On Wed, 22 Jun 2011 13:18:51 +0200 Stefan Assmann <sassmann@xxxxxxxxx> 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? Thanks -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>