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Date: Fri, 27 Feb 2009 12:26:02 -0800
From: Yinghai Lu <yinghai@...nel.org>
To: Jeremy Fitzhardinge <jeremy@...p.org>
CC: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] x86: add brk allocation for very, very early allocations
Jeremy Fitzhardinge wrote:
> [
> I'd like to add a mechanism like this so I can dynamically allocate some
> Xen-related structures, rather than statically allocating them in the bss,
> both so that Xen has less overhead when it isn't being used, and so I can
> scale better to things like memory size.
>
> I think this is more widely useful; it would supplant dmi_alloc_data[], for
> example, and I'm sure there's other cases.
>
> This is fundimentally the same as head_32.S's extension of the bss to build
> the initial kernel mapping, but 64-bit doesn't currently do anything
> analogous
> to this.
>
> Unfortunately when I use this code as-is I'm getting crashes when the slab
> allocator starts up. I think this is all correct, but I'm wondering if
> there's something I'm overlooking which is broken in principle.
>
> So, what am I missing?
>
> Thanks,
> J
> ]
>
>
> Add a brk()-like allocator which effectively extends the bss
> in order to allow very early code to do dynamic allocations.
> This is better than using statically allocated arrays for
> data in subsystems which may never get used.
>
> The amount of space available depends on how much the initial
> kernel mappings have covered, and so is fairly limited.
>
> Not-Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>
> diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
> index 66801cb..e6b754b 100644
> --- a/arch/x86/include/asm/setup.h
> +++ b/arch/x86/include/asm/setup.h
> @@ -99,6 +99,11 @@ extern struct boot_params boot_params;
> */
> #define LOWMEMSIZE() (0x9f000)
>
> +/* exceedingly early brk-like allocator */
> +extern unsigned long _brk_start, _brk_end;
> +void init_brk(unsigned long start);
> +void *extend_brk(size_t size, size_t align);
> +
> #ifdef __i386__
>
> void __init i386_start_kernel(void);
> diff --git a/arch/x86/kernel/head32.c b/arch/x86/kernel/head32.c
> index ac108d1..fa9ae31 100644
> --- a/arch/x86/kernel/head32.c
> +++ b/arch/x86/kernel/head32.c
> @@ -34,6 +34,8 @@ void __init i386_start_kernel(void)
>
> reserve_ebda_region();
>
> + init_brk((unsigned long)__va(init_pg_tables_end));
> +
> /*
> * At this point everything still needed from the boot loader
> * or BIOS or kernel text should be early reserved or marked not
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index f5b2722..4b29802 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -91,6 +91,8 @@ void __init x86_64_start_kernel(char * real_mode_data)
> if (console_loglevel == 10)
> early_printk("Kernel alive\n");
>
> + init_brk((unsigned long)&_end);
> +
> x86_64_start_reservations(real_mode_data);
> }
>
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 0d051b4..8899cfa 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -113,6 +113,7 @@
> #endif
>
> unsigned int boot_cpu_id __read_mostly;
> +__initdata unsigned long _brk_start, _brk_end;
>
> #ifdef CONFIG_X86_64
> int default_cpu_present_to_apicid(int mps_cpu)
> @@ -335,6 +336,26 @@ static void __init relocate_initrd(void)
> }
> #endif
>
> +void __init init_brk(unsigned long brk)
> +{
> + _brk_start = _brk_end = brk;
> +}
> +
> +void * __init extend_brk(size_t size, size_t align)
> +{
> + size_t mask = align - 1;
> + void *ret;
> +
> + BUG_ON(align & mask);
> +
> + _brk_end = (_brk_end + mask) & ~mask;
> +
> + ret = (void *)_brk_end;
> + _brk_end += size;
> +
> + return ret;
> +}
> +
> static void __init reserve_initrd(void)
> {
> u64 ramdisk_image = boot_params.hdr.ramdisk_image;
> @@ -727,11 +748,7 @@ void __init setup_arch(char **cmdline_p)
> init_mm.start_code = (unsigned long) _text;
> init_mm.end_code = (unsigned long) _etext;
> init_mm.end_data = (unsigned long) _edata;
> -#ifdef CONFIG_X86_32
> - init_mm.brk = init_pg_tables_end + PAGE_OFFSET;
> -#else
> - init_mm.brk = (unsigned long) &_end;
> -#endif
> + init_mm.brk = _brk_end;
>
> code_resource.start = virt_to_phys(_text);
> code_resource.end = virt_to_phys(_etext)-1;
> @@ -897,6 +914,9 @@ void __init setup_arch(char **cmdline_p)
> acpi_numa_init();
> #endif
>
> + if (_brk_end > _brk_start)
> + reserve_early(__pa(_brk_start), __pa(_brk_end), "BRK");
> +
> initmem_init(0, max_pfn);
it seems reserve _brk_end is some late?
init_memory_mapping(0,...) could get some for direct mapping page table. and it could start from _end...
YH
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