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    SubjectRe: [PATCH v2 0/3] support for broken memory modules (BadRAM)
    On Wed, 22 Jun 2011 13:18:51 +0200 Stefan Assmann <> wrote:

    > Following the RFC for the BadRAM feature here's the updated version with
    > spelling fixes, thanks go to Randy Dunlap. Also the code is now less verbose,
    > as requested by Andi Kleen.
    > v2 with even more spelling fixes suggested by Randy.
    > Patches are against vanilla 2.6.39.
    > The idea is to allow the user to specify RAM addresses that shouldn't be
    > touched by the OS, because they are broken in some way. Not all machines have
    > hardware support for hwpoison, ECC RAM, etc, so here's a solution that allows to
    > use bitmasks to mask address patterns with the new "badram" kernel command line
    > parameter.
    > Memtest86 has an option to generate these patterns since v2.3 so the only thing
    > for the user to do should be:
    > - run Memtest86
    > - note down the pattern
    > - add badram=<pattern> to the kernel command line
    > The concerning pages are then marked with the hwpoison flag and thus won't be
    > used by the memory managment system.

    The google kernel has a similar capability. I asked Nancy to comment
    on these patches and she said:

    : One, the bad addresses are passed via the kernel command line, which
    : has a limited length. It's okay if the addresses can be fit into a
    : pattern, but that's not necessarily the case in the google kernel. And
    : even with patterns, the limit on the command line length limits the
    : number of patterns that user can specify. Instead we use lilo to pass
    : a file containing the bad pages in e820 format to the kernel.
    : Second, the BadRAM patch expands the address patterns from the command
    : line into individual entries in the kernel's e820 table. The e820
    : table is a fixed buffer that supports a very small, hard coded number
    : of entries (128). We require a much larger number of entries (on
    : the order of a few thousand), so much of the google kernel patch deals
    : with expanding the e820 table. Also, with the BadRAM patch, entries
    : that don't fit in the table are silently dropped and this isn't
    : appropriate for us.
    : Another caveat of mapping out too much bad memory in general. If too
    : much memory is removed from low memory, a system may not boot. We
    : solve this by generating good maps. Our userspace tools do not map out
    : memory below a certain limit, and it verifies against a system's iomap
    : that only addresses from memory is mapped out.

    I have a couple of thoughts here:

    - If this patchset is merged and a major user such as google is
    unable to use it and has to continue to carry a separate patch then
    that's a regrettable situation for the upstream kernel.

    - Google's is, afaik, the largest use case we know of: zillions of
    machines for a number of years. And this real-world experience tells
    us that the badram patchset has shortcomings. Shortcomings which we
    can expect other users to experience.

    So. What are your thoughts on these issues?


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