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    SubjectRe: [PATCH 2.6.24] mm: BadRAM support for broken memory
    On Sun, 2 Mar 2008 13:42:21 +0000 Rick van Rein wrote:

    > This is the latest version of the BadRAM patch, which makes it possible to
    > run Linux on broken memory. The patch supports the use of a lesser grade
    > of memory, which could be marketed more cheaply and which would thereby
    > decrease the environmental stress caused by the process of (memory) chip
    > manufacturing.

    Patch needs to be made against (i.e., applyable against) the latest
    linus-mainline kernel, not a few-weeks old kernel.
    Of course, you could be lucky and it applies to either one.

    > diff -pruN linux-2.6.24.orig/Documentation/badram.txt linux-2.6.24/Documentation/badram.txt
    > --- linux-2.6.24.orig/Documentation/badram.txt 1969-12-31 19:00:00.000000000 -0500
    > +++ linux-2.6.24/Documentation/badram.txt 2008-02-05 23:29:49.000000000 -0500
    > @@ -0,0 +1,275 @@
    > +INFORMATION ON USING BAD RAM MODULES
    > +====================================
    > +
    > +
    > +Initial checks
    > + If you experience RAM trouble, first read /usr/src/linux/memory.txt
    Incorrect path. Insert "Documentation/". But we usually don't include
    the full path (i.e., drop "/usr/src/linux" or say
    "read Documentation/memory.txt in the kernel source tree".

    > + and try out the mem=4M trick to see if at least some initial parts
    > + of your RAM work well. The BadRAM routines halt the kernel in panic
    > + if the reserved area of memory (containing kernel stuff) contains
    > + a faulty address.
    > +
    > +Running a RAM checker
    > + The memory checker is not built into the kernel, to avoid delays at
    > + runtime. If you experience problems that may be caused by RAM, run
    > + a good RAM checker, such as
    > + http://reality.sgi.com/cbrady_denver/memtest86
    > + The output of a RAM checker provides addresses that went wrong. In
    > + the 32 MB chip with 512 faulty bits mentioned above, the errors were
    > + found in the 8MB-16MB range (the DIMM was in slot #0) at addresses
    > + xxx42f4
    > + xxx62f4
    > + xxxc2f4
    > + xxxe2f4
    > + and the error was a "sticky 1 bit", a memory bit that stayed "1" no
    > + matter what was written to it. The regularity of this pattern
    > + suggests the death of a buffer at the output stages of a row on one of
    > + the chips. I expect such regularity to be commonplace. Finding this
    > + regularity currently is human effort, but it should not be hard to
    > + alter a RAM checker to capture it in some sort of pattern, possibly
    > + the BadRAM patterns described below.
    > +
    > + By the way, if you manage to get hold of memtest86 version 2.3 or
    > + beyond, you can configure the printing mode to produce BadRAM patterns,
    > + which find out exactly what you must enter on the LILO: commandline,

    Drop ":".

    > + except that you shouldn't mention the added spacing. That means that
    > + you can skip the following step, which saves you a *lot* of work.
    > +
    > + Also by the way, if your machine has the ISA memory gap in the 15M-16M
    > + range unstoppable, Linux can get in trouble. One way of handling that
    > + situation is by specifying the total memory size to Linux with a boot
    > + parameter mem=... and then to tell it to treat the 15M-16M range as
    > + faulty with an additional boot parameter, for instance:
    > + mem=24M badram=0x00f00000,0xfff00000
    > + if you installed 24MB of RAM in total.
    > +
    > +
    > +Capturing errors in a pattern
    > + Instead of manually providing all 512 errors to the kernel, it's nicer
    > + to generate a pattern. Since the regularity is based on address decoding
    > + software, which generally takes certain bits into account and ignores
    > + others, we shall provide a faulty address F, together with a bit mask M
    > + that specifies which bits must be equal to F. In C code, an address A
    > + is faulty if and only if
    > + (F & M) == (A & M)
    > + or alternately (closer to a hardware implementation):
    > + ~((F ^ A) & M)
    > + In the example 32 MB chip, we had the faulty addresses in 8MB-16MB:
    > + xxx42f4 ....0100....
    > + xxx62f4 ....0110....
    > + xxxc2f4 ....1100....
    > + xxxe2f4 ....1110....
    > + The second column represents the alternating hex digit in binary form.
    > + Apperantly, the first and one-but last binary digit can be anything,

    Apparently
    s/one-but last/next to last/ ?

    > + so the binary mask for that part is 0101. The mask for the part after
    > + this is 0xfff, and the part before should select anything in the range
    > + 8MB-16MB, or 0x00800000-0x01000000; this is done with a bitmask
    > + 0xff80xxxx. Combining these partial masks, we get:
    > + F=0x008042f4 M=0xff805fff
    > + That covers everything for this DIMM; for more complicated failing
    > + DIMMs, or for a combination of multiple failing DIMMs, it can be
    > + necessary to set up a number of such F/M pairs.
    > +
    > +Rebooting Linux
    > + Now that these patterns are known (and double-checked, the calculations
    > + are highly error-prone... it would be neat to test them in the RAM
    > + checker...) we simply restart Linux with these F/M pairs as a parameter

    End above sentence with period (".").

    > + If you normally boot as follows:
    > + LILO: linux
    > + you should now boot with
    > + LILO: linux badram=0x008042f4,0xff805fff

    Does the choice of bootloader matter?

    > + or perhaps by mentioning more F/M pairs in an order F0,M0,F1,M1,...
    > + When you provide an odd number of arguments to badram, the default mask
    > + 0xffffffff (only one address matched) is applied to the pattern.
    > +
    > + Beware of the commandline length. At least up to LILO version 0.21,
    > + the commandline is cut off after the 78th character; later versions
    > + may go as far as the kernel goes, namely 255 characters. In no way is
    > + it possible to enter more than 10 numbers to the badram boot option.

    x86 command line length is now 2048.
    I don't know if bootloaders can handle that.

    > + When the kernel now boots, it should not give any trouble with RAM.
    > + Mind you, this is under the assumption that the kernel and its data
    > + storage do not overlap an erroneous part. If this happens, and the
    > + kernel does not choke on it right away, it will stop with a panic.
    > + You will need to provide a RAM where the initial, say 2MB, is faultless

    End with period (".").

    > +
    > + Now look up your memory status with
    > + dmesg | grep ^Memory:
    > + which prints a single line with information like
    > + Memory: 158524k/163840k available
    > + (940k kernel code,
    > + 412k reserved,
    > + 1856k data,
    > + 60k init,
    > + 0k highmem,
    > + 2048k BadRAM)

    > +Known Bugs
    > + LILO is known to cut off commandlines which are too long. For the
    > + lilo-0.21 distribution, a commandline may not exceed 78 characters,
    > + while actually, 255 would be possible [on x86, kernel 2.2.16].

    Ancient kernel alert.

    > + LILO does _not_ report too-long commandlines, but the error will
    > + show up as either a panic at boot time, stating
    > + panic: BadRAM page in initial area
    > + or the dmesg line starting with Memory: will mention an unpredicted
    > + number of kilobytes. (Note that the latter number only includes
    > + errors in accessed memory.)
    > +
    > +Future Possibilities
    > + It would be possible to use even more of the faulty RAMs by employing
    > + them for slabs. The smaller allocation granularity of slabs makes it
    > + possible to throw out just, say, 32 bytes surrounding an error. This
    > + would mean that the example DIMM only looses 16kB instead of 2MB.

    loses

    > + It might even be possible to allocate the slabs in such a way that,
    > + where possible, the remaining bytes in a slab structure are allocated
    > + around the error, reducing the RAM loss to 0 in the optimal situation!
    > +
    > + However, this yield is somewhat faked: It is possible to provide 512
    > + pages of 32-byte slabs, but it is not certain that anyone would use
    > + that many 32-byte slabs at any time.
    > +
    > + A better solution might be to alter the page allocation for a slab to
    > + have a preference for BadRAM pages, and given those a special treatment.
    > + This way, the BadRAM would be spread over all the slabs, which seems
    > + more likely to be a `true' pay-off. This would yield more overhead at
    > + slab allocation time, but on the other hand, by the nature of slabs,
    > + such allocations are made as rare as possible, so it might not matter
    > + that much. I am uncertain where to go.
    > +
    > + Many suggestions have been made to insert a RAM checker at boot time;
    > + since this would leave the time to do only very meager checking, it
    > + is not a reasonable option; we already have a BIOS doing that in most
    > + systems!
    > +
    > + It would be interesting to integrate this functionality with the
    > + self-verifying nature of ECC RAM. These memories can even distinguish
    > + between recorable and unrecoverable errors! Such memory has been

    recoverable

    > + handled in older operating systems by `testing' once-failed memory
    > + blocks for a while, by placing only (reloadable) program code in it.
    > + Unfortunately, I possess no faulty ECC modules to work this out.
    > +

    > diff -pruN linux-2.6.24.orig/Documentation/kernel-parameters.txt linux-2.6.24/Documentation/kernel-parameters.txt
    > --- linux-2.6.24.orig/Documentation/kernel-parameters.txt 2008-01-24 17:58:37.000000000 -0500
    > +++ linux-2.6.24/Documentation/kernel-parameters.txt 2008-02-05 23:33:55.000000000 -0500
    > @@ -322,6 +323,8 @@ and is between 256 and 4096 characters.
    >
    > autotest [IA64]
    >
    > + badram= [BADRAM] Avoid allocating faulty RAM addresses.

    See Documentation/badram.txt for parameter details.

    > +
    > baycom_epp= [HW,AX25]
    > Format: <io>,<mode>
    >
    > diff -pruN linux-2.6.24.orig/Documentation/memory.txt linux-2.6.24/Documentation/memory.txt
    > --- linux-2.6.24.orig/Documentation/memory.txt 2008-01-24 17:58:37.000000000 -0500
    > +++ linux-2.6.24/Documentation/memory.txt 2008-02-05 23:39:04.000000000 -0500
    > @@ -18,11 +18,22 @@ systems.
    > as you add more memory. Consider exchanging your
    > motherboard.
    >
    > + 4) A static discharge or production fault causes a RAM module
    > + to have (predictable) errors, usually meaning that certain
    > + bits cannot be set or reset. Instead of throwing away your
    > + RAM module, you may read /usr/src/linux/Documentation/badram.txt
    > + to learn how to detect, locate and circuimvent such errors

    circumvent

    > + in your RAM module.
    > +
    > +
    > diff -pruN linux-2.6.24.orig/include/asm-x86/page_32.h linux-2.6.24/include/asm-x86/page_32.h
    > --- linux-2.6.24.orig/include/asm-x86/page_32.h 2008-01-24 17:58:37.000000000 -0500
    > +++ linux-2.6.24/include/asm-x86/page_32.h 2008-02-05 23:43:00.000000000 -0500
    > @@ -189,6 +189,7 @@ extern int page_is_ram(unsigned long pag
    > #define pfn_valid(pfn) ((pfn) < max_mapnr)
    > #endif /* CONFIG_FLATMEM */
    > #define virt_to_page(kaddr) pfn_to_page(__pa(kaddr) >> PAGE_SHIFT)
    > +#define phys_to_page(x) pfn_to_page((unsigned long)(x) >> PAGE_SHIFT)

    Use tab(s), not spaces.

    >
    > #define virt_addr_valid(kaddr) pfn_valid(__pa(kaddr) >> PAGE_SHIFT)
    >
    > diff -pruN linux-2.6.24.orig/include/asm-x86/page_64.h linux-2.6.24/include/asm-x86/page_64.h
    > --- linux-2.6.24.orig/include/asm-x86/page_64.h 2008-01-24 17:58:37.000000000 -0500
    > +++ linux-2.6.24/include/asm-x86/page_64.h 2008-02-05 23:44:26.000000000 -0500
    > @@ -126,6 +126,7 @@ extern unsigned long __phys_addr(unsigne
    > #endif
    >
    > #define virt_to_page(kaddr) pfn_to_page(__pa(kaddr) >> PAGE_SHIFT)
    > +#define phys_to_page(x) pfn_to_page((unsigned long)(x) >> PAGE_SHIFT)

    Ditto.

    > #define virt_addr_valid(kaddr) pfn_valid(__pa(kaddr) >> PAGE_SHIFT)
    > #define pfn_to_kaddr(pfn) __va((pfn) << PAGE_SHIFT)
    >
    > diff -pruN linux-2.6.24.orig/mm/page_alloc.c linux-2.6.24/mm/page_alloc.c
    > --- linux-2.6.24.orig/mm/page_alloc.c 2008-01-24 17:58:37.000000000 -0500
    > +++ linux-2.6.24/mm/page_alloc.c 2008-02-06 00:03:28.000000000 -0500
    > @@ -4378,6 +4381,91 @@ EXPORT_SYMBOL(pfn_to_page);
    > EXPORT_SYMBOL(page_to_pfn);
    > #endif /* CONFIG_OUT_OF_LINE_PFN_TO_PAGE */
    >
    > +
    > +#ifdef CONFIG_BADRAM
    > +
    > +
    > +void __init badram_markpages (int argc, unsigned long *argv) {
    > + unsigned long addr, mask;
    > + while (argc-- > 0) {
    > + addr = *argv++;
    > + mask = (argc-- > 0) ? *argv++ : ~0L;
    > + mask |= ~PAGE_MASK; /* Optimalisation */

    Optimisation ?

    > + addr &= mask; /* Normalisation */
    > + do {
    > + struct page *pg = phys_to_page(addr);
    > + printk(KERN_DEBUG "%016lx =%016lx\n",
    > + addr >> PAGE_SHIFT,
    > + (unsigned long)(pg-mem_map));
    > + if (PageTestandSetBad (pg))
    > + reserve_bootmem (addr, PAGE_SIZE);
    > + } while (next_masked_address (&addr,mask));
    > + }
    > +}
    > +
    > +
    > +
    > +static int __init badram_setup (char *str)
    > +{
    > + unsigned long opts[3];
    > + BUG_ON(!mem_map);
    > + printk (KERN_INFO "PAGE_OFFSET=0x%08lx\n", PAGE_OFFSET);
    > + printk (KERN_INFO "BadRAM option is %s\n", str);

    No space after function name (2x).

    > + if (*str++ == '=')
    > + while ((str = get_longoptions (str, 3, (long *) opts), *opts)) {
    > + printk (KERN_INFO " --> marking 0x%08lx, 0x%08lx [%ld]\n",
    > + opts[1], opts[2], opts[0]);
    > + badram_markpages (*opts, opts+1);
    > + if (*opts == 1)
    > + break;
    > + };
    > + badram_markpages (*badram_custom, badram_custom+1);
    > + return 0;
    > +}


    ---
    ~Randy

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