Re: [PATCH RFC] ARM: option for loading modules into vmalloc area

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Nov 18, 2014 at 08:21:46PM +0400, Konstantin Khlebnikov wrote:
> Usually modules are loaded into small area prior to the kernel
> text because they are linked with the kernel using short calls.
> Compile-time instrumentation like GCOV or KASAN bloats code a lot,
> and as a result huge modules no longer fit into reserved area.
> 
> This patch adds option CONFIG_MODULES_USE_VMALLOC which lifts
> limitation on amount of loaded modules. It links modules using
> long-calls (option -mlong-calls) and loads them into vmalloc area.
> 
> In few places exported symbols are called from inline assembly.
> This patch adds macro for such call sites: __asmbl and __asmbl_clobber.
> Call turns into single 'bl' or sequence 'movw; movt; blx' depending on
> context and state of config option.
> 
> Unfortunately this option isn't compatible with CONFIG_FUNCTION_TRACER.
> Compiler emits short calls to profiling function despite of -mlong-calls.
> This is a bug in GCC, but ftrace anyway needs an update to handle this.

It also isn't compatible with the older architectures which don't have
"blx".

> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> index 9d2cdda..9e0c4f4 100644
> --- a/arch/arm/mm/mmu.c
> +++ b/arch/arm/mm/mmu.c
> @@ -1161,16 +1161,15 @@ void __init sanity_check_meminfo(void)
>  
>  static inline void prepare_page_table(void)
>  {
> -	unsigned long addr;
> +	unsigned long addr = 0;
>  	phys_addr_t end;
>  
>  	/*
>  	 * Clear out all the mappings below the kernel image.
>  	 */
> -	for (addr = 0; addr < MODULES_VADDR; addr += PMD_SIZE)
> -		pmd_clear(pmd_off_k(addr));
> -
>  #ifdef CONFIG_XIP_KERNEL
> +	for ( ; addr < MODULES_VADDR; addr += PMD_SIZE)
> +		pmd_clear(pmd_off_k(addr));
>  	/* The XIP kernel is mapped in the module area -- skip over it */
>  	addr = ((unsigned long)_etext + PMD_SIZE - 1) & PMD_MASK;
>  #endif

This seems to be an unrelated change.  In any case, it looks wrong to
me.  Please describe what you are trying to achieve here.

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