Bugzilla Bug #242648: kdump broken on x86_64 describes a fix in 5.1 that is needed to get the kdump kernel working on some systems. in the interests of keeping the set of rhel-rt packages as lean as possible, it may be necessary to provide the RHEL5.1 (beta?) kdump kernel through the yum repo for RHEL-RT. the howto needs to be updated with instructions
Note that that patch should not be necessary, and in fact won't apply to kernel-rt-2.6.21-25.el5rt. The upstream kernel, and therefore the -rt kernel, is less restrictive when checking the "crashkernel=" command line argument, just making sure that they're not the same value: #ifdef CONFIG_KEXEC if (crashk_res.start != crashk_res.end) { reserve_bootmem_generic(crashk_res.start, crashk_res.end - crashk_res.start + 1); } #endif whereas the broken, but more restrictive, RHEL5.1 code does this: if ((crashk_res.start < crashk_res.end) && (crashk_res.end <= (max_low_pfn << PAGE_SHIFT))) { reserve_bootmem_generic(crashk_res.start, crashk_res.end - crashk_res.start + 1); } where the RHEL5.1 patch replaces "max_low_pfn" with "end_pfn" for x86_64. So if you just leave things as they are, the -rt kernel should load the secondary (RHEL5.1) kernel OK... ;-)
This patch affects the boot kernel (it involves the parsing of the crashkernel= option), not the kdump kernel. The RHEL5 kernel can be used as a kdump kernel even with this bug. The patch which introduced the bug has not been carried into the RT kernel, so it does not affect the crashkernel= parsing in the RT kernel.