If the size of RAM is not an exact power of two, we may not have covered RAM in its entirety with large 16 and 4 MiB pages. Consequently, restrict the top end of RAM currently allocable by updating '__initial_memory_limit_addr' so that calls to the LMB to allocate PTEs for "tail" coverage with normal-sized pages (or other reasons) do not attempt to allocate outside the allowed range.
Signed-off-by: Grant Erickson <[EMAIL PROTECTED]> --- This bug was discovered in the course of working on CONFIG_LOGBUFFER support (see http://ozlabs.org/pipermail/linuxppc-dev/2008-October/064685.html). However, the bug is triggered quite easily independent of that feature by placing a memory limit via the 'mem=' kernel command line that results in a memory size that is not equal to an exact power of two. For example, on the AMCC PowerPC 405EXr "Haleakala" board with 256 MiB of RAM, mmu_mapin_ram() normally covers RAM with precisely 16 16 MiB large pages. However, if a memory limit of 256 MiB - 20 KiB (as might be the case for CONFIG_LOGBUFFER) is put in place with "mem=268414976", then large pages only cover (16 MiB * 15) + (4 MiB * 3) = 252 MiB with a 4 MiB - 20 KiB "tail" to cover with normal, 4 KiB pages via map_page(). Unfortunately, if __initial_memory_limit_addr is not updated from its initial value of 0x1000 0000 (256 MiB) to reflect what was actually mapped via mmu_mapin_ram(), the following happens during the "tail" mapping when the first PTE is allocated at 0xFFF A000 (rather than the desired 0xFBF F000): mapin_ram mmu_mapin_ram map_page pte_alloc_kernel pte_alloc_one_kernel early_get_page lmb_alloc_base clear_page clear_pages dcbz 0,page <-- BOOM! a non-recoverable page fault. arch/powerpc/mm/40x_mmu.c | 16 ++++++++++++++-- 1 files changed, 14 insertions(+), 2 deletions(-) diff --git a/arch/powerpc/mm/40x_mmu.c b/arch/powerpc/mm/40x_mmu.c index cecbbc7..29954dc 100644 --- a/arch/powerpc/mm/40x_mmu.c +++ b/arch/powerpc/mm/40x_mmu.c @@ -93,7 +93,7 @@ void __init MMU_init_hw(void) unsigned long __init mmu_mapin_ram(void) { - unsigned long v, s; + unsigned long v, s, mapped; phys_addr_t p; v = KERNELBASE; @@ -130,5 +130,17 @@ unsigned long __init mmu_mapin_ram(void) s -= LARGE_PAGE_SIZE_4M; } - return total_lowmem - s; + mapped = total_lowmem - s; + + /* If the size of RAM is not an exact power of two, we may not + * have covered RAM in its entirety with 16 and 4 MiB + * pages. Consequently, restrict the top end of RAM currently + * allocable so that calls to the LMB to allocate PTEs for "tail" + * coverage with normal-sized pages (or other reasons) do not + * attempt to allocate outside the allowed range. + */ + + __initial_memory_limit_addr = memstart_addr + mapped; + + return mapped; } -- 1.6.0.1 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev
