On Wed, 22 Jan 2014, Wang Nan wrote:
> This patch allows the kernel to be loaded at the middle of kernel awared
> physical memory. Before this patch, users must use mem= or device tree to cheat
> kernel about the start address of physical memory.
>
> This feature is useful in some special cases, for example, building a crash
> dump kernel. Without it, kernel command line, atag and devicetree must be
> adjusted carefully, sometimes is impossible.
With CONFIG_PATCH_PHYS_VIRT the value for PHYS_OFFSET is determined
dynamically by rounding down the kernel image start address to the
previous 16MB boundary. In the case of a crash kernel, this might be
cleaner to simply readjust __pv_phys_offset during early boot and call
fixup_pv_table(), and then reserve away the memory from the previous
kernel. That will let you access that memory directly (with gdb for
example) and no pointer address translation will be required.
> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 3.4+
> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Geng Hui <hui.geng@xxxxxxxxxx>
> ---
> arch/arm/mm/init.c | 21 ++++++++++++++++++++-
> arch/arm/mm/mmu.c | 13 +++++++++++++
> mm/page_alloc.c | 7 +++++--
> 3 files changed, 38 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 3e8f106..4952726 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -334,9 +334,28 @@ void __init arm_memblock_init(struct meminfo *mi,
> {
> int i;
>
> - for (i = 0; i < mi->nr_banks; i++)
> + for (i = 0; i < mi->nr_banks; i++) {
> memblock_add(mi->bank[i].start, mi->bank[i].size);
>
> + /*
> + * In some special case, for example, building a crushdump
> + * kernel, we want the kernel to be loaded in the middle of
> + * physical memory. In such case, the physical memory before
> + * PHYS_OFFSET is awkward: it can't get directly mapped
> + * (because its address will be smaller than PAGE_OFFSET,
> + * disturbs user address space) also can't be mapped as
> + * HighMem. We reserve such pages here. The only way to access
> + * those pages is ioremap.
> + */
> + if (mi->bank[i].start < PHYS_OFFSET) {
> + unsigned long reserv_size = PHYS_OFFSET -
> + mi->bank[i].start;
> + if (reserv_size > mi->bank[i].size)
> + reserv_size = mi->bank[i].size;
> + memblock_reserve(mi->bank[i].start, reserv_size);
> + }
> + }
> +
> /* Register the kernel text, kernel data and initrd with memblock. */
> #ifdef CONFIG_XIP_KERNEL
> memblock_reserve(__pa(_sdata), _end - _sdata);
> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> index 580ef2d..2a17c24 100644
> --- a/arch/arm/mm/mmu.c
> +++ b/arch/arm/mm/mmu.c
> @@ -1308,6 +1308,19 @@ static void __init map_lowmem(void)
> if (start >= end)
> break;
>
> + /*
> + * If this memblock contain memory before PAGE_OFFSET, memory
> + * before PAGE_OFFSET should't get directly mapped, see code
> + * in create_mapping(). However, memory after PAGE_OFFSET is
> + * occupyed by kernel and still need to be mapped.
> + */
> + if (__phys_to_virt(start) < PAGE_OFFSET) {
> + if (__phys_to_virt(end) > PAGE_OFFSET)
> + start = __virt_to_phys(PAGE_OFFSET);
> + else
> + break;
> + }
> +
> map.pfn = __phys_to_pfn(start);
> map.virtual = __phys_to_virt(start);
> map.length = end - start;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5248fe0..d2959e3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4840,10 +4840,13 @@ static void __init_refok alloc_node_mem_map(struct pglist_data *pgdat)
> */
> if (pgdat == NODE_DATA(0)) {
> mem_map = NODE_DATA(0)->node_mem_map;
> -#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
> + /*
> + * In case of CONFIG_HAVE_MEMBLOCK_NODE_MAP or when kernel
> + * loaded at the middle of physical memory, mem_map should
> + * be adjusted.
> + */
> if (page_to_pfn(mem_map) != pgdat->node_start_pfn)
> mem_map -= (pgdat->node_start_pfn - ARCH_PFN_OFFSET);
> -#endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
> }
> #endif
> #endif /* CONFIG_FLAT_NODE_MEM_MAP */
> --
> 1.8.4
>
> --
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