed crash-kernel wants to decompress itself it does so in the
    available memory which is the region reserved by the crash-kernel parameter
    to the first-kernel.


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