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memmap option causes inexplicable hang
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  1. #1
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    memmap option causes inexplicable hang

    My system has 16GB RAM. With memtest86+, I determined that some RAM between 12621M and 12622M is bad, so I tried to block it out using the memmap option on the linux line in grub2. I did so by adding this option to the 'linux' line at bootup:

    memmap=4M$12620M

    The reason I blocked a larger chunk away than just 1k or 1M is just in case there is any kernel pickiness about what memory boundary granularity is acceptable, and 4M seems like an absolute largest reasonable value for such things.

    Anyway, when I tried to boot, the machine got stuck at 'Loading initial ramdisk ...".

    Any idea what's going on? Why would this additional option cause a hang?

  2. #2
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    Re: memmap option causes inexplicable hang

    Test your memory modules separately (one at a time) and replace the faulty memory module ;-)

  3. #3
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    Re: memmap option causes inexplicable hang

    I know that's one approach. It's not available to me because the replacement hasn't arrived. I'm looking for a real solution to the question.

  4. #4
    stevea Guest

    Re: memmap option causes inexplicable hang

    Quote Originally Posted by george_toolan
    Test your memory modules separately (one at a time) and replace the faulty memory module ;-)
    Neither a borrower nor a lender be.
    A stitch in time saves nine.
    Use yum clean, uninstall pulse and try again.

    This is the george_toolan trite-advice-to-rack-up-post-count thread - no ?
    Luvya George, but why post such drivel ?
    ==========

    The hang in initrd when you add this kernel option suggests that the kernel is failing on that option (as in a kernel bug). Does it get any farther if you remove the option ?

    OK I tried on a VM and I found something very interesting.
    If I add "memtest=4M$500M" in the grub.cfg file ,then the /proc/cmdline shows "memtest=4MM" !!! It seems grub2 is gobbling up the "$500" part.

    Changing this to "memtest=4M\$500M" produces the correct result.
    So backslash-quotng the '$' dollar sign is needed.


    Also - if the memory error is pretty reliable you might want to try "memtest=n" to perform n' memory tests and map out bad pages.
    Last edited by stevea; 7th November 2012 at 02:55 AM.

  5. #5
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    Re: memmap option causes inexplicable hang

    It works fine without the option. Do you mean that when I edit the line in grub during bootup, I have to escape the '$' by putting a '\' in front? Let me try that.

  6. #6
    stevea Guest

    Re: memmap option causes inexplicable hang

    Yes, it's also needed when you modify grub from the command line
    Verify with
    cat /proc/cmdline

  7. #7
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    Re: memmap option causes inexplicable hang

    The actual option I ended up adding to /proc/cmdline is 'memmap=1m$12621m'. In order to make grub2 use this, I have a line in /etc/default/grub that looks like this:
    GRUB_CMDLINE_LINUX="... memmap=1m\\\$12621m ..."

    Then, when I run grub2-mkconfig, the generated cfg has 'memmap=1m\$12621m', which is also what shows up if I try to edit the command during boot. After I boot with this, /proc/cmdline has 'memmap=1m$12621m'. Also, /proc/meminfo suggests that I've achieved the intended effect.

    (The "..." just represents other things in the line.)

    So, in summary, all this confusion is about the escaping requirements in grub2.

    Thanks, stevea.

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