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

From: [email-masked] (Konstantin Khlebnikov)
Date: Tue, 18 Nov 2014 21:13:56 +0300

On 2014-11-18 20:34, Russell King - ARM Linux wrote:
> 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".

Ok, I'll add "depends on CPU_V6 || CPU_V7" I don't think that it is 
necessary for older cpus.

>> 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.
>

My patch changes  MODULES_VADDR, if option enabled it becomes alias for 
VMALLOC_START

It's used as border between two loops, before patch this code looks like:

     /*
      * 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
     /* The XIP kernel is mapped in the module area -- skip over it */
     addr = ((unsigned long)_etext + PMD_SIZE - 1) & PMD_MASK;
#endif
     for ( ; addr < PAGE_OFFSET; addr += PMD_SIZE)
         pmd_clear(pmd_off_k(addr));

It's written with assumption that MODULES_VADDR < PAGE_OFFSET,
but this no longer true if modules are placed inside vmalloc area.

After patch:

     unsigned long addr = 0;

     /*
      * Clear out all the mappings below the kernel image.
      */
#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
     for ( ; addr < PAGE_OFFSET; addr += PMD_SIZE)
         pmd_clear(pmd_off_k(addr));

Without XIP there is no reason for splitting it into two loops.